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Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.

PCB Material selection

Gary Hunt


PCB Material selection | 21 June, 2000

I need to construct test boards for which will be repeatedly subjected to Highly Accelerated Stress Screens, (HASS), and am looking for recommendations for materials. Would an Epoxy Glass or an Epoxy Glass Composite be least suceptible to exposure to repeated exposure to environmental extremes, (high temp, low temp, vibration at low temp)?

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Re: PCB Material selection | 21 June, 2000

Gary: Regular FR-4 will HASS.

HASS has largely fallen from favor, because: * Process is difficult to control, so not to strip the life from components * Failures under HASS have nothing to do in use failures.

If you insist on continuing with this, defining your screening limits will help in determining the required laminate and "resin system"

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Re: PCB Material selection | 26 June, 2000

Gary: Allow expand on this topic a bit.

Back in the gray skied past when people wore calculators on their belts, rather than pagers n Palms, and didn�t drink California spill, they used HASS that you mentioned and HALT (Highly Accelerated Life Testing) as product screening methods. The thinking was � "Hey let�s stress test the blank out of the product over a wide range of environmental conditions and if the unit passes, it�ll be good to go." The logic was difficult to argue against. Hey, what�s argue about??? Some products failed the screening and some products passed the screening.

Many downed space flights, aircraft, and other high $$$ efforts later (and may be some of that spill), people began pondering the logic of shipping highly stressed screened units. People found that the process was not without problems, which included: � Process is difficult to control, so not to strip the life from components. � Failure under HASS / HALT has nothing to do field failures.

Today, practical accelerated testing, or environmental stress screening, is seen as a product reliability engineering tool. Best applied during the design phase, HALT / HASS stimulate component weaknesses and failures in an accelerated test chamber far exceeding those in the field. HALT / HASS combine extremes of temperature, temperature change, and multi-axis vibration to rapidly expose design weaknesses and process flaws. So the process goes: design, HALT /HASS, redesign, HALT / HASS ... you get the picture.

This is not to say that accelerated testing is a not a element of an effective pre-shipment product evaluation. Accelerated testing is product screen for damage mechanisms that could occur during product use, but are accelerated to cause failures in less time that in service. Here the damage mechanisms result from shorter cycles and more severe loading, but avoid damage mechanisms that are beyond the normal product operating range.

References to these two approaches are: � "Accelerated Reliability Engineering: HALT And HASS" Hobbs, John Wiley, 2000, (047197966X) [I suggest this based on a SMTA Book Store recommendation, rather than first hand experience] � "IPC-SM-785, Guidelines For Accelerated Testability Testing Of Surface Mount Solder Connections"

Please e-mail me the name of our company name so that I can avoid your products.

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Re: PCB Material selection | 27 June, 2000

Hey Brainiac!!!! Those were slide rules, not calculators!!! Let's keep the historical context of this drivel accurate !!!!

Dr Vibrato

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