Graham,
Now have I opened my TechNet eyes. The study was done in the beginning of
the nineties, i.e. before BGAs were used. What we found was that the
reliability of solder joints to LCCCs were improved by conformal coatings
most notably by Parylene and some silicone coatings but also to some extent
by other conformal coatings. It should be pointed out that only a fraction
of the space beneath the LCCCs were filled with coating material.
We have later performed a study for ESA in which we applied a solventless
silicone conformal coating as underfill under ceramic BGAs and CGAs monted
on polyimide/glass and polyimide/aramide (Thermount) boards. We could not
see any significant influence of the silicone "underfill" on the fatigue
life of the solder joints, i.e. it neither improved nor worsened the fatigue
life.
Regards
Per-Erik Tegehall
-----Ursprungligt meddelande-----
Från: Graham Naisbitt [mailto:[log in to unmask]]
Skickat: den 21 maj 2002 22:40
Till: [log in to unmask]
Ämne: Re: [TN] Conformal coating BGAs
Peter, Linda et al (whoever he/she/is?) :-)
I have a study completed by IVF in 1988, regarding the influence of
conformal coatings to the reduction in lead pad fracturing by >80%.
I may be able to send you copies but, if Per-Erik Tegehall has his
TechNet eyes open, maybe he can enlighten me to the availability of this
study. Let me know please.
With specific regard to BGA underfill, and with due recognition of SIR
Doug's posting, I think it might be feasible to utilise some of the
newer coating formulations - e.g. water based - that may do a very
acceptable job in one operation. Curing will be an important criterion
that requires proper control as Doug says.
If you would care to assist me, I will be willing to receive your
samples here, coat them using production methodology and return them for
test evaluation FOC. Let me know please?
Regards Graham Naisbitt
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> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of
> [log in to unmask]
> Sent: Tuesday, May 21, 2002 06:34
> To: [log in to unmask]
> Subject: Re: [TN] Conformal coating BGAs
>
>
> Linda,
>
> I woud be interested to see the information you have that
> says underfilling a BGA with Conformal Coating reduces life
> cycle. I epoxy underfill BGA's at present, but there are
> mutterings from bean counters, etc about this extra process,
> and why can't we just underfill with coating? I have argued
> about his, but from a rather hypothetical standpoint -
> comparative thermal conduction properties of the two
> materials, ease of rework of my underfill versus difficulty
> in removing parts stuck down with coating, stated improved
> life of BGA joints using epoxy versus "I expect coating
> underfill to have no beneficial effect on life cycle". I've
> had no words of wisdom to back up my wanting to keep epoxy
> underfilling and my rejecting coating underfilling.
>
> I suspect the reason for reducing life cycle is if the
> coating underneath the component is thick and hasn't cured
> properly - after all, it's supposed to be a thin coating, not
> a thick filler. The chemisty of uncured coating is bound to
> be different from that of cured stuff. Perhaps Graham N. or
> Brian E. can step in at this point (?).
>
> Peter
>
>
>
> Linda Woody <[log in to unmask]> 20/05/2002 11:53 PM
> Sent by: TechNet <[log in to unmask]>
>
> Please respond to "TechNet E-Mail Forum."; Please respond to
> Linda Woody
>
> To: [log in to unmask]
> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
> Subject: [TN] Conformal coating BGAs
>
>
>
>
>
>
>
>
> I am searching for any data out in the industry referencing
> the affects of conformal coatings on BGA devices? How much
> coating should be allowed under the device? Should the
> underside of the BGA be completely dammed or underfilled? I
> have collected data that shows standard spraying of coating
> material does not affect life cycle, but allowing complete
> underfill of the coating material greatly reduces life cycle.
> What concerns are there for leaving exposed adjacent uncoated
> conductors (BGA balls) under the BGA devices?
>
> Regards,
>
> --------------------------------------------------------------
> -------------------
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