We await your expert rebuttal, with regards to your conclusive statements
about HASL reliability, after multiple passes, Mr. Engelmeier.
LK
On Sat, 28 Nov 1998 [log in to unmask] wrote:
> Thanks, Franklin D Asbell. Such ellagence in a simple statement. Thanks Bev Christiansen for helping with my personal oxidation problem.
>
> I don't want to beat a dead horse or bore industry experts, but this subject deserves more consideration for and by us rookies – and it is far from
> being dead as evidenced by repeated questions about it. Also, I don't want to make a mountain out of a mole hill, but this matter is of no small
> concern to many as is evidenced by my email lately. That is why I submit the following:
>
> The original question simply asked about re-running boards through the solder coating process and whether this would be good or bad. From my
> point of view, concerning solder wetting, if it didn't wet the first time – find out why, fix the problem, and prevent it from recurring. Even if such
> small areas do not wet, they will impact solder joint quality later. My fix and preventive method is some alternative to HASL as many in industry
> are striving to achieve but, as HASL still remains, it must be dealt with properly. The best way is first to ensure solder surfaces are clean before
> processing (in HASL not an easy task). Then, the process must be effectively managed instead of the results (the defect in question), and to re-
> solder is results management.
>
> Concerning PCB initial quality and reliability, I echo what most already know. Thermal stress and/or shock is not good for you. It's not good for
> me, and certainly is not good for PCB's – especially when repeated. It must be minimized where and whenever possible.
>
> As many know, to reveal possible problems or determine initial multilayer PCB quality, MIL-P-55110 (now IPC-6012/5013?) requires (ed) the
> "B" coupon to be thermally pre-conditioned and subjected to a solder float at 550 degrees F. for 10 seconds. For thermal shock, no pre-
> conditioning is done. Both tests emulate HASL/reflow/wave soldering conditions as boards see both stress and shock during these operations
> though not at such a high temperature or long dwell/contact time (hopefully). Hence, a good test.
>
> After specimen testing, x-sectional analysis is performed. If no defects, as indicated in the aforementioned documents (in laminate and thermal
> evaluation zones), are found, board quality is acceptable to the extent tested. Many of us have seen many defects over time – consisting of
> delamination, pad lifting/rotation, hole wall cracking, and other thermally induced failures. Again, thermal stress and/or shock should be
> minimized where and whenever possible.
>
> The Navy (Indianapolis?) in the early 1980's, ran tests to determine what was causing polyimide boards to fail and cause severe problems in
> aircraft digital engine controls that caused bigger problems with aircraft and crew losses. I seem to remember Hamilton Standard (Windsor
> Locks, CT) was the contractor. To cut this short, it was found that foil bond strength was reduced from about 7 lbs. as supplied, to less than 1 lb.
> after about 100 thermal excursions from – 55 to 125 degrees C. Jerry Kirchenbaum (Trace Labs) gave me the report though I don't remember all
> details and don't recall the results concerning inner laminar bond strength. Phil Hinton may shed more light as I remember he was/is very involved
> with polyimide material types. One of the fallouts of all this was suppliers recommending relamination cycle modification to reduce the Tg from
> its fully cured 270 degrees C. to about 250 to provide better bond characteristics over material and board life.
>
> I know other studies have been done with other resin systems. I don't know all the results, but IPC and many having participated in round robin
> testing, probably has much data already published. The problems associated with polyimide are not as severe with epoxy resin systems though
> thermal stress and shock does reduce foil and inner lamnar bond strength also.
>
> And on it goes,
>
> Earl Moon
>
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