Hi Torsten,
Because the maximum temperature in your test is limited to 140°C I expect you could find materials that would survive to 3500 cycles. The dwell time at temperature may be more degrading to the material than the number of thermal excursions.
If there are thermal excursions to simulate, or as a result of, lead/free assembly and rework, say 6X260°C, prior to testing then it would be harder to find a robust material but they are available. I expect design will have an influence in the ability to pass this test. HDI designs have a greater challenge to pass at 3500 cycles after lead/free assembly.
My advice is to ask your PWB fabricator what high reliability material they use and, if possible, perform some comparative tests using that material. Choosing a high reliability material with which the vendor has no experience can cause unexpected problems. Maybe it would be better to say "expect problems" with any new material until the fabrication process has been tweaked to accommodate that new material.
Field life can be anticipated using acceleration testing of representative coupons at three or more temperatures below Tg. Using the three data points a curve could be plotted which allows one to calculate the number of thermal excursions a PWB can survive at a lower temperature. If you can anticipate the number of thermal excursions a day and the maximum temperature, for example turning on a computer in the morning and off at night is one thermal excursion, where the internal temperature reaches say 35°C, then you can anticipate the field life of the PWB.
We will be giving a PWB reliability workshop at EIPC in Nuremburg this June if you are planning to attend. We will be addressing thermal cycle testing, although not to the parameters you are using here. The workshop includes an overview of thermal cycling induced failure modes in copper and dielectric materials which are common to all thermal test methods.
Sincerely,
Paul Reid
Program Coordinator
PWB Interconnect Solutions Inc.
235 Stafford Rd., West, Unit 103
Nepean, Ontario
Canada, K2H 9C1
613 596 4244 ext. 229
Skype paul_reid_pwb
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Torsten Hagge
Sent: March 16, 2010 11:55 AM
To: [log in to unmask]
Subject: [TN] thermo shock cycles
Dear Technos,
we have a customer who recommended 3500 thermoshock cycles -40 to +140°C with 15°C/min gradient. Any ideas/suggestions which PCB material and lead free solder can be used to fulfil this?
As other customers require much less thermo cycles for the same application any hints how to calculate the required thermo cycles out of a mission profile and when the formula for this kind of ALT test does not make any sense (as I think 3500 cycles is normally not reachable for standard PCB material and especially for lead free solder)?
Best thanks in advance,
Torsten
Dipl.Ing.(FH)Torsten Hagge
tech. projectmanager
Kristronics GmbH
Gewerbegrund 5 - 9
24955 Harrislee
Tel.: +49 461 7741 624
Fax: +49 461 7741 642
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