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Bleeding Edge: Extreme Printed Circuit Boards -- "Down the Hole"

"Down the Hole" oil-well sensing is one example of an extreme printed-circuit application in severe conditions, featuring very high temperatures of 200* C and high pressures of 2000 psi. The biggest problem to solve is the laminate's ability to withstand high temperature without delaminating and without cracking the vias. A few companies, such as Rogers and Panasonic, sell a new very-high-TD laminate at 440* C, featuring good copper-bond strength and improved thermal-cycle crack resistance.

For years, the "Down the Hole" industry used polyimide laminate. However, polyimide can suffer from early delamination caused by poor wetting and prepreg flow. Some of the new laminates use phenolic resin, which flows like water to wet the cores fully. The high temperature in a "Down the Hole" environment means that we no longer worry about T/G, which measures the point at which the Z axis starts to expand at a higher rate. The T/G is not a specification of the laminate's ability to withstand temperature but rather a point at which hole cracking can become a problem. The true specification of a laminate's highest temperature is the T/D (decomposition temperature). T/D is the temperature at which the laminate loses five percent of its mass. To increase the high-temperature bonding strength as well as the life expectancy of the circuit, we can increase copper bonding to the laminate by custom-laminating high-oxide heavier copper to high-resin prepreg. The heavier copper traces with their slight overhang lock in the cores and the prepreg. 

Very high temperature stresses the copper in the hole to the point of failure. Unlike repeated large-thermal cycling, which tears the hole copper apart through many small stretches, extreme high temperature causes the holes to fail through a few large expansions of the copper, past the elastic point to failure. The thermal cracking problem experienced in extreme delta temperature cycling was solved through slightly changing the plating parameters by applying Sierra Proto Express's patented High Reliability Technology. Originally, the high-reliability plating was designed to prevent copper-hole wall cracking through 2000 lower-temperature delta cycles. Lead-free solder processing presented a new challenge in cracking of the vias due to a few higher-temperature cycles caused by the higher process temperatures of lead-free solder. The lead-free high-reliability plating technology was applied to the Down the Hole circuits, showing an increase from 3-5 cycles to failure (at 350* C) to over 30 cycles with no failures. The end-result of the laminate change and the application of high-reliability technology to the Down the Hole circuit boards is a new product.

With a little R&D, a little imagination and a lot of sweat, most extreme printed circuits can be manufactured successfully.

Ken Bahl is CEO of Sierra Proto Express

Robert Tarzwell is Director of Technology at Sierra Proto Express bobt@protoexpress.com