The Story After Plating

DEDICATED TO MY FRIEND SOFTER FRAME WHOSE ONE STATEMENT HAS CHANGED MY ENTIRE LIFE

Before Getting In… My dear friends! PCB manufacturing is one of the most practical subjects of our syllabus. This is a must-know subject in the course of Electronic Engineering. Its style of studying is a bit different. Fortunately, we have a complete set of PCB manufacturing devices and machines in our PCB LAB. I have compiled this report in order to give you a preview of what we do in our PCB LAB. This report is written about the processes which come after Cutting and Drilling of PCBs. So remember, that I have assumed that you have already cut and drilled your PCB. In this report, I have tried to be as simpler as possible, even then you may require some technical guidance from your subject teacher. Diagrams are given to illustrate almost every aspect that is written in this report. Remember, that this report was first written by me as a description of what I saw and experienced in my session of PCB LAB. This report is the revised version of the same report with some more figures and details about the processes. Furthermore, this report is the blend of my personal research and the lessons of my teachers. Most of the pictures here are made by me and some of them are taken from various websites. I would like to thank my PCB teachers Sir Ghous Baksh and Sir Babar Ali Shah for teaching me so effectively that today im able to write this report. I will appreciate if you will report me any mistake in this report if found. I am confident that this report will serve its purpose Insha Allah. Enjoying Reading Fabrication !!

Prepared By Shaiq Bashir Student of Electronic Engineering (Batch 2004-2005) NED University of Engineering & Technology, Karachi

Reviewed By Syed Muhammad Babar Ali Department of Electronic Engineering NED University of Engineering & Technology, Karachi

Copper Plating Ok guys! Now you have a PCB Board in your hand, which may be drilled or undrilled. But as you can see that it is made up of a material that cannot conduct electricity. So, how we are going to make it for conducting electric current. Well that is the reason why a PCB is copper plated. In this process, we intend to plate the board from both sides with copper. And this will make the whole board conducting. The machine that we use in our lab for Copper Plating is called: Bungard Compacta L30 ABC

The reason for which it is called COMPACTA is due to its small size and high intelligence. This machine offers a complete GALVANIC PLATING SYSTEM for a maximum board size of 210 x 300 mm. Lets have a look at some of the features of this machine: • • •

• •

The speed of board movement is adjustable as required. That ensures a good chemical flow and excellent chemical contact especially in the drilled holes of the PCB. There are thermostat controlled baths with large surface teflon heaters for gentle temperature adjutments. Digital timers are available with each bath with countdown, accoustic end signals and auto reset functions. There is a feature of bath circulation by air injection which is also available.

•

Rectifieres with low residual ripples, and with constant current and constant voltage output are given on the machine. • There are large analog Amperes and Voltage displays present. As you can see in the figure, the treatment baths that are given in the machine. It is very important to understand the sequence and purpose of each of these baths. There are total 6 baths that are provided in this machine. Lets first have a quick look at these baths: Bath Bath Bath Bath Bath Bath

1: 2: 3: 4: 5: 6:

Cleaner Conditioner Pre-Dip Catalyst Intensifier Spare Bath Copper Plating

Before discussing these baths in detail, we must know about the rinsing facility provided in this machine. COMPACTA L30 ABC provides two types of rinsing baths: 1. 2.

Static Rinsing Spray Rinsing

Rinsing is usually done with water. We will rinse our board after each step except for Pre-Dip. Now lets discuss these baths in detail: Bath 1 (Cleaner Conditioner): Chemical Code name: Time: Temperature: Rinsing:

DS270 7 min 65-70 degree celsius Static Rinsing (1min), Spray Rinsing (1min)

This is the first bath in the machine. We dip our PCB board in this bath containing the above mentioned chemical. DS270 is basically an alkaline cleaner-conditioner that is used for cleaning and conditioning of PCB substrates. It contains certain conditioning agents that have the ability to activate glass fibres and dielectric so that a proper and reliable absorption of the catalyst is possible. Bath 2 (Pre-Dip): Chemical Code name: Time: Temperature: Rinsing:

DS400 1 min 20-25 degree celsius Not Required

The second bath contains a chemical called Pre-Dip. This chemical has a special additive which prepares the panerls for the activation process step and it protects the catalyst (which will be used in the next step) from harmful drag-in (contaimination). This ensures superior performance and extends the life of catalyst.

Bath 3 (Catalyst): Chemical Code name: Time: Temperature: Rinsing:

DS500 7 min 20-25 degree celsius Static Rinsing (1min), Spray Rinsing (1min)

This is the third bath and the chemical that is used here is called Activator DS500. It is a low acid colloidal Palladium catalyst that is specially formulated to seed the non-conductive surfaces of PCBs for complete and uniform desposition of copper. It actually creates a dense, uniformed distribution of drill hole activation necessary to achieve the highes conductivity and consistent perfect coverage. Bath 4 (Intensifier): Chemical Code name: Time: Temperature: Rinsing:

DS650 4 min 45 degree celsius Static Rinsing (1min), Spray Rinsing (1min)

This fourth bath contains a soultion which is called Intensifier DS650. It is also called an Accelerator. Its important function is to modify absorbed catalyst to allow quick and uniform metal desposition of copper. It strongly improves board platibility by creating a conductive metallic layer inside the holes. It promotes Cu-Cu bonds and protects the copper from potentially harmful catalyst drag-in. Bath 5 (Spare Bath): Well, this bath can be used for other spare purpose. Bath 6 (Copper Plating): Chemical Code name: Time: Temperature: Rinsing:

CU400 24 min 20-25 degree celsius Static Rinsing (1min), Spray Rinsing (1min)

This is the last bath in this machine and the most important one. Here, actually your board gets plated with copper. It is extremely important to understand this bath. CU400 is the chemical that is contained in this bath. This chemical produces highly levelled, bright smooth copper deposits on properly prepared and conductive PCB at both high and low current density levels. There are two copper anodes in this bath which are enclosed in bags. The mixture is agitated by the injection of air. Please keep this thing in your mind that COMPACTA L30 ABC machine has the capability to plate a board at a speed of 0.7 um/min at 3A/dm2 of current settings. This will take almost 24min to plate a board of size 210 x 300mm as mentioned above.

Photo-Plotting You have prepared the layout in any software like OrCAD, P-CAD, Allegro and etc. So now its time to get this artwork printed on your PCB. But for that you have to get this artwork in hard form, i.e. from the screen of your monitor to some photographic film. And this is done through Photo-Plotter. Well! My dear friends! For photo-plotting, we use obvioulsy a Photo-plotter and that one is present in our PCB lab is: Bungard Filmstar Photo-Plotter

As far as its specifications are concerned, you can read them from the manual provided. Let’s get straight back to the point. This photo-plotter comes with three software packages:

1) GERB2BITMAP-----used to covert the GERBER and EXTENDED GERBER files into high resolution Bitmap format which serve as FPF (Foto-Plotter Files). 2) VIEW MATE------used to view your artwork. 3) RUN_PLOTTER-----used to export the bitmap file to the photo-plotter for the plotting purpose. Let’s come back to the plotter. Now you have to use a photographic film which is manufactured by many graphic industries like Agfa, Kodak and Bungard too. When you will have a close look at this film, you will find that its one side is dull and other is shiny than full side. The dull side EMULSION side, you have to put this emulsion side down when placing it around the drum of the plotter. You can use the adhesive paper tape in order to ensure perfect contact between the surface of the drum and the film. The paper tape also makes it possible to eliminate any inclusion of dust particles in between surfaces. Now move your drum back to the initial position. This is determined by the two white markers, out of which, one is present on the fixed body of the plotter and the other on the drum. When these two markers coincide, it means that this is the starting position of the drum. Now close the cover and come back to your computer. So here is what the simple procedure you will follow:

1) Import you GERBER or Extended GERBER file in GERB2BITMAP and then export it into the bitmap format or FPF format. 2) Use Run_PLOTTER to import these bitmap/FPF files. In this software, you can vary the intensity of the laser light to be used in the plotter as well as you can select X-resolution of the four that are provided. Remember that Y-resolution is fixed here. And finally you export the file to the plotter for plotting the artwork. Warning! Remember that our Plotter is connected with a parallel port with the PC. After plotting, we open the cover and take the film out. Now its time to develop our artwork. Remember the following configuration for developing the artwork. 1) 2) 3) 4)

Developing Rinsing Fixation Rinsing 5) Drying Now remember one thing here, for the first three steps, it is more effective to use vertical treatment tanks, but that doesn’t mean that horizontal ones can not be used. We have used horizontal treatment tanks for these processes. we will use three trays actually. The first one is filled with the “Developer” solution, the second one is filled with Water, and the third one is filled with “Fixer”. We will pass our plotter mask (the film containing that artowork) through each of these trays and our image will be developed and become visible. Remember one point here, all this photo-plotting and developing is done in a Dark Room with GREEN safe light. Now remember that two types of Plotter mask can be prepared:

1) Positive Art Work-------It contains tracks and conductive paths in black color and other non-conductive areas in transparent.

2) Negative Art Work------It contains tracks and conductive paths in transparent and the other non-conductive areas in black color.

Remember this difference with full clearity because this concept will be much used in the later sections of PCB manufacturing. Here I have given two example figures to illustrate this difference.

Lamination Before understanding Lamination Process, it is better to know that what actually the lamination is? Lamination means “to bond some layers together” and the sheet which is used in this process is called a “Laminate” Photo-Resist Laminates (for PCB only) are basically of two types:

1. Photo-resist Laminate blue Etch Resist 2. Photo-resist Laminate Solder Mask (Also available in different colours) Now a laminate is composed of three layers. These are 1) Peel Sheet It’s a transparent Polyester covering sheet which covers the laminate sheet inside. It is removed automatically by the laminator during lamination. 2) Resist Film This is the main laminate sheet. It is sensitive to light, can be developed in the solution called “Developer” and possess a very high viscosity. 3) Cover Sheet This is a heat-resistant sheet. Its main function is to block oxygen from effecting the laminate inside. It is not removed by the laminator so it stays there after lamination. It is removed just before developing the board after exposing. Since by know we have discussed that what basically a laminate is. So its time to discuss the lamination process. In our lab, the machine that we are using for the lamination is called Bungard RLM 419-P Laminator

In this laminator, you can adjust Pressure, Transport Speed and Temperature with the help of separate controlls given on it. Its maximum throughput is 20 square meters per hour. It looks like a Plastic Coating machine, you have to insert the board from one side and the board gets out from the other. It has got rollers that contain the laminate sheet on them. One of two protective coverings of the laminate sheet is stripped off by the laminator itself. Look for its roller diagram in the manual provided. Remember that since the photo-sensitive material is used as the laminate therefore, this process is carried out in the dark room with yellow safe light. We used a 30 sec time for our double sided PCB. The PCB is entered and we get a fully coated PCB (coated from both sides) in our hands from the other side of the laminator. Temperature settings proposals are: 105 degree celsius for Photo-Resist Lamination 115 degree celsius for Solder Mask Lamination Now you may get one question in your mind that what will happen when the temperature will get lower or higher than the proposed settings: The answer is pretty simple: If the temperature will be too low then the laminate sheet will not stick to the PCB. If the temperature will be too high then the laminate sheet will produce vapors because it will become too hot. Further, it can loose its sensitivity and can get damaged. Exposing Now, after applying this lamination, we head towards another prcoess which is the exposing process. In this process we expose our PCB towards UV light. Now have a look at the state of our PCB just before Exposing!

Now you can easily see that what is now present on the copper plated PCB right now. There is a laminate sheet above which there is the 2nd protective covering present. Now its time to expose the board. But remember that we will not remove this top protective covering before exposing. The reason; I have already discussed. Now my dear friends! Let us suppose that we have applied the Photo-resist laminate during the lamination process. This photo-resist is sensitive to light. We will make use of this property of photo-resist in order to print our artwork on the

PCB. But before that, it is very necessary that you should know some basics about the Photo-resist films. Remember that there are two types of Photo-resist Films: 1) Positive Photo-resist 2) Negative Photo-resist Now you already know that there are two types of Plotter Mask: 1) Positive Plotter Mask 2) Negative Plotter Mask I have already discussed the plotter masks in details so now lets move towards the details of photo-resists films. Positive Photo-resist When UV light falls on a positive photo resist film, the underlying material gets removed. This thing must be kept in the mind, the positive photo-resist film will be used when you want to remove the material from those areas where the light is falling. Now can you answer me one question: Tell me which type of plotter mask will be used with a Positive Photo-resist laminate? Ans: Positive Plotter Mask The reason is very simple, in positive plotter mask you have you tracks in black color, so they will block the UV light to pass through them. As a result the photoresist film beneath them will remain as it is and will not be removed by the developer. While those areas which are transparent, light will pass through them and hence the photo-resist beneath them will be removed by developing. That is the main thing to understand. Chemistry of Positive Photo-resist Film The positive photo-resist consists of a polymer called “Novolak”, which is a polymer made up of Phenols. Being Phenols, it is soluble in water but with a very low dissolution rate. That is whty a small amount of diazonapthaquinone is dissolved in Novolak as an additive. When the resist is exposed to light, diazonapthaquinone produces Carboxylic acid which is more soluble than the Novolak, thus increasing its rate of dissolution. This leads to a Positive Photo-resist Film.

Negative Mask

Plotter

when UV light falls on a negative photo resist film, the underlying material gets harder and difficult to be removed. This thing again must be kept in the mind. The negative photo-resist film will be used when we want to remove those areas which are not exposed to UV light. So what sort of plotter mask will be used with this film. That’s right a negative Plotter Mask. Chemistry of Negative Photo-resist Film Actually the negative photo-resist consists of Poly (vinyl cinnamate). The cinnamate groups undergo [2+2] cycloaddition when exposed to light, leading to a cross-linked polymer and hence becomes extremely difficult to dissolve. This leads to a negative Photo-resist Film.

Remember the following configuration for printing the artwork on the PCB: Positive Photo-resist Film works with Positive Art Work Negative Photo-resist Film works with Negative Art Work

Now we should move towards the topic of Exposing: Exposing in our lab is done through the machine called Bungard Hellas Exposure Unit Have a look at this machine:

this machine, you have an “Emission Control Display” which keeps you informed about the condition of the UV lamps. There is an integrated timer also given which makes it very easy to select exposure timings. There is a vacuum pump also provided with the ability of creating a strong vacuum within a few seconds which is essential for ensuring a good contact between the artwork and the PCB surface. There is a vacuum frame also given which is transparent and can be used to check the position of the board containing artwork before starting exposing. There is an integrated fan also given which keeps the body of the machine cooled. UV light is used for exposing. Right now, it is assumed that we have used positive Photo-resist and Positive Plotter Mask. All the next processes will be described keeping this assumption under consideration. Let’s have a look at the state of our PCB just after the exposing process:

Developing After exposing, you will get a PCB like the fig above. Now you can see that there are both exposed and unexposed Photo-resist films present and Cu is present all over the PCB. Now remember that the Cu under the unexposed Photo-resist is the one which would be later converted into tracks. i.e. The required areas of Cu. But those Cu areas which are present beneath the exposed Photo-resist are unwanted

Cu areas and should be removed. It is to remember that Cu should be present on the tracks and conductive paths of our circuit not on the whole PCB. So let’s have a look at the areas that we now have on our PCB after the process of exposing:

1) 2) 3) 4)

Exposed Photo-resist Unexposed Photo-resist Cu under Exposed Photo-resist Cu under Unexposed Photo-resist

Now my dear friends! Out of these four areas, we want only one to remain on our PCB and that is Cu under unexposed photo-resist. The reason you know very well now. It means that the remaining three areas are to be removed. Now these three areas will be removed by three separate processes, each process will remove one of them. Lets first have a look at these processes:

1) Developing--------------------to remove the exposed Photo-resist 2) Etching------------------------to remove the Cu under exposed Photo-resist 3) Caustic Soda Operation-----to remove the unexposed Photo-resist Remember that these processes will be carried out in the same sequence as described above. So now, let’s discuss the Developing Process! As I told you already that developing is done in order to remove the exposed photoresist film so that Cu beneath it (i.e. unwanted Cu) should be uncovered. This will allow the etchant in the next process should be able to remove it. One thing you must remember and that is to never match this developing with the developing of the Plotter Mask. The machine that we used in our PCB lab for the developing is called: Bungard Splash Splash is a machine that can be used for both developing and etching purposes. It contains a bigger tank which has the capacity of about 24 litres and inside this tank a chemical called “developer” is sprayed from two sides on the PCB. Remember that vertical spraying is done in this machine because research shows that the vertical spraying is very effective in developing. Then you can take your PCB out with the help of stand. Rinse it in a treatment tank given on the right side of the machine. This tank has the capacity of around 10 litres. On the bottom side of the machine there are some valves given which ensure safe discharge of fluids. There is a timer also given which helps you to select the spraying time according to the requirements.

So now let’s have a look at the PCB after developing:

As you can see that the developer has only removed the exposed Photo-resist film areas. So now the Cu beneath them is uncovered and can be removed by etchant in the next process. Etching Etching is the process which is used to remove the unwanted Cu areas. The chemical that we used in this process as an etchant is Ferric Chloride. It directly attacks on Cu and removes it. Now one may ask a question here that why only unwanted Cu it removes, why not the wanted one as well. The answer is very simple, the unwanted Cu areas are uncovered and open but wanted Cu areas are still covered with unexposed Photo-resist which was unable to be removed by the developer. That is the reason that makes it impossible to the etchant to remove the wanted Cu areas. Now let’s talk about the machine which in our lab is used for etching purposes. This machine is called: Bugard Splash Center It is important to clearify here that Splash and Splash center are two different discrete machines. Some students take them the same but these are two different machines. Let’s have a look at the difference which these two have:

1) Splash has only two tanks, one spraying tank and the other treatement tank 2) Splash Center has got five tanks, one spraying tank, and the other four are treament tanks. Let’s talk about these tanks first. The first one is a vertical spraying tank; exactly the same is the one in Splash. The working is same spraying of fluid from two sides. But mind it this time the fluid used is Ferric Chloride which serves as an etchant. Then it come the remaining four treatment tanks. Remember this configuration; this is the one which we used in our lab: 1) the first treatment tank is for the Pre-rinse 2) The second treatment tank is for both spray and static rinse

3) The third treatment tank is for the Caustic soda operation (will be discussed later) 4) The fourth treatement tank is for the Chemical Tinning. Besides these, there is a drying roller present for drying up the PCB and the same sort of timer that you have seen in splash is also given to control the etchantspraying time. The procedure is very simple. First you put the PCB in the spraying tank; etchant will remove the unwanted Cu areas, and then let the residue to drip off. But why we allow this residue to drip off? B cau e

e s

this minimizes the amount of rinsing water needed. Then put the PCB In the prerinse tank, then in Spray and Static rinse tank. So after that, your PCB will be of something like this: So this is what the etchant has done. Now look at your PCB, only one of the three wanted are remaining and that are UNEXPOSED PHOTO-RESIST. So its time to remove that as well! and this can be removed by using Caustic Soda. I m giving here two diagrams that will summarize the processes that we have discussed so far. They will also prove to be helpful to describe the difference between the positive and negative films.

Caustic Soda Operation Remember that caustic soda operation is not a very special type of proces that will require a separate machine for it. Infact, we use the same machine called Splash Center, and this is the time to use its third treatment tank which contains Caustic Soda. Just Drop your PCB in that tank, and take it out after the required time by pulling the handle of the basket type tray. Then pass it through the spray and static rinsing steps, and here you are. Let’s have a look at the PCB after this:

So now its almost done. Now your PCB is free from all those unwanted areas. You can see that only the Cu that you wanted as tracks and conductive patterns is now present. But still there are some steps to be followed: let’s quickly summarize these steps.

1) Chemical Tinning----To protect the Cu circuitry from being etched. 2) Solder Mask Application-----------To save your PCB from being damaged from solder. 3) Silk Screen Application------------The wirtings and component references. We also apply in the last a coating LACKER solution on our PCB in order to protect it from oxidation. Finally, your PCB is ready, its time to put on the components and enjoy the electronic creation. -The End-