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? 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