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Marcy's Musings: Engineering Economics
Punching Out: Should You Sell Your Company to a Private Equity Firm?
Estimated reading time: 15 minutes
SMART Group Seminar: Harsh Environments & Electronics
Moving one step along the supply chain, it was the turn of Dennis Price, quality director at Merlin Circuit Technology, to discuss high-temperature PCBs and solderable surface finishes from a printed circuit fabricator’s viewpoint. With reference to the critical laminate parameters previously detailed by Alun Morgan, he described three methods for measurement of glass transition temperature: Differential scanning calorimetry (DSC), thermal mechanical analysis (TMA), and dynamic mechanical analysis (DMA). All gave different results, sometimes quite significantly so, as demonstrated by examples he showed, and it was important to define which method had been used when quoting Tg values.
Price commented that the continuous maximum operating temperature of a laminate was difficult to quantify meaningfully. The UL relative thermal index (RTI) was derived from a mathematical extrapolation of data from accelerated ageing measurements and defined a temperature at which the material would operate for 100,000 hours and still retain at least 50% of its original physical or electrical properties. In his opinion, the RTI might not represent a PCB designer’s ultimate material temperature choice, but it was a good starting point. He went on to describe the tests used to establish the various UL flammability classifications for vertical and horizontal burning, which gave a preliminary indication of their suitability for a particular application.
Price then discussed the coefficients of thermal expansion, in-plane and through-plane, of various laminates in the context of thermal cycling reliability and described the principles of the interconnection stress test (IST), an accelerated method creating uniform strain from within a multilayer daisy-chain pattern of holes and tracks by DC electrical heating and forced convection cooling. The technique identified and assessed the severity of post separation and barrel cracks in plated through holes.
So much for the thermal characteristics of laminates--he turned to the practicalities of managing and removing heat from real PCBs and handed round many examples of boards with external and internal heat-sinking and expansion-restraining components, from simple bonded-on heat ladders, through copper-invar-copper constructions, carbon cores and modern insulated metal substrates.
He concurred with Morgan that polyimide was the most popular high-temperature laminate choice and materials were available from a range of suppliers. But it had certain shortcomings--brittleness leading to chipping on final-stage drilling and routing and a tendency to high moisture absorption. The ultimate high-temperature laminate choice was ceramic, either low-temperature co-fired (LTCC) or high-temperature co-fired (HTCC), but these technologies were outside the scope of mainstream PCB manufacture.
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