Tech Question of the Week: “How do I find the right laminate material within IPC-4101?”

Tom Newton’s YouTube video with the above title is fine as far as it goes. Unfortunately, neither this video nor IPC-4101 are sufficient for selecting a proper material slash sheet out of the 55 slash sheets in the B-revision and the 66 slash sheets in the upcoming C-revision, let alone the proper base material for a given assembly sequence/application. 

Tom correctly points out that there are 60 search key words; they are listed in Section 7 but not on the slash sheets in an effort to discourage using them for specification purposes.  Unfortunately, the key words are somewhat generous in describing attributes like ‘Low Z-axis CTE,’ ‘High Decomposition Temperature’ and ‘Lead-Fee FR-4 (meaning RoHS capable).’

My emphasis of course is reliability, which has gotten much more critical with the higher soldering temperatures required with Pb-free solders.  I have found that three key properties are mostly responsible for the physical survival of PCBs during the soldering processes. They are glass transition temperature, Tg, decomposition temperature, Td, and thermal expansion, TE, in the z (Thickness)-direction.  I have proposed, for search purposes only just like the key words, a Soldering Temperature Impact Index (STII) for inclusion into first the B-revision as well as the new C-revision; to no avail.  The STII is defined as

STII = {(Tg[TMA] + Td[5%])/2} – {(% TE[50->260C])x10},

with the recommendation of STII≥215 for thicker PCBs and STII≥205 for thinner PCBs. 

The STII addresses issues like PTH/via reliability, thermal degradation and to some extent delamination (excluding moisture), but does not address failure modes like pad cratering. 

Even with this added, the search for the right material requires much more knowledge than is contained in IPC-4101. It is for this reason I produced the White Paper: “Recommendations for PCB FAB Notes and Specifications in Printed Circuit Board Drawings for SnPb and Lead-Free Soldering Assemblies, the Qualification of PCB Shops and Activities to Assure Continued Quality.”

Please contact me at engelmaier@aol.com regarding the white paper.

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