 Hello and welcome to this course on advanced machining techniques for microsystems fabrication. Today we are going to learn a very interesting topic of fabricating the printed circuit board PCB which is used for most of the electronic applications related to these microsystems devices. Typically there are two different modalities in which this printed circuit board is fabricated. One of them is called the body mounted scheme of the printed circuit board, the other one is called the surface mounted scheme of the printed circuit board. Printed circuit boards these days are very common place in all microelectronics and there is a huge potential in assembling the different MEMS devices or maybe even all other microelectronic devices in a manner so that there is a intercommunication between these devices based on some connections etc. And particularly it is more so important for MEMS because the MEMS devices produce signals which are of very low strength and there is always a requirement of signal processing from which you can get to know about what exactly are the different aspects of that signal is it actually a signal or a noise what it is able to represent. So typically the strategy that is followed in most of the micro systems are either ASIC which is called application specific integrated circuit it is a terminology used for a completely sort of buried or embedded electronic module which will do the signal processing for the MEMS or the micro systems which is fabricated and it sits typically on the top of this ASIC. The other simpler way of producing these devices are through these PCB routes where you can make and build a circuit which you otherwise what to do on completely semiconductor level on a much more simpler form called the printed circuit board. So this printed circuit board illustration is you can represent you know you can see this is actually how the board would look like before it is started to fabricate. This in fact is a PCB where you can see a lot of these small dots here and in fact there are holes which are drilled in this PCB which allows the components to be placed crisscross across this board so that there can be soldering or there can be joining on the other side of the board which is the back side of the board. So typically you make the PCB in a manner so that you mount the components on this side on this particular side and mount all the connections or all the you know interconnections between the components you do on this other side. So this is called the body mounted fabrication strategy for the PCB. You have another option where you can do the even surface fabrication of whatever components electronic components you want to lay out and interconnect and that is called the surface mounted design of the PCB. I will just show you one illustration of that. Typically if you look at this small teeny tiny PCB here you can see that there are hardly except some for some portions where you need to probably mount some kind of a more complex integrated circuit the other portions are not drilled. And in this kind of a system the interconnections are all made on the same side as placing the component. So you are mounting the component on the surface and you are also interconnecting them on the same surface itself through different soldering strategies or different other joining strategies. So today we are going to give you an illustration of how this whole fabrication process of generating microelectronics you know so that there can be signal conditioning or processing can be done and realized on a scale like this using the different you know machines we have at the 4i lab in IIT Kanpur. So the first thing that is important for me to tell you is the substrate because that is where the PCB build up actually happens the exercise happens and as I already told you that there are two different strategies of mounting of the components. One is the surface mounted components and other is the body mounted components. Therefore one needs to first of all see what are the kind of substrates which would be able to provide interconnects between these components if they are on the surface or on the body or the particular PCB. So there are different methodologies for doing this PCB fabrication. So as I told you there are several type of different type of PCB boards which are used as input material in this particular case. I think I shown you this double sided PCB board with double sided copper coating similarly you have a single sided board which is actually on one side you have the copper the other side is the plastic as you can see here. There are even boards which are related to flexible PCBs this for example is a flexible board and you can see this is also a single sided flexible board. So you have a copper coating on one side and the other side is a flexible plastic. So you know depending on the requirements of the different kind of PCBs for example supposing you were to make a body mounted component of the PCB or a solid mounted component of the PCB you will have to use different boards accordingly. This is a double sided board which you can be used probably for a case where you can use both the SMD and BMD components on the same PCB board. So in one instance you can make the interconnects through this copper coating by micro milling this coating on the top as well as the bottom side of the board using a micro mill cutter. I am going to demonstrate the whole assembly line for this PCB process here at the 4I lab where you will see that there is a very specific tool which works on a Gerber data data file and is able to print or is able to sort of machine micro machine the copper coating so that it can formulate interconnects. So if supposing you were to place the components on this top side of this particular plate and it had a dual SMD, BMD kind of a PCB system. So in some instances you would like to retain the connections on the top side. In some other instances you would like to retain the connections on the bottom side and the connectivity on this particular case is made through micro drilling of certain holes which would be able to take or transport the connectors of different components across the PCB board from one side to the other. So if supposing the connections were to be on the back side for the BMD case then the connectors would typically go in such drilled holes in the PCB and the connector would be or the copper strip connection would be done at the bottom surface so that you can interconnect. Similarly on some cases if the solid mounted component or surface mounted component was present you would do whatever connections on this particular side. So this is a very flexible kind of system where both sides the machining can be done and both kind of components can be mounted. The other one is a single side where obviously as you may think that it can be either a surface mounted or a body mounted in case of surface mounted maybe you will use this as a front side because it is copper and in the body mounted case you will use this as a front side which is highly insulating and the connectors would be done on the bottom side based on that. So there are also even multi layered PCBs these days because naturally the requirements of microelectronics imposes a lot of yield and utility issues about the various components of a circuit. So there can be a possibility that instead of a single layer PCB you have many layers as many as about 28 to 32 layers of PCB where each layer is having a circuit interconnect and the idea is that the components are mounted only on one side that is the top side of these kind of laminated layer of PCB boards where each layer would then have a circuit which would join the components in a specific manner. So in the layer one maybe there may be a different circuit plot, the layer two maybe there may be a different circuit plot and each layer of interconnect is separated from each other through a insulated material or insulator material like this. So supposing you were to draw some lines here using a milling cutter which would be connector and then you would like to place another PCB board on the top of this layer. So typically you will cover this layer through this dielectric and then you would like to put this on the top of this layer and this way you can have a press forming of this different layers where each layer would have some circuit and the circuits can work either in series or in parallel series circuits are sometimes designed in a manner so that the voltages at one layer, the output voltages and the input voltages are the various layers of the PCB boards. So in one layer probably the output voltage whatever is commensurate with that the input output characteristics is commensurate with that on a second layer. So in that even you can use probably a series connection. Supposing there is a difference in the voltages between the various layers then you might have to use a parallel connection system. So I am going to help you to sort of demonstrate a protocol where we can actually fabricate this design on the PCB which is very critical the copper wires which comes out of this material and this is of course a dry process for doing the PCB. PCB in fact can be also edged using wet processes where there can be a photolithography step, there can be a masking step, there can also be a redox reaction or a etching step and so the copper can be patterned and removed selectively in that particular case. We are not going to discuss that here this is more of a dry PCB fabrication run that we are going to illustrate today. And so talking about the various equipments which are in the laboratory for using this whole you know for trying to fabricate the PCB the first important tool of course is a computer which can be used to draw the CAD file of the design that the circuit design which is there and we are using a couple of softwares here one of them is called the Eagle 6.1.0 version where we can actually lay out the components and interconnect them. We also have several other softwares like for example the board master or sometimes the circuit cam 6.1 version where we can actually after the components are selected together and joined together we can make a sort of layout design on the PCB with the an option of drilling the PCB either on the so either the drill plating the PCB on the back side or you know make a unplated condition where there is no plating needed and it is for the SMD case on the front side. So this software used to program the basic drawing the file of the design and then it is imported as a Gerber data file into this particular machine right over here and you can see this is a micro tool where there are a couple of controllers which would be controlling in a very precise manner the action of a rotating tool head the tools in this particular case that are being utilized are represented or are there they can be seen particularly here. So the tool size of about close to 200 microns about 0.2 mm minimum size all the way to about close to 2 mm all sort of possibilities are here in this particular tool tip all you need to do is to select a tip commensurate with your machining activity that you would like to do when mounted on the machine and then with a Gerber file you basically operate the XYZ stage on this machine in order to take care of the machining of the copper in a selective manner. So wherever there is going to be an interconnect there is no machining wherever you are wanting to remove the interconnect and create a gap between several such lines or several such interconnects you remove the copper in that particular area. So that you can do later on the silver paste etcetera over particularly in the surface mounted case and so what essentially this step is about is that once you have this kind of a drilled sample with you with some kind of a copper tracks on the other side. So one of the purposes why you need a protection layer on this top of the copper as you were seeing there is a copper layer turn in the last step is that you want to prevent the atmospheric corrosion to damage the copper interconnects which have already been created by the micro milling tool earlier as you know the layer of copper which is involved in making this interconnects is quite thin and there also is a possibility because there are going to be several wet steps in between and there is going to be a application of temperature particularly for soldering etcetera there is always a tendency of the copper to go away because of this erosion action. So at least one aspect that we need to take care in a PCB manufacturer is to somehow be able to protect and deprotect selectively so that the interconnects part which really do not do any connectivity between the components as such and the copper tracks they are protected and there are only the pad part where there is really the connection happening should be deprotected. So there is one small zone of the pad which is actually slightly bigger in size as well where you would actually eventually where would you would eventually need to connect the connectors of the electronics that portion has to be exposed and the remaining portion has to be covered. So there are several steps which are followed in this process basically there is a lacquer coating which comes for this PCB manufacturer. There is a lacquer coating and there is a particular product box which is given as can be seen here called the quick start pro mask pro legend kind of you know protocol where what you would do is actually you will start with this PCB board and there is always a design which is already there for this PCB board remember in the last step we had done this design. So we would at the outside print a mask and this mask would be made in transparency printed by a local laser printer and they are going to be selective depositions of printed and unprinted zones in the mask. The idea is that the lacquer which we are going to use here is photo curable. So when there is exposure to a UV radiation of a frequency there is going to be a change in the lacquer in some zones where there is no exposure as opposed to exposure. So I think I had earlier illustrated about negative and positive photo resist and let me just recall what I have said is that whenever in a negative tone resist there is exposure there is hardening of the resist and in a positive tone resist wherever there is exposure there is going to be a removal selective removal of the resist from that particular portion. So in this case the resist is where wherever there is no exposure you can easily develop it off and the exposure makes it harden out or cross band. So it is some kind of a replica of the negative tone resist although it is not a photo resist it is some kind of a lacquer by itself and basically there are different chemicals which come with this kit. Some of the chemicals that we use are these for example jelly like materials and they come in two different parts which you have to pre-mix and they are what formulates after hard baking the lacquer film on the top of the PCB material. So you basically mix these two packets together and then in a soft manner you apply the lacquer on the surface of the PCB board using this roller made out of a soft material and if you were to really do a hard coating there is a possibility that whatever drilled holes are there the lacquer may go inside those holes and that may create problems. So you want to avoid that at the first place and you want to just use a soft polishing mechanism so that the lacquer can be applied over the whole PCB board. Several steps of processing where you can first probably hard bake the lacquer. So it is done in a gravity fed convection oven called protoflow which is actually heated to a temperature of 80 degrees Celsius and the material which is coated is kept inside that oven for about close to 30 minutes so that the initial solvent which is there in that paint would evaporate and there is going to be a hardened film on the surface left over because of that. The other aspect that this lacquer has is that it is actually a UV curable lacquer as I told you and you need a photo mask for the purpose of selective exposure. Now supposing I were to draw a PCB something like this where you have these copper lines as well as there are certain places which have the copper pads where there would be the interconnect. So you have to now do what you have to expose the pads and cover the lines for protection sake and for doing that you print a mask like this. If you look into very closely at this particular illustration in this particular area you can find out a lot of black and white areas or regions. The black regions are typically ones which block the UV from going all the way into the lacquer and the transparent regions are one which passes the UV light. So it is all like photolithography which has been illustrated and also videographed and shown to you before and basically the idea is that whenever there is light transmission across this this would be a masking layer over the lacquer. So some zones of the lacquer are not exposed and some are exposed. So whichever zones are not exposed in this particular case there would be a tendency of those zones to come off or get developed and expose the pad in those zones whereas the areas which are exposed to the light would be hardened and they would remain as such. So then the question of development comes and once the exposure has been done there is another kit which is there provided in this pro mask strategy where you basically dissolve this this is like a powder it is a powder material and you dissolve it in a certain amount of ml of water and you immerse the exposed PCB on the solution so that the portions which are now unexposed that means they were covered by the black on the mask and the UV did not hit that portion those portions would be slowly dissolved into the solution thus exposing the bottom plate the copper plate in those small zones. Now if you with the power of CAD work at a much more magnified scale and are able to print very small regions which you want to deprotect then the idea is that only a small region of copper would be deprotected which would be helping us because we do not want to expose the whole copper because of protection and because of the life of the PCB. So once this development happens you now have small vias opened in that particular area where you can make the contact pads and once this process has been done you need a heat curing of one more step where you put this particular exposed and developed a PCB into the protoflow oven again for heat for a temperature of 160 degrees now for 30 minutes and the intention there is just to harden out the material which is remaining. So the portions which are not coming off or the portions which are already there on the the UV exposed regions of the mask they need to cross bond well so that they can harden. Temperature is merely a catalyzer there the same thing could happen if you put this in a room temperature condition for 2-3 days but the process of hardening would be slower in that particular case. But in this case we want to accelerate the process so we heat it and act the temperature acts as a catalyst and does more cross bonding action and in just about 30 minutes the whole lacquer which is exposed to the UVV now has a completely hardened structure and those regions which are now which had been developed off are now exposed and this gives you the complete processing related to the PCB board cut. So once the hard baking step has at 160 degrees has completed then you need to clean the PCB substrate before the electronic soldering etcetera and the component placement can start to take place and also before you apply the solder paste typically through another masking strategy that I will show a little bit later. So there is another pouch which is there in this pro mask kit which again is dissolved in water for doing this cleansing action. So you can either spray the solution on the surface of this completely hardened PCB for some time so that you can remove off all the small dust particles and other organic impurities collected on the surface during this whole process of lacquer application and exposure and development. So that you have a completely clean board which is again fit for or amenable for the next step of the component pick and place and soldering. So the next step that is there in the PCB fabrication after particularly the cleansing has been done and the PCB is ready for the component mounting stage is the silver paste application. So as I told you earlier that in the solid mounted as a surface mounted case of the components there is always a tendency to interconnect between the components using some kind of a silver paste on the same side as the mounting of the components. So the interconnects are through coppers and the paste is typically applied for joining the component to the copper interconnected through the pad and it is applied on the pad. You already know that we had sort of exposed the pad to the open environment and this PCB board that was being illustrated here you can see this is the PCB board that we had talked about earlier. We now need to apply particularly in those areas where you have the copper pads visible in this particular case in those areas you had to apply the silver paste. So for that you have to do another masking process or a masking strategy where you had to take some kind of a self-in-paper or a butter paper like this and using the same drawing file that you had illustrated earlier you develop a mask like this you can see some of the perforations in these regions which have been done by otherwise laser machining. Laser machining has been illustrated in one of the earlier modules where we showed that with the drawing data with the drawing data you can actually print with laser and try to a blade with laser. So selective machining can be done. So this actually is a selective machining case in this particular zone here as you can see of the specific kind of paper used for the masking strategy and it has been framed up nicely in this particular metal frame which is now used to mount on this machine. So this is again mounted on this rail which is meant for this paper and you have on both sides sort of wheels which would be able to very easily move and mount this frame. So for selective coating of the silver paste over such a mount. So we have shown how this mask is now mounted. The other aspect is to sort of align this PCB over these stands which are mounted through these allen bolts into the stage and the stage is mobile in a way that it goes all the way down here so that it can go and align with the mask itself. So as you can see here the mask has now been aligned very well to the PCB which is actually at the bottom side and through these XY knobs etc which are actually here in this particular zone you can shift over this plate here in a manner so that you can do proper alignment between the mask at this center this mask and the PCB. Also what is important for me to tell you is that in this particular PCB you can see now only the copper pads which had been fabricated and exposed in the last step they are coming out through this mask and the drawing file shared by the machine which do the copper pad milling and mask making with laser they are the same file. So therefore you can overlap them very well and the copper pads can start coming out of this butter paper this mask paper and they get exposed selectively to the outside environment. Now there is a scope for introducing this silver paste which actually comes in a premixed form and comes in a syringe needle like this so basically the idea is to be able to apply this paste on the whole of this region and then try to cover the surface properly with this small flutter and you can actually this can be something like you know spread maker so whatever silver paste has been dropped you can spread on the whole surface and cover the whole area enabling thereby the copper pads to be just covered with the silver paste in the remaining area which is covered by the butter paper to be not having any silver paste. So, you are now selectively coating the silver paste on the top of the PCB and one of the reasons why you do that is the silver paste is used for the next stage of the component getting connected electrically to the copper circuit. So, the silver paste is sort of a conducting as a adhesive between the components terminals and the copper interconnect which is there on the PCB by means of the pad so you already saw how to do the surface mounted components part with silver paste on the same side as the interconnects and sharing the same side with as well the mounted coppers the components on the substrate. The other body mounted components can be actually done using not using the silver paste and the masking methodology, but just solder wires and there is a machine that we have while show subsequently where we can actually solder the tips of those electrical electronic components on the other side of the micro milled PCB board so that the connection can be on the bottom side and the component can be mounted on the top side. So, now the next step is basically to pick and place the small miniaturized components that would go on the top of the PCB board and for doing this we use this machine called protoplace which is right around this place. The idea behind this machine is that there is a very small suction created at a tip of this particular you know you can see this tip coming down here all the way so this tip is actually used for going up all the way to a component and applying a certain pressure so that you can pick up this particular component. So, now you can see that this component has been sort of you know touched by this vacuum tape and with the pressure on condition there would be a possibility of the component to get lifted as you can see now the component has been held by suction and this particular tip and then you can bring the tip all the way to the PCB board. So, the component can be brought all the way up to the PCB board so that you can actually place it on the top of the particular area that you would need this component to be placed. So, typically you can rotate the component as per the guide line as per the orientation of the PCB itself and then you can actually go ahead and place this component on the top of the portion where you would like this component to typically sit. So, the idea behind this whole exercise is a precise nature of the pick and place of the individual components from the stage which is mounting these components through vacuum suction and placing it over those small regions of the PCB where you would really need the component to get soldered already as you know the copper pads are pre exposed to the solder paste and then the picking and placing of the component itself would ensure that the components are connected in that particular case this is the case of the SMD or the surface mounted components. A different protocol slightly different protocol would be for the body mounted we are going to show it in the next step actually. Some of the features about this proto place machine is that you know if you look at the different aspects of this machine you have this display here the LCD display here which talks about various commands and in fact you know there are there are commands like manual pick and place like the turntable etcetera there is typically a turntable in this particular zone you can see the various chambers of these are filled with the micro electronic components and you can actually easily go all the way to the to the turntable. So we go to the menu and we go to the turntable and we can either go clockwise direction or counter clockwise there is a set of toggle keys which are there on this side of the proto flow machine right over here and with this toggle key we can select the various command options in the turntable in the in the display in the LCD display like turntable counter clockwise clockwise so on so forth. So here for example we are going to select the clockwise option the clockwise option by moving the toggle on the left and you can see that there is going to be a movement of the stage in a manner. So that by the repeated motion of counter clockwise and clockwise you can place the particular stage containing the electronic component just underneath the picking up bed which is actually this perforation right here on the acrylic sheet. So once this is done then the other option that is there is to use this XYZ stage to position this all the way and then you know in fact in the video as you can see here position the stage all the way and then you know go to the selective location where this miniaturized component is and just as shown before you can probably pick this component using the vacuum and then drag it all the way to the PCB as has been illustrated just a few moments earlier. So for picking up components of various sizes you can see there are different size nozzles which are around this particular place and you can vary the nozzle tip diameter in a manner so that you can pick bigger components just because there would be a the larger area to grab by vacuum on the surface of the component. The component is big you need a larger area for creating a force good enough for dragging the component all the way from the stage to the PCB. The PCB is as you can see kept right on here in summary the nozzle is right around this corner and then this nozzle goes and talks to this particular zone here which can be found out in this camera the video camera here you know and then you can actually be able to position the nozzle in accordance as you may deem appropriate. What is also very important for me to tell you is that there is another line which is there in this particular zone where we are talking about you know there is one nozzle here which is lifting the component. Now the purpose of the other nozzle which you can see here which can be actually put back into the syringe at any point of time is to draw up the excess silver paste which has been applied in the last masking step and by any chance there may have been a spread over because of incorrect masking strategies. So from an area which is otherwise not needed to have the silver paste if you want to draw you as well draw using the same vacuum of the system. So all you need to do is to sort of again once you have put this nozzle in place you can actually illustrate the you know you can actually take the particular stage all the way up to the PCB as can be seen here. So that now you can actually draw up the excess paste from a region that you would like you know something like that by going to different places and drawing up the excess silver paste. The only reason why the excess silver paste is needed to be removed is that sometimes there is a connectivity problem because of the spillover and the area which is not needed to be coated with silver paste gets coated with silver paste as a result of which you always try to do this alternate actions of drawing up the paste from some zone and then trying to pick and place the components at some other zone for a proper electrical connectivity between the PCB and the components. So now you have a PCB which is mounted with some components and these components are now sitting on the silver paste and you need to dry up the silver paste so that there is proper adhesion of the electronic component with the PCB board. So as I told you the warming tool is a reflow oven which is actually a gravity fed kind of a convection kind of a system which is called the proto-flow and there is a controller which is there on this oven which ensures that the door can open and close in an automated manner. There is of course a stage for setting up the PCB which now has the pasted component from the earlier step and we need to reflow. So there are three steps in this particular process. One is actually a warming up of the oven where the temperature has to go all the way to about 190 degrees Celsius before you know the sample can be put in. That is typically the curing temperature of the silver paste which is being applied here. The other aspect is that the solder should not only sort of go to that stage but also reflow into the various gaps which are created and even prior to that it is important that whatever oxygen whatever air bubbles are there in the silver paste which comes during the time of you know application of the paste itself has to be removed. So therefore the first step of this it is a programmable oven as I told you so the first step of the program is to sort of heat it for about 140 seconds at 190 degrees Celsius so that it gets rid of all the air bubbles which are enclosed within the paste. The second step is to sort of be able to reflow it and the reflow condition happens at 270 degrees where typically the solder starts melting off and then goes around those places where the leads of the components the input output of the components are being placed. So, there is some kind of a fusion between the reflowed solder and those particular places or gaps and once that is done then you actually automatically cool the system down and the oven opens automatically for doing after doing all this processing so that whatever components have been laid out just on the silver paste itself in the previous step has been actually now completely hardened solidified and electrically connected to the copper connections which are there interconnects which are there. So, let us look at the oven how it is done so this controller right here has an option of a profile selection. So, we need to sort of enter the profile selection and then go to the open close option for the oven and then we you can see the stage of the oven comes out automatically because of the controller the controller is fully programmed of course and then what we are going to do is to take on the PCB component which had been earlier made by the protoflow system with the picking and placing operation and place this PCB component all the way to the oven right about here. And after doing this you basically close the oven and start the program so that now the whole curing action can happen of the silver paste and the electrical connection between the component and the PCB can be established in this manner after a certain amount of time is spent.