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The Occam Process: One PCB Fabricator's Perspective
October 19, 2007 |Estimated reading time: 10 minutes
I read with some trepidation the many views and opinions from respected and knowledgeable PCB and assembly leaders regarding the Occam Process. I love the concept and, as developer Joe Fjelstad and his advisors have stated, there is work to be done before this technology becomes a mainstay in the industry.
Now, the first thing is, I don't want anyone to get the impression that I'm focusing on the negative or that I am knocking the Occam Process. Rather, I think it's important that we (the industry) take a real practical look at the process. We need to add our observations and discoveries to the Occam development process to ensure nothing gets missed along the way. The sooner we get all the potential issues out on the table, the more quickly we can find solutions.
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Over the last couple of months, I've taken a fairly close look at the technology and have come up with some areas which, I believe, need work.
We know how to laser drill microvias; and, yes we know how to place components. Some of the Occam Process steps, as described by the White Paper, are fairly obvious. And, although the tendency is to jump to the conclusion that Occam is the best thing since sliced bread, we have to stop and look more carefully at some of the issues which need to be addressed and solved over the coming months and years before the Occam Promise can be realized. We have lots of hard (industry-changing) work ahead.
The Occam Process will not easily integrate into a standard PCB factory. Rather it's a new idea which needs to be researched (how to actually make an Occam board, if there is one). We need real Occam boards environmentally tested, probed for unknown failure modes and pushed to the limit in the lab before anyone uses them in production. Joe (Fjelstad) has told me that there is an Occam Test Vehicle in the design stages right now, so we'll all have something to work with soon.
To take a look at a few of the more complicated areas of Occam, we first need to look at the actual components we will be burying inside the encapsulant to make a base for the rest of the circuitry. Remember that we will still need 6 to 20 layers of HDI circuitry to get the signals, power and ground to all the components. There are some "powermesh" ideas that Happy Holden suggested which might dramatically reduce the number of layers, which lends itself well to the Occam Process, but this needs to be addressed. We also need some way to get the signals, power and wires out to the world (out of the encapsulated Occam "brick"); remember we cannot solder, and the thin HDI layers cannot support SMT type connectors, so we have to come up with some type of connector for this. Still, is possible that traditional edge card connections could serve the needs of many early applications.
Embedded Chips
Since most of the silicon world can be buried with Occam there are some issues with thermal dissipation and overall height. When the height of the components gets too high we have problems in the amount of encapsulant used. Thick encapsulant gets expensive to fill and could cause issues with shrinkage. Any slight crack or pull away of the encapsulant from the component leads would allow corrosive liquids from plating and etching to weep in causing future damage to the connection. Many larger computer processor chips are too thick or have a large thermal profile which requires a heat sink. The outboard chip/heat sink would require an unknown form of socket we can embed, yet we need a way to create subsequent layers above the socket and yet still get to the pins. So we have some work to do, here. Of course, if there is a need and value is perceived those solutions will be worked out over time.
Because one of the fundamental objectives of the Occam Process is to eliminate solder (leaded or lead-free) we therefore cannot solder to the top HDI circuitry. It is this idea of eliminating solder that limits Occam to a single layer of components, at least until we work this through.
The Encapsulant
I actually found an encapsulant which meets many of the requirement of the Occam Process which are: very small shrinkage numbers, high operating temperature, high Tg, low temperature or UV cure and, the most difficult, the ability to take electroless copper or additive copper. The only problem is that this suitable epoxy costs about $1000.00 per liter. I'm sure there are chemists already looking at alternatives which will provide a reasonably priced substitute and, given the long history of molded interconnection devices (MIDs), it is likely that workable formulations already exist.
Resistors/ Capacitors
Small resistors and capacitors are a potential problem to work around. The issue is with the thickness of the copper plating on the ends of the chips. I cross-sectioned a 201 resistor and the end-plating was only .0003 inch thick or .3 mils (8 micron). This thin copper plating presents a difficult target for a laser to stop on. So, there's some work to do here.
There is also the question of tin contamination from the coating on the components when we laser through tin to the copper. Our (Sierra's) Micro Circuits shop needs at least 10 microns of copper to stop the laser on or it damages the pad or punches through. A laser punching through or falling off the side of the resistor end-plating would damage the resistor or the HDI circuitry below it.
We use an ESI 5330 yag laser with a 30 micron beam. Typically we laser down through only 2-5 mils of dielectric or encapsulant to land on a 10 micron thick copper pad. With Occam the encapsulant may be thicker due to the thicker chips and components therefore the laser needs more power to drill through from the top but this remains to be seen. The extra laser heat for thicker dielectric material requires even thicker stop copper, limiting the ability to put HDI circuitry on the top of the Occam board. If we laser through the FR4 mounting base or thin HDI circuits, lower power can be used making the job easier. The very thin 3 mil wide wedge of metal on the top and bottom end of a 0403 resistor or capacitor would heat up quicker than the larger 9 mil round pads we normally hit with the laser in HDI. Additionally, the very small landing pad of only 3 mils by 5 mils on a 201 resistor is not an easy hit, even without accounting for part misalignment. Remember we need to align the components, the laser holes and the HDI layers over 24 by 18 inches with a accuracy of +/- 1 mil to hit the resistors, which isn't an easy job.
Typically, when we assemble components as long as these, 50 percent or so of the lead is touching the pad, we would pass the placement and proceed to solder. However, with laser microvias, we would need to hand rework the misaligned glued parts with a much tighter tolerance. To laser a microvia through 100 mils of encapsulant direct from the top to a small resistor or chip lead is impossible. Alternatively, coming up from the bottom through 5 mils of FR4 is not easy, but not impossible, requiring a lot of research and development.
When we did our assembly test to place components on a 5 mil core we ran into problems with placement accuracy caused by the thin core moving around. Again, not impossible; just more R&D needed.
Typical misaligned components 3 mil laser via in relation to a 0201 chip
notice the thin laser pad on the chip no margin for error
Component Height
Component height is a major limiting factor for the Occam process. You cannot try to encapsulate a 1.5-inch high electrolytic capacitor or a 2-inch transformer. The extra height would make encapsulating too expensive, have too much shrinkage and weigh too much. Yet we need large capacitors, transformers and other large, thick and tall components to make our products work. So, it's another area to work on.
If a typical maximum component height for Occam is, say .100 inch, this eliminates a significant portion of the electronic components used today. If you do not believe me, look at pictures of finished boards in electronics magazines or thumb through an electronics parts catalog. You will see many higher and bigger components used on boards which we won't be able to use for Occam--at least, today--until we figure out how to.
Outside world connection is also a problem to solve for Occam, as the Occam design encapsulates the components in the center of the circuit and places circuits on one or both sides. How do we get potentiometers, LCD, LEDs, connectors, test points, big capacitors, transformers, power resisters, heat sinks from transistors to the outside world to turn, switch and read? How do we incorporate big parts such as IGBT or speakers and microphones on the less structured HDI surface which we cannot solder to? Also problematic are parts that do not have easily lasered flat leads such as round leads and through board connectors. Remember, I am not saying we cannot do Occam; just thinking out loud about the problems we need to solve.
Occam on the Factory Floor
In the Occam process we, the board manufacturers, will now be handling electronic components, something we have not done much of. The biggest concern for me is the poor interaction between PCB processes and electronic components. For example we need to laser drill microvias in the encapsulant and base FR4. We cannot use plasma to either drill or clean out microvias as the large, high voltage fields generated in a plasma machine will destroy most chips. The effect of static electricity on components is well know and understood. PCB shops can easily add wrist straps, bench conductive mats as well as conductive floors. There are other concerns with existing PCB machinery and their static or process voltages. Dry film laminators separate two isolative films and create huge static voltages. Plating tanks develop 2 volts at huge currents for plating that can easily damage silicon chips. Etching and striping machines have redox voltages generated from chemical and metal reactions with high enough voltage to damage silicon. Electric circuit testers use voltages to test, some thing we cannot do with the on board silicon chips with Occam with today's testers.
Environmental situations exist inside PCB shops with processes that are too hot, generate too much static, have too much pressure and too many chemicals for most electronic components. I have tried for 2 years to encapsulate components inside FR4 multilayer boards with little success. The pressure and temperature of a typical FR4 hydraulic press cycle destroys the chips. I have since found a way to attach the chips and encapsulate them without a hydraulic press lamination cycle.
I also believe the biggest problem for Occam to overcome is that it's trying to change the world. Troubled PCB manufacturers and assemblers will not want to invest heavily in new equipment and technology. Present assemblers who will be out of business if the board shops start to do assembly, will not go quietly into the night. Expect a fight. A majority of present larger components do not directly fit into the Occam process so companies using bigger components will not disappear, they will stay with their current solder assembly process. To ask component manufacturers who make billons of chip resistors and capacitors to add more plating or bigger plated ends to the components will not be an easy sell.
Market Potential
I believe the number of boards that can actually be converted to Occam, today, is around two to five percent. A major problem is component height and outside world connection.
Having said all this, I find the Occam process interesting, with its simplicity. I know our company will be watching its progress carefully. I guess only time will tell if it's successful. If Occam captures just 2 to 5% of global electronic products, it will be very successful indeed.
Ken Bahl is the CEO of Sierra Proto Express.
Robert Tarzwell is the Director of Technology for Sierra Proto Express.