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My career began at General Electric in 1978 as a young chemist. My first project was to develop and scale-up a new composite product designated by NEMA as CEM-3. GE was late to the party as several other CEM-3 products were already in the marketplace. The product was 1.52 mm double-sided copper-clad laminate which could be used as a cheap replacement for FR-4. CEM-3 was unusual as the reinforcement was a combination of woven fiber-glass fabric and fiber-glass paper. The resin system was a dicy-cured epoxy resin yielding a Tg the same as FR-4 at the time, of 110–120°C range. Because it was all epoxy and all fiberglass, the properties were electrically identical to those of FR-4. The mechanical properties also were similar although the flexural strength was lower due to the lack of stiffness generated by woven fiberglass.
Figure 1: FR-4 1.52 mm Construction
Figure 2: CEM-3 1.52 mm construction
If this CEM-3 was that similar to FR-4 what was the advantage of using it other than as a cost reduction? The big feature was that holes could be punched in the CEM-3 panel whereas the FR-4 had to be drilled. A thousand holes could be put in the board at one time while the FR-4 had to be drilled hole by hole. Even though punched, a viable plated-through-hole (PTH) could be generated. General Electric called this product General Electric punchable epoxy glass (GEPEG). Other competitive product names were Norplex’s NP424, Oak’s AL910, and Westinghouse’s 65M72. At that time, the constructions and the resin systems for all the competitive CEM-3s were virtually identical.
The first use of GEPEG was as a double-sided PWB for an Atari game board. These boards plugged into an Atari game console and were about two inches on a side. One side had fingers to be snapped into the edge connector of the console. What it lacked in sophistication, the Atari game board made up in volume and this GEPEG product consumed a lot of base material real estate. For a long time I put a hook through an Atari game board and hung it on my Christmas tree just to remind me what was supporting my delicate status of employment. Later Delco planned to use GEPEG as the board for their “Engine Command Control Module” for General Motors but the opportunity went poof when the design was changed to a four-layer multilayer and I went on to another GE product line.
Another common feature that was not well-known at the time was that CEM-3 contained filler—and a lot of it. Because the fiberglass paper was very light in terms of density, the resin content was typically 85% as opposed to 40–45% resin in the woven fiberglass layers. To reduce cost and to make the core layers more robust, the suppliers added alumina tri-hydrate (ATH), clay, talc and mixtures of various fillers all with a cost at that time being less than US$10 per pound. The core would contain between 15 and 30% by weight of these fillers.
Then comes the Underwriters Laboratories scam. Each of the CEM-3s developed had another common feature, the woven-glass/epoxy surface prepregs were all FR-4 surface sheets. The IR scans of the new CEM-3s all matched that of the original FR-4 laminates that had been put through the long term thermal aging (LTTA) at UL. UL granted recognition and listing based on the IR scan match. From that point on, UL assumed that the CEM-3 products where devoid of filler and were identical to FR-4 terms of the epoxy resin system, even though the core was loaded with fillers.
CEM-3 and its earlier brother, CEM-1, disappeared from the U.S. manufacturing scene in the 1980s. One of the reasons for its demise was that the glass paper never was produced in enough volume to make the price for CEM-3 a significant cost reduction. Board shops found it easier to buy and stock only one inventory item in FR-4. Most of the double-sided shops had enough drills anyway and did not find the gang-punched hole-generating process to be much of a cost savings. Those shops that wanted to use CEM-3 bought cheaply from Nanya as a viable alternative.
Over the years since, CEM-3 has been manufactured in and consumed in Asia. The price of the product came down to the point where it occupied its own cost niche. Companies such as Shengyi Technology, Panasonic, Nanya, Kingboard and others still make CEM-3 today. The applications for CEM-3 have expanded beyond just a punching alternative to FR-4. The fillers in the core have been optimized to improve performance properties such as thermal conductivity and comparative tracking index (CTI). The epoxy resin-rich product made it excellent for high humidity environments as well as applications where high power is utilized. Appliances, automotive and gaming are normal end-use applications for CEM-3.
Additionally, further property enhancements have been generated by incorporation of fillers into the surface layers as well. CEM-3 can be filled in the surface in the same fashion as the FR-4 products. The CTI and thermal conductivity are further enhanced by the addition of filler in the surface plies. Alternative resins such as cyanate esters, epoxy novolaks and BT can also be added easily to provide low-Dk/Df, high-Tg and more robust CEM-3 products. These modifications to CEM-3 make it a higher performance product than FR-4 for these applications. Thinner versions of CEM-3 can also be used as innerlayers for a multilayer board. But that is a discussion for another column. Let’s agree that CEM-3 continues to evolve and reinvent itself as a customizable, copper-clad laminate which keeps the forecast for this grade on a positive trend.
From the UL point of view, there is currently a proposal to acknowledge that fillers are a part of the composition of the CEM-3. The proposal is that fillers can be used up to 80% by weight when the reinforcement portion is subtracted. Approval of this change within the UL746E, will allow more of the CEM-3 products to be used on an interchangeable basis. Voting closes on February 22. I will keep you posted.
Doug Sober is the president of Essex Technologies Group and may be reached by clicking here.