Dave
Even Parylene (R) is only a retardant if applied over anything
hygroscopic. It is not totally impervious, although it is possibly
better than most other coatings. I have a theory that its main advantage
lies in the conditions under which it is deposited (ie a reasonably high
vacuum with a light plasma to activate the surfaces) more than the
quality of the film itself.
Brian
Dave Hillman wrote:
>
> Hi Seth! Your vision of how the acrylic is applied to the BGA components is
> correct and at first a number of the process engineers were concern about
> the potential "trapped air" concern. However, using correctly applied,
> consistent coating methodology we are not getting a gas-tight "seal' and we
> still pass the saltfog testing, environmental testing, etc. My impression
> is that we (e.g the industry) have a preconceived notion of our conformal
> coating processes being much more gas-tight than they really are (with the
> exception of paralyene). When the underfill testing progresses a bit
> further and I have some real data, I'll send out a status report - there
> are a few bottles of Mountain Dew riding on who's hypothesis is correct on
> whether underfill is a benefit or detriment!
>
> Dave Hillman
> Rockwell Collins
> [log in to unmask]
>
> Seth Goodman <[log in to unmask]>@ipc.org> on 03/28/2002 09:43:10
> PM
>
> Please respond to [log in to unmask]
>
> Sent by: TechNet <[log in to unmask]>
>
> To: [log in to unmask]
> cc:
>
> Subject: Re: [TN] Conformal Coating BGA's
>
> Hi Dave,
>
> Just curious on this one. When you apply Humiseal to a board with BGA's
> with no underfill, I take that to mean the coating just bridges the gap
> between part and board around the perimeter of the BGA without wicking
> underneath very far. This traps air under the BGA. Obviously this works
> fine, as you have been using it in avionics for some time and that is a
> tough environment. My naive question is why doesn't the coating around the
> BGA perimeter blow out when the trapped air under the BGA heats up due to
> power dissipation in the BGA itself? If it was going to happen at all,
> your
> application is possibly a worst case with the combination of high ambient
> temperature and low ambient pressure.
>
> I would also be interested to know the results of your test to see how the
> underfill affects the failure rate due to temperature cycling. Since
> neither the underfill nor the conformal coating will really keep moisture
> out, it would be nice to know if there are any real benefits conferred by
> this extra processing step.
>
> Seth Goodman
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Dave Hillman
> Sent: Thursday, March 28, 2002 5:06 PM
> To: [log in to unmask]
> Subject: Re: [TN] Conformal Coating BGA's
>
> Hi Peter! Cool, now your underfill rationale falls into place, especially
> since you were reacting to a less-than-robust design. I have test boards in
> the thermal cycle chamber right now in an attempt to understand just how
> much of an "improvement" the underfill operation adds to the solder joint
> thermal cycle fatigue life. And yes, underfill adds an extra processing
> step thus increasing time and cost. If the underfill operation doesn't
> result in a value added benefit for our use environments then it most
> definitely won't be a suggested process addition. The EMMA project has
> published an extensive data set on the impact of vibration on area array
> components (BGA, CSP, FC) which might be useful to you. Contact Lee
> Whiteman for access to the data ([log in to unmask]).
>
> Dave
>
> [log in to unmask] on 03/27/2002 06:22:10 PM
>
> To: <[log in to unmask]>
> cc:
>
> Subject: Re: [TN] Conformal Coating BGA's
>
> Hi, Dave,
>
> Got a Coke to hand? OK. Reason for underfilling BGA's was that the first
> BGA boards we had were designed and assembled for us by a s/c. The first
> time we put them near any form of vibration, and it was gentle, they
> failed. Investigation followed, and I learned more about BGA's, ENIG boards
> and the poor manufacturing quality of whoever assembled the things.
>
> I had a couple more boards assembled myself, full of sheer terror, as you
> correctly guessed, of the ENIG and BGA horror stories I'd heard about, and
> heard about what Underfills were supposed to do. So I opted to use it,
> partly to fillin the air gap but mostly to add support to the solder joints
> against creep fatigue.
>
> I do know, though, that many high reliability appliactions do not underfill
> their BGA's, but they've maybe had more money, resources, time and
> experience to prove it isn't necessary with the processes they use. If I
> could be confident enough, I wouldn't use it either - it's one more process
> to take time and cost, right?
>
> Peter
>
> <ddhillma@rockwellco
> llins.com> To: [log in to unmask],
> DUNCAN Peter/Asst Prin
> Engr/ST Aero/ST Group@ST
> Domain
> 03/28/02 06:40 AM cc:
> Subject: Re: [TN]
> Conformal Coating BGA's
>
> Hi Peter! You have me confused (which is a normal state for me if I don't
> have a Coke in my hand). Collins has been using BGAs in avionics use
> environments, not underfilled, with acrylic conformal coating very
> successfully for a couple of years. What was the reason/rationale for using
> BGAs with underfill? Was is an issue with solder joint thermal cycle
> fatigue for a given use environment? Corrosion? Customer requirements?
> Residual flux issues? Sheer terror of the unknown?
>
> Dave Hillman
> Rockwell Collins
> [log in to unmask]
>
> [log in to unmask]@ipc.org> on 03/26/2002 07:48:52 PM
>
> Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
> to [log in to unmask]
>
> Sent by: TechNet <[log in to unmask]>
>
> To: [log in to unmask]
> cc:
>
> Subject: Re: [TN] Conformal Coating BGA's
>
> Hi, Bruce,
>
> We apply Humiseal acrylic coating to boards that fit your description and
> application precisely, except that we do underfill the BGA's with a
> reworkable epoxy, applied after assembly. The only exception was for the
> case of two boards destined for temperature cycling measurement, for which
> we couldn't get a supply of underfill material at the time. We substituted
> thinned Humiseal as the boards would not be flying, as we were concerned
> about entrapped air and also no suuport for the BGA joints. Humiseal is not
> a proper substitute for the specialised proper epoxy, but was sufficient
> for our particular purpose.
>
> The main issue as I see it would be moisture and other contaminants in the
> entrapped air, but I have no first hand data to help you decide if that
> causes problems or not.
>
> Good luck
>
> Peter
>
> "Misner,
> Bruce" To: [log in to unmask]
> <Bruce_Misner cc: (bcc: DUNCAN Peter/Asst
> Prin Engr/ST
> @ATK.COM> Aero/ST Group)
> Sent by: Subject: [TN] Conformal
> Coating BGA's
> TechNet
> <[log in to unmask]
> ORG>
>
> 03/20/02
> 03:48 AM
> Please
> respond to
> "TechNet
> E-Mail
> Forum.";
> Please
> respond to
> "Misner,
> Bruce"
>
> My question of the day: Is anyone conformal coating (not parylene) plastic
> BGA's on FR-4 in Hi-Rel applications (aircraft; -20 to +71C ambient
> operating temperature requirement) without benefit of an underfil? If yes,
> has entrapped air been a major issue? Anyone care to comment on this
> approach?
>
> Regards,
> Bruce Misner
>
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