PC boards: Materials and processing are now a hot technology
Engineers' traditional rational for printed circuit boards (PCBs) was to save cost over hand-soldered wiring. The earliest PCBs were made from paper saturated with phenolic resin. This is now known as FR-2 (flame-retardant #2). Since phenolic resin is a thermoset resin like epoxy or Bakelite, it has good structural properties. Unfortunately, the paper foundation of the board is subject to cracking and tears when you subject it to high loads. In 1974, GMC Truck mounted ignition ballast resistors on paper phenolic boards. The board worked fine under the normal design mechanical and thermal loads. What the designers did not foresee was the force auto assemblers used when they had to push a wire harness connector onto the board. That act, before the vehicle was even competed, broke the PCBs and caused immediate warranty problems.The structural limitations of paper-phenolic boards were not confined to General Motors. In 1980 the least reliable part in a Ford vehicle was the ignition module. The module was a 5 x 8-inch zinc die casting with a paper phenolic board holding the timing, logic, and drive circuits to fire the spark plugs. The modules worked fine in the lab, but in a real production environment, the paper phenolic board would warp under temperature variation and the circuit traces would crack. This would cause a catastrophic failure of the ignition system, stranding the car in or alongside the road. Ford management considered the cost of switching to a fiberglass-based FR-4 (flame retardant #4) board to be prohibitive. Instead, they had the division making the module partially fill it with sand before potting the paper phenolic board. This increased the thermal mass and provided a cushioning effect that reduced the failure rates to fewer than 5% in the warranty period. It was still the worst reliability Ford part of the era.
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PC boards: Materials and processing are now a hot technology
The module was a 5 x 8-inch zinc die casting with a paper phenolic board holding the timing, logic, and drive circuits to fire the spark plugs. The modules worked fine in the lab, but in a real production environment, the paper phenolic board would
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