Thermal profiling is always necessary — no matter what. Or is it? If this sounds like a contradiction, there are currently ways of profiling that provide alternatives to the most time-consuming or labor-intensive methods. For example, oven verification may in some cases replace repeated board profiles, but only if strict parameters are met. The trick is in knowing when to profile the board or verify the oven performance.
PCBs often contain a mix of active and passive components, varying widely in size, functionality and heat tolerance. J-STD-075, issued by the IPC in December 2008, illustrates that even “penny-priced” passive components such as capacitors can be damaged during reflow — so much so that these components can, as far out as two years after production, cause failure in a product worth thousands of dollars. These latent failures do not show up even in the best of assembly test and inspection processes. When they do show up, the OEM faces warranty, or even worse, liability issues.
During ECD-University profiling classes, we show that thermal profiling consists of both characterization and verification, and liken the entire process, which we call Thermal Quality Management (ThQM™), to maintaining the health of the human body — perhaps the largest and most complicated assembly of all. There are times to have a complete physical, including an EKG, to determine whether you are in the correct range for your age, size and weight. This is similar to profile characterization.
Then there are times you need only regular checks to make sure you are still within the set boundaries (blood pressure reading, for example) which is similar to profile verification.
And finally, there are times when you need a full reexamination because some things have changed (weight gain or loss, for example). This would be similar to reprofiling after maintenance. The profiling tools may vary for each stage of PCB assembly and for each type of physical checkup, but the goals are the same — assuring the health of your assembled end product.
An ECD survey at APEX and online showed that while 90 percent of EMS and OEM companies are diligent about initial thermal profiling, nearly 60 percent admit much less diligence in verifying successive, simultaneous, or repeated production runs. Knowing that a lack of profiling can result in field failure, why isn’t profile verification a routine part of every quality-conscious EMS and OEM company?
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