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We will continue with the Printed Wiring Board Technologies module. We have finished four
hours in this particular chapter and if you have observed, what we have basically done
is, taken you through the basic process steps of fabricating and understanding the technologies
involved in the manufacture of a printed wiring board. It can be a simple single-sided board
or a double-sided board plated through hole board or a multilayer board or a high density
interconnects structure.
Basically, what we have seen now is, the individual processes have been described. So, that these
basic processes recur at every layer manufacture. It is going to be repeated, if you are going
to do a multilayer structure and therefore, it is very essential to know the basics of
the key processes that we are now discussing.
Now in the previous stock, we have seen about the laminate selection. What kind of laminates
you have to choose, the quality of the laminates and then, we spend a lot of time on imaging.
Before that, we talked about cleaning and preparing the substrate for the manufacture
or the imaging process. Then the imaging and image transfer including photolithography
was discussed. Now we are going to see, how a circuit is established on the copper surface
after the developing process is completed in the image transfer process. If you recollect,
we have used dry film or a wet film photoresist; we have applied UV light and then we had cured
the board. We have developed the board and now, the board is ready for the etching process.
Now, etching as the term indicates is the removal of unwanted or non-circuit copper
areas from the board. You can see in this figure at the top, what you see here, the
red lines are the copper areas; that means, these are a typical double-sided copper structure.
On the top, imaging has been done using a dry film photoresist material and then it
has been developed, opened and then you can see some areas of copper have been etched
out and some copper areas are remaining. Typically, in this particular cross section, what you
should understand is that the area behind the mask or the resist is the circuit areas
the non-circuit areas have been removed. This is typically depicting the etching process
in the printed wiring board sequence. Now the term etch resist, you should understand
because, you can have organic and metallic resist. As the term indicates, resist is the
term for resisting the etching process. The resist material should not be eaten away or
react with the etchant chemical. So that is why the term etch resist comes into usage.
You can have an organic resist like a dry film material or a wet film material. Typically,
a photoresist material can behave as a etch resist or you can have metallic resist. In
the case of metallic resist, typical examples can be tin, which is plated on copper then
you can have gold; gold is usually plated after an under coat of nickel is plated, so
you can have nickel, gold plating which can act as a etch resist.
Then you can have other alloys typically, based on silver and so on, but in general
practice, more than 90 percent of the etch resist - in the classification of metallic
resist - is tin or nickel gold. The reason why we use nickel as an under coat of a gold
plating is that, if you directly plate gold on to copper, after some time or during its
operation and based on the environmental factors and so on and also due to the inherent crystal
structure and properties of copper and gold; gold can dissolve into copper interstices
and therefore, after sometime, you will see that the thickness of the gold that you have
plated is gradually reduced. The reason why we plate gold is, because of
the good contact resistance, good conductivity and you need to plate only a very thin film
of gold compared to larger thicknesses of tin, for example if you want to protect copper.
So, nickel is known as a under coat or very basic essential requirement. If you want to
plate gold on copper, the thickness of nickel can be around 3 to 5 microns and gold can
be about a micron or less than 1 micron. Now, after the etching process is over the
circuit gets defined, you are removing all the unwanted copper. Now, the etchant chemicals
are corrosive in nature, typical examples of etchant chemical, as you can see at the
bottom of the slide are ferric chloride, cupric chloride, ammonium persulfate - which is a
micro etchant, chromic acid and sulfuric acid combination and so on. For the through whole
processes, we also use ammoniacal - alkaline ammonia solution. These of the basic resist
etchants that you will see and also the etch resist given here typically organic and metallic.
Now, etching is the process which is dependent on time and temperature. If you take an etchant;
let us say cupric chloride, you have to maintain suitable working temperatures for this cupric
chloride. Then, if you are using machine based conveyorized etching, then there are other
factors that come into the picture that are based on the equipment - the machine. Therefore,
those controls have to be taken care of, maintenance is a crucial issue, if you want to get very
high performance etching and especially, if you are doing fine lines of the order of 4
mills and so on. Your etching process has to be well controlled because, time and temperature
are going to be closely monitored. If you want to etch away a few microns of copper
perfectly with very clean edges, but etching normally, whatever be the best controls that
you do, etching is not a perfect process, there will always be defects because, it is
a very fast process and it is only the resist that protects your copper, nothing else. Process
control is required therefore. There is a term called undercut, so undercut
issues in etching are very important. We are going to discuss, what undercut is. Basically,
as the name indicates, if the etchant is going to sweep under the photoresist material here
and remove copper in those areas which are supposed to be protected then, it is known
as an undercut. Undercut can be fairly small in number or it can be large, if you are etching
controls or not well monitored. So the idea is, you should have minimum undercut on the
lines and the pads that you have defined. Now, the environmental question of a recycling
of spent etchants is not a normally well considered, but if you look at the current legislations
of the usage of certain chemicals and also the local environmental regulatory body, they
will issues the guidelines, how you have to recycle yours spent etchant? But, today it
is somewhat straightforward; that means, if you buy an etchant from a company A; the company
A is willing to take back your spent etchant at a fairly nominal price. So in that sense,
you do not have to do the recycling. The manufacturers themselves are willing to take back the spent
etchant; spent etchant means that you cannot regenerate that etchant any further.
The next point is, regeneration of etch solutions is required frequently, because when you makeup
an etch solution, the concentration of the basic etchant ions or the etchant free radicals
or whatever is generated based on the etchant, it has to be regenerated. Now, the rate of
etching comes down slowly, as you progress using the same etchant. Therefore, you need
to monitor regularly the concentration of the copper that goes into the solution, because
the basic reaction of an etchant with the copper surface on the PCB is that the copper
has to dissolve into the etchant. Therefore, as the copper concentration increases
in the etchant your reaction slows down; the rate of etching goes down. Therefore your
etch time, which is actually a very key factor in large volume manufacturing, so the time
will increase which will again indirectly show up in the undercut. Regeneration of etch
solution is considered at most necessary in large volume manufacturing and typically,
you are going to make an extent in the etchant life, but after sometime, you will understand
that it has been classified as a spent etchant and you have to make up a new solution.
Typically, in large volume manufacturing, you will use conveyorized operations because
etchant chemical is corrosive. You have to take care of the pollution factor and also
the safety of the personal working in the lab or the industry. We have seen in our previous
class, a very important factor that determines the design from manufacturing aspect; that
is aspect ratio, which is defined as laminate thickness by whole dia. We have seen, how
a designer needs to know whether a particular aspect ratio in a design can be manufacture
or not, because smaller holes are difficult to plate. Therefore, the plating solution
will not wet the small holes therefore, the reliability is a concerned. We have said that
to keep the aspect ratio as small as possible in the range of 2 to 4.
Similarly, another key parameter that you have to consider with more is concerned with
the manufacturer - here not really with designer, is that etch factor is a key issue because
no etchant gives perfect straight cuts. Etching always will have difficulties, you will always
observe undercuts. If you carefully look into the microscope of the side wall of the copper
track that you have etched, you will always see defects.
Therefore, you have to provide an etching allowance of 5 two 8 percent on the track
width. That is where I say designers needs to understand, what an etch factor is or the
manufacture can suggest to designer that if you have kept track at 6 mills, you can give
an etching allowance of 5 to eight 8 on the track width because incase, the manufacture
has observed that consistently in his manufacturing, he is getting an etch profile like this. What
you see here in this picture the red one is the copper ,the top blue one is the mask or
the resist. It can be an organic resist or it can be a metallic resist.
After the etching process is over normally, we would be delighted to see a perfect cut
but, if you look into the microscope, you will see that there is a deviation. You can
see that the area steepering down from the top surface of the copper and extending to
the complete width of the copper that you have taken; you will see progressively copper
has been eaten away. This distance that you see here, can play a major part in defining
the electrical performance of that particular track. Therefore, if you get an etch profile
like this; the extent of undercut, so the copper that is eaten away here, it is known
as the undercut, you have to minimize the undercut.
If you minimize the undercut then, that directly reflect on the etch factor. Etch factor is
a number that defines the quality of etching of a particular etchant. For example, material
like cupric chloride will have an etch factor, alkaline ammonia will have an etch factor
number; then ferric chloride will have a number and so on. So do not forget to discuss with
the manufacturer about the aspect ratio and etch factor.
They are two very important processes not to be ignored. Let us try to define what an
etch factor is? The ratios of depth of etch - so what you see here. This is the depth
of etch to the amount of lateral etch; that you see, there is a ratio of conductor thickness
to the amount of undercut. Basically, it gives an idea about the extent of undercut that
has taken place despite having a metallic resist or an organic resist, which is a fairly
resistant to the etchant that you have used. If you look at this picture here. You can
understand, this is a cross section of the base core structure or the FR-4. For example,
this is the plated copper and you have protected on top with tin or tin lead; this is the undercut.
You have to measure it using a microscopes, how do you look at etch factor? You are not
going look at etch factor every day in manufacturing. If you prepare a new etchant or if you have
working consistently with the same etchant, you can do it once in a while but, if you
are moving to some new etchant then you have prepare a sample, test board to perform the
etching and then do a cross section and then into the microscope, you go to the exact area
where you have observed the undercut, measure the distant and then you know the thickness;
you know the starting thickness of copper. So, the ratio of the conductor thickness to
the amount of undercut will give you the etch factor. The etch factor has to be very high,
which means the undercut is going to be very very low. So that is what you actually desire.
As a designer, you are aware of this problem and you can give a certain amount of allowance
on the conductor thickness. You can increase the conductor thickness by 2 to 5 percent
and then really not worry about the undercut. Now, we talking about wet etching where all
the problem comes. Wet etching is where the material that is the copper; this case, the
PCB is immersed into the chemical solution - that is the etchant - where it dissolves
completely in the etchant material. Now, the etch factor is V by X as I mentioned here.
If you want an equation for etch factor, you know the conductor thickness and you know
the extent of undercut which is X, in this case. So, V by X will give you the etch factor.
The etch factor is again different for different chemicals.
Coming to wet etching and now to dry etching, where the material will be sputtered or dissolved
using reactive ions that are generated in an equipment typically, in the vapour phase
etchant chamber. There are lots of differences between using a dry etchant, you can also
use a dry plasma; that means, in a reactive ion chamber, you can create reactive ions
which form plasma and then they react with a copper surface and remove the copper from
the surface of the PCB, so that a dry etching process. There is going to be a lot of difference
between a dry etching and wet etchings. Wet etching is typically very useful advantages
for large area removal of copper, whereas dry etching, typically does not work well
for large volume manufacturing and you can use a dry etching for prototyping.
Now, we are going to look at a video of the etching process and also final step of removal
of photoresist which has behaved as a etch resist material in this case, the photoresist
is behaving as a etch resist. This video as been shot at CEDT, you can see here. We have
equipment which uses cupric chloride as the etchant and typically this is an aqueous based
solution - cupric chloride dissolved in water and other chemicals added. Typically an acid
cupric chloride; so use hydrochloric acid to the required percentages. So, basically
this is an acid etchant. You also have alkaline etchants, which we will also discuss later.
After etching, the important process is to remove the photoresist which acted as an etch
resist. You can see here, etchant is a very corrosive chemical even the equipment that
you have used in this case, it is actually a PVC material, even then you can see that
the leakages from the spray of etchant and the fumes that comes out from the conveyorized
etching machine or the batch etching machine will create a lot of corrosion. So, the personal
safety is very important and also we have to look at how you can save the equipment
for extended life. Now, typically in etchant equipment you start
with the power supply; then you have the conveyorized equipment in this case, for example, you can
see that this particular board is now imaged and developed. You can see here the areas
that need to be etched are exposed; that is, the copper areas here are exposed to the etchant.
The pink area that you see here are the protected areas pertaining to the circuitry. Now, the
board enters into the conveyorized chamber and then typically the equipment will have
spray nozzles that spray the etchant. Then finally, you will remove the protective photoresist
and then you will get the completed circuit.
So, I will switch on this video once again for your attention. Now, this is the stripling
chemical which is basically a hydroxide - potassium hydroxide which removes the photoresist material
that completes the etching process.
Now, let us briefly look into the chemistry involved in PCB etching that is copper etching.
If you take ion ferry chloride, Fe is present as Fe 3 plus and it reacts with the copper
on the surface of the PCB and then you get a ferrous ion Fe 2 plus and Cu goes into cupric
2 plus. So, ferric chloride reacts with the copper at the PCB to give ferrous ions and
cupric ions. Now, this concentration will keep on increasing
and you will see that the ferric ion concentration will keep on reducing that is why the rate
of etching gradually decreases. So, regeneration is required in this typical case. So, what
you do is, you add regenerate chemical which is basically a source of oxygen; let us say
hydrogen peroxide, now to convert to the Fe 2 plus to Fe 3 plus. So that the reaction
can progress further to react with copper on the PCB and then produce the required rate
of etching. In the case of cupric chloride where copper is in 2 plus and it react with
copper on the PCB copper 0. You get cuprous ion cu plus. Therefore here again, as I mentioned
earlier, you have to regenerate Cu plus to Cu 2 plus to start the reaction or increase
the self-life or extend the bath concentration of this particular cupric chloride. Now, the
problem with ferric chloride in the first case is that Fe 3 plus is not easily regenerated
from the ferrous ion. Therefore, the reaction between ferrous and
the oxygen air is very slow. Therefore, you have to again use hydrogen peroxide based
supplier of oxygen, so that you can get Fe 3 plus and then water. This is a very slow
reaction that is why ferric chloride is very difficult to regenerate; whereas, in the case
of cupric chloride the reaction between Cu plus and oxygen is virtually instantaneous
and you get back Cu 2 plus very fast. So, the regeneration of cupric chloride is well
understood compared to the complex Fecl3 that is ferric chloride. If you look at the recycling
process for ferric chloride and cupric chloride in a spent etchant of ferric chloride, you
have Fe2plus and Cu 2 plus. Therefore, the removal of Cu 2 plus which is a commodity
that is worth removing from such as spent etching is going to be very complex; whereas,
in this case, where you have only copper it is going to be recovered fairly straight forward.
Now at the bottom, what you see here is the progress observed in the cupric chloride etchant.
For example, you see here at the extreme left; this is a very new solution of cupric chloride
which is mild green in color. As the time progresses; as you start using the etchant
material, you see it becomes dark and then finally, without regeneration, you see it
becomes black in color, almost brownish black. This is the time when you have to regenerate
the material. So at the bottom of the slide, I have put two very important issues that
you need to always look at as a manufacture, etch rate verses temperature and etch rate
verses cuprous ion concentration. What you mean by this? The etch rate will be dependent
on the temperature of the etchant. Typically, for cupric chloride the temperature
normally is set at 45 degree centigrade so try to maintain this temperature to get a
good rate of etching; whereas, in ferric chloride it can be 40 or 39 and so on. So, look at
the manufacturer's recommendation or look at some of the industry practices that produce
best results for using cupric chloride or ferric chloride. The other important thing
is Cu plus concentration so you need to monitor the cupric chloride for example, in the case
of a cupric chloride etchant you need to monitor the copper ion concentration. Therefore, try
to regenerate as early as possible into cupric Cu 2 plus, so that your etchant life is extended.
This slide will give you some simple illustration of what kind of etching defects that you can
see normally in a finished PCB after etching. The first one indicates for example, here
in this case, this is a typical electrical short; there are two lines and you can see
a short that is a copper not properly etched. One of the reasons could be that strip of
mask or photoresist is present there which you have not noticed. There is a problem with
a developing process during the photoresist application or image transfer or it could
be not undergone the right exposure in that particular area or there could have been a
bubble which basically as effected the exposure and it has got translated finally into copper.
Then the next one is this kind of mouse bite kind of structures that you see where copper
has been eaten away. This is again a process problem; then you have islands like this,
fortunately it is not shorting but definitely not an accepted standard or an accepted quality
after the etching process. I told you before that at every step whether it is image transfer
or etching or developing; you have to have visual inspection.
Then you have zigzagging open areas, you can see typically, these can be due to debris
- external debris sitting on your board. So one clear point that images from this kind
of defect is that you have to work in a clean room atmosphere or a clean room and your materials
have to be handled very carefully and stored in perfect clean conditions. Then you see
a typical example here is, zigzagging again indicates a poor control on your etching process.
Then the last one is over etching; that means, you are although it is somewhat over emphasized
or exaggerated here, what it basically you see here is, the timing is not ok. Temperature
could be above normal set temperature. Therefore, you can see the lines have been tin down compared
to your design requirements.
So finally, a brief summary on the etching process: Etching is removing unwanted metal
or material from a substrate. Different etching is used for different metals or materials.
You can even etch gold, for example the etching for gold is aqua regia - a mixture of hydrochloric
acid and nitric acid in a ratio of 1 is to 3. Then you can etch copper, typically in
PCB using cupric chloride, ferric chloride, alkaline ammonia. You have different etchant
for steel. You can even etch glass using hydrochloric acid for example.
Therefore, etchant can be employed for various materials and metals. Here, we are interested
in copper; you have different etchants for silicon, in the semiconducting manufacturing
that is typically potassium hydroxide is used. Different etching rates for different etchants
towards different material; so constant agitation, continuous air supply, time, temperature are
the key factor for good etching process. Mechanical etching is basically a mechanical
milling using a milling machine. Chemical etching or wet etching is always a question
of debate but, people are more inclined to wet etching in printed circuit board industry,
because the volumes are very large and they require at faster rate. Whereas, in the case
of a dry etching; let us say using a dry plasma, you require expensive equipment and typically
they are use for small area surfaces. You can also use electrochemical etching; in the
case of chemical etching or wet etching, so that you can speed up the process. So, it
is somewhat the reverse of a normal electrochemical cell or electrolysis process; whereas in this
case, the PCB will be the anode instead of the cathode; instead of depositing copper
on to the PCB, you are going to remove copper from the PCB.
So dry etching, as I mentioned, it can be a reactive ion etching using plasma or inductively
coupled plasma using RF magnetic field. You can get more isotropic etching using dry etching
process compared to a wet etching process, but the key issue here is undercut has to
be minimized during etching. We have seen that it is minimal in dry etching compared
to wet etching but, there are the ways to get a better quality control in wet etching.
Etch factor should be high between small undercut; vertical etching and horizontal etching are
the two processes. In horizontal etching, the board is kept horizontal and it is moving
through conveyorized equipment, whereas in a vertical etching a board is kept vertical
and it is rotated as it is typically a batch process and you have sprays from both sides
attacking the copper. Isotropic and anisotropic etching, you see in this particular figure
here at the right; example of an anisotropic etching and isotropic etching.
So, isotropic etching is more has a good control in terms of the smaller undercut that you
can observe; whereas, an anisotropic etching the rates are different at different areas
compare to isotropic etching. Undercut shapes vary with conditions and protective resist,
so you have to be a very clean observer if you want to look at the undercut. As you can
see here in this figure.This is an isotropic etching and this is anisotropic etching. The
rates are almost the same if use an isotropic etching, but that depends on the material
also to be etched in terms of the structure of the material and the etchant that you will
use for this. This is anisotropic etching; on the top surface
is the photoresist or the mask. You will observe more isotropic etching in the case of plasma
etching, also in the case of a silicon KOH semiconductor etching, because there are different
structures available and you can use suitable etchant for that. The undercut shapes in the
case of PCB etching can vary with the condition and these kinds of resist that you are using.
Now, the question is if etching is going to pose so much of problem, can additive technology
help in solving problems with wet etching. In the case of PCB instead of removing copper
from the board or more thicknesses of copper from the board, you can add copper using electroplating
to the exact requirement in terms of thicknesses and so on. That is why additive technology
is becoming very popular, you can mask certain areas on the PCB that is a non-circuit areas
with a metallic resist or a photoresist and then plate copper onto the pattern areas that
you require to the required thicknesses.
Final step in the board manufacturing in general terms would be to strip the photoresist, because
the photoresist which acted as a etch resist has done its job and now you are going to
strip it. You can use aqueous solution - hydroxide based solution and then you can protect the
copper with anti-tarnish treatment. It is a basically a simple dip of a board into a
anti-tarnish chemical a thin layer will be protective, will be used as a protective agent
for the copper surface and that can be removed later, if the board is going to be processed
into the next stage. Quality assurance a very key issue at every
stage; visual examination at every step - in all the steps - that we have discussed so
far is very important. You have to measure thicknesses, if you are doing plating for
every batch, including thicknesses of the plating of the through hole walls. Microscopic
examination for residues which can affect the board after the etching and before the
etching. We have seen etch defects, so those can be avoided if you look at some microscopic
examination, visual examination on the board. Typically, before etching every panel has
to be examined carefully before it goes for etching, because at that point of time you
can still rectify the board. Test coupons for every batch of boards will usually made
for quality check, because a quality check will define how well your various chemicals
performing at every stage and that can go for as a feed back to the manufacturing process.
So, that completes the etching process.
Now, we will go into the new topic called screen printing for printed circuit boards.
Now screen printing is a very commercial activity that most of you would have seen screen printing
of various inks on to various surfaces like your t-shirts or a simple printing like your
invitation cards or simple printing inks on to walls or any surfaces to provide a legend.
The same technology can be used for printing your tracks and pads on to the surface of
the laminate structure, which contains a copper and the dielectric material and you clear
the ink and then expose the unpatterned areas to copper for etching.
So, it is simple print and etches the process if you adopt screen printing for printed wiring
boards. Obviously, you cannot do fine line circuitry with screen printing, this is normally
used for commercial electronics applications; boards which have larger line widths, which
obviously it cannot used for high density interconnects. Although screen printing can
be used for solder mass printing and legend printing as a finishing process in the high
density interconnect or any kind of PCB structure or any system level printed wiring board.
This picture shows you two types of situation: one is the printed electronics and other is
the conventional electronics. The printed electronics are using printing - screen printing
as a step in the manufacturer of PCBs it will be lower cost, because it involves a simple
screen printing process. Long switching time, low integration density, large areas, flexible
substrate can be used, simple fabrication process as I said, you simply print and then
etch. Extremely low fabrication costs, but the important thing here is it is low end
- in terms of performance - it is low end. If you go to high end performance, high end
electronics, which is obviously high cost. Here, you have extremely short switching times,
extremely high integration density small areas can be done, rigid substrate can be used,
sophisticated fabrication technologies, new chemistries, new materials, high fabrication
costs because, you are talking about fine line circuitry.
Screen printing is a technique that uses a kind of woven mesh a nylon or a polyester
woven mesh to support an ink-blocking stencil. So basically, you are preparing a stencil
using a screen plastic or a polymeric screen and using this stencil, you are going to do
a printing process and this mesh is called a screen, therefore this process is known
as a screen printing process. Obviously, the surface has to be very clean, the copper surface
has to be clean or any surface for any form of printing. This stencil forms open areas
in the mesh; that means, in the mesh, you are creating an image and that image will
have open and block areas. The open areas in the PCB can be your non-circuit areas;
the ink areas which go or flow through the mesh can pertain to your circuit, because
the next step you are going to clear the ink therefore, they will act as a etch resist.
In this process, you are going to use a mesh and you are going to image it on to the substrate
by printing, how do you do that? You use a roller or a squeegee and it is moved across
this screen stencil. As you can see here, there is a printed circuit board before that
I will explain to you what a screen is? You can see that there is frame of aluminum which
contains the polymeric mesh. Now you create a stencil, here the word screen printing is
depicted here. The areas where the letters are - are the open areas and the blue areas
are closed. Now, if you run an ink from top to bottom using this squeegee here; the ink
will flow into the open areas were screen and printing is written here; and it will
print or the ink will flow through the mesh onto the base substrate.
Now, what you see here after the printing is done or after moving this squeegee, your
lifting the stencil and you can see on the base substrate the words 'screen' and 'printing'
have been printed. You can use this as an analogy for printed circuit board. Here, you
have a printed circuit board; you have the stencil, which depicts the copper and the
non-copper areas of your design which needs to be translated on to the printed circuit
board. You take the ink and you squeegee it through this stencil so, this is known as
the squeegee. It can be a silicon rubber, which will have a very sharp edge at 45 degrees
to the stencil here, moving the ink through the stencil areas - open areas in the mesh.
So, the ink will be transferred on to the base substrate which is the printed circuit
board, you will move it only one direction, so assume this is the direction of print,
you will get the print on to the PCB surface.
Screen printing is a stencil method of print making in which a design is imposed on a screen
of silk or other fine mesh material with blank area coated with an impermeable substance
and ink is forced through the mesh on to the surface or the substrate and also known as
silkscreen serigraphy or serigraph. I will describe here the complete process steps for
a screen print and etch technique. You select the laminate, clean the surface in the normal
methods which you have described. Screen print the resist ink followed by baking in an oven.
So you use a suitable stencil, image it and then create a mask for that and then keep
the stencil ready, print it. Now, after the baking is over, you introduce it into the
copper etchant; this will etch copper in non-circuit areas. Now you can strip the resist ink from
the copper areas which protected your circuitry. Now, you can drill the holes for component
mounting. Protect copper with tin, because as you know copper react with environmental
oxygen or moisture. Therefore, immediately you have to protect copper with tin or tin
lead in the earlier case, but today, we are discouraging people to use lead, so tin lead.
You can do plating is also here at this stage, but you can also use a simple tin coating
- molten tin on top of the copper surface. Post-process is like a solder mass, printing
and legend printing can be done later to complete the single-sided board. So note here, I have
emphasized that screen print and etch technique should be used for manufacture of PCBs only
if it is a single-sided board. Do not use this technique for double-sided board and
other processes because is not going to work; we have to focus on additive technology there.
This is an example of a subtractive process, where you simple or focusing your work on
removal of copper.
What are the defects that you will see in screen printing for printed circuit boards:
board surface preparation is very important. Quality of ink, the flow properties it should
not be too thick or too thin. Look at the viscosity; look at the type of structures
that need to go on to the surface and the time taken for drying, what kind of solvent
is used there? What temperature is need to be used for curing the material? Is there
a shrinkage of the ink after curing? If you look at all of these things, you can fix the
exact thickness of ink. Right mesh size of the screen of the stencil that you have used.
Quality of the squeegee blades typically, this squeegee blades must have perfect edges.
Otherwise, the pressure of transferring the ink to the surface of the PCB will not be
perfect. The angle should be at 45 degrees to the stencil
that is holding your squeegee blade. Right volume and spread over the entire surface
for a particular area that you are working with, ensure that you have minimum wastage
of the ink, this will come with experience. Cleaning of mesh and blade regularly to avoid
drying. Typically let us say, in volume production of single-sided PCBs, you will use some 500
boards and then you will have to remove mesh, remove the blade, clean them thoroughly and
use them again or preferable use a new stencil or a new mesh. Clean and dust free environment
as always very much required, so that these are not translated to etching defects that
we have seen earlier. This is again a figure showing you a screen
table, a PCB is mounted which holds the PCB the cleared area between the stencil or the
screen mesh and the PCB. So, introduce the PCB sorry introduce the stencil on to the
surface of the PCB, load the ink and start the stencil the squeeze towards one direction.
Using enough force to transfer the ink through the mesh openings and then lift the stencil,
so that it detaches itself from the surface of the PCB. There should be no ink sticking
between the surface of the stencil and the PCB surface; so that is where you have some
experience on the quality of ink. We will look at a video of a semi-automatic
screen printing for printed wiring board applications. You will understand how important it is for
the operator or the technician to understand this process? This is the mesh that we have
seen and the blade - squeegee blade that you have seen and this is the semi-automatic printer,
where you have the squeegee loaded or fitted on to the machine. Here, you can see that
the ink in this case, it is a solder mask for example, it is loaded on to the screen
printing machine.
Now, basically you are trying to create a nice hold using vacuum between the stencil
surface and the PCB. Here you can see, this is the squeegee blade mounted on to the screen
printing machine. The ink is dread or pushed through the mesh openings on the stencil on
to the surface of the PCB. If required, you can also do a second motion of the squeegee
blade in the reverse direction to the original starting point and then finish the process.
Now, switch off the vacuum and lift the holder - place holder of the stencil. Now, you can
see at the bottom there is PCB surface which has been printed with the solder mask material.
This is a photo imaginable liquid solder mask that has been used in this particular case.
Now this covers the entire area; this is the circuit area. We are going to use a mask for
the solder mask after this material has been cured in the oven at let us say, 100 degree
centigrade for about 10 to 15 minutes. Now, you can place a solder mask photo tool and
then open up the unwanted areas and remove it by developing. So, the areas that is the
pads and the tracks or the pads will be open, other than the pads all other areas will be
covered with this solder mask material. So, these screen printings is very useful for
print and etch process, solder mask process or legend printing for printed wiring boards.
Now you can see here pictorially, defects that can be observed during screen printing
process. This is the stencil; this is the PCB surface. You can see, if you have the
controls right in your equipment and the process steps, you can get perfect dispensing of the
material, it can be an ink, it can be a solder mask, it can be a white ink for legend printing
or today screen printing is also used for solder paste dispensing during surface mount
assembling. In this case, this picture typically depicts metal particles embedded in epoxy
medium and what have basically shown here is a screen printing process for solder paste
dispensing on to the lands that have been generated already on the printed wiring board.
At the bottom picture shows you typical defects. This is a well define structure of a line
that has been screen printed; this is irregular, sometimes you get this, sometimes get perfect
lines. Here what you do is, basically you have to check the mesh openings, you have
to clean the mesh opening and the stencil regularly to avoid these kinds of defects
and instead of getting these kinds of prints. This is again saw-toothed, you can see consistently
you get these kinds of defects. So make sure that you get a well-defined line or an image
after screen printed process.
Now, we will look at another aspect of the PCB process. Remember, we have seen that mechanical
drilling is a final step in single-sided board manufacture; whereas, mechanical drilling
will be the first step in the double-sided board manufacture. Today, with all the advanced
methods, we can also drill first for a single-sided board and then register your photoresist material
and mask for single-sided PCB also. So mechanical drilling of holes for PCB, why do you require
holes? You require holes for registration during manufacturing. If you are working on
large volume production, you have to register you photo tool to the copper surface of the
PCB at various times during let say a multi-layer PCB manufacture.
Therefore, you are not going to use manual registration, we will do an automated registration
for that you require holes. Mounting holes for assembling the PCBs or in your PCB, you
definitely have mounting holes which need not be plated but, these are going to be mounted
to your chassis of your product. Stacking holes during drilling; in a CNC drilling machine,
the panels have to be stacked before the drilling process. Therefore, you require stacking holes
and holes are required basically to inter connect two copper layers and make them conductive
by plating the whole walls of a mechanically drilled hole.
In our case we are more focused on how to make a whole wall conductive from a mechanically
drilled hole of a structure like this; where you have copper on both sides, a dielectric
in-between sandwiched, you are doing a mechanical drilled hole and you are going to copper plate
this. We are going to look at, how well a mechanically drilled hole can be made suitable
in all aspects for electro plating.
Double sided plated through hole printed wiring board. In this case, the inter layer inter
connection is very important. The earliest technology was riveting the holes and making
it conductive; in the olden days, people also tried filling it with solder, although it
tripped -- very immature for us today, but earlier days, this was simply an easy method
of filling holes with molten solder and then providing some kind of an interconnection,
but never reliable one. Filling with conductive polymer paste; this
is again well utilized today. If you are making hide density interconnects, where you are
making thin inner layers, you can fill the hole with conductive paste and then polish
off the top and bottom area and then get a very reliable conductive hole, but the more
reliable one is metal deposition on the hole wall by electro plating methods. Electroless
and electroplating methods - this was done or brought into the focus by Shipley in 1964
and we are still working on these technologies of electroless and electro plating combined,
to get a very reliable hole. So, we will discuss more on these aspects in the next hour.