Hi Gause,
We recommend that you precondition the IST coupons to simulate assembly (3X) and assembly and rework (6X) prior to testing at 150°C. Tin/lead preconditioning is to 230°C; lead free preconditioning is at 245°C or 260°C. Preconditioning at 260°C is the default lead free temperature. Preconditioning on the IST tester will give a tighter data set (and has better control) than preconditioning in a thermal oven but methods are used.
The reliability data we have is based on 150°C testing (hundreds of thousands or tests) so 150°C is the default test temperature. You can establish a reliability profile for your company at any test temperature.
Most IST reliability testing is performed at 150°C. Microvias are tested at 190°C (powering on the microvia). Flex circuits are tested at 210°C. Survivability testing is preformed at 220°C or above. Field life testing is performed at three temperatures usually just below Tg (ex. 150°C, 160°C and 170°C).
You can expect that a well made circuit will have a 50% reduction in reliability due to lead/free assembly and rework at 260°C. Because of that observation many people are trying to reduce the assembly temperature to 245°C if at all possible.
It is important to note that delamination extends thermal cycles to failure so you have to be vigilant to monitor for delamination and crazing. We monitor for delamination and crazing electronically and confirm with microsections. Delamination is dramatically increased with lead free assembly and rework. Grid size has an inverse relation to propensity to delaminate.
Werner has observed, and I concur, that lead free assembly has had a large impact on electronic reliability and the reliability of circuit boards has been the hardest hit. It is important to do your home work when you switch to lead free assembly. Design, material and copper quality work must all be optimized to meet the lead free challenge.
Please feel free to contact me directly if you need more information.
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 HuGause
Sent: Friday, January 11, 2008 1:49 AM
To: [log in to unmask]
Subject: [TN] IST for lead free assembly PCB monitoring
TechNeters,
We used IST data for PCB supplier process health monitoring, for conventional stardard Tin/lead assembly, we follow PWB recomeend cycling temperature from room temperature to 150 degree C. Now, our product transition to lead free assembly,the assembly reflow temperature go up to 245--260 degree C. We are considering to increase the IST cycling temperature to 230 degree C. The pass/fail criteria as 20 to 50 cycles.
I would like to know anyone has the experiece of higher cycling temperature for IST, and any comment on the peak temperature and criteria. Thanks.
Best Regards,
Gause Hu
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