Hi Earl,
There is one other matter to consider regarding repeated HASL and long
term reliability, namely cleanliness. It is a well known fact that
ingredients in fluxes used for HASL are absorbed into the epoxy resin in
the PCB laminate and into solder mask coatings, both inorganic (chloride
and bromide for example) and organic substances such as polyglycols.
These will affect SIR properties and may also cause CAFs (Conductive
Anodic Filaments) along glass fibres in laminates. Since the flux
ingredients are absorbed, it is not possible to completely remove them.
Repeated HASL will cause larger amounts of flux to be absorbed with
increased risk for SIR and CAF problems. Fluxes used for HASL are
normally water soluble and more strongly activated than fluxes used for
assembling. Therefore, they may have a large impact on the reliability
of the final product, especially if no-clean processes are used in the
assembling of the boards.
Per-Erik Tegehall
IVF
SWEDEN
> ----------
> Earl Moon 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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