The one coating material that works well as an underfill is SR.
In a previous life we used to coat most of our boards with SR coating
and we made sure there were no bubbles since some of the boards were
used in airplanes. A bubble under the BGA or QFP was a potential
disaster due to the pressure difference as the airplane gained altitude.
One factor that may help in avoiding bridging the under the component
gap is the viscosity of the coating. Spraying with under 20 second Zhan
cup #2 coating viscosity may do the trick.
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Fox, Ian
Sent: Wednesday, July 20, 2005 7:13 AM
To: [log in to unmask]
Subject: Re: [TN] Coating under BGA's
Many thanks for your comments Graham. Parylene was in the back of my
mind.
Up 'till recently (intro of a PVA spray machine) we've always dipped and
manually sprayed so PGA's have been adequately coated underneath. We
probably therefore don't have the precedent you do. I didn't want to
revert to an underfill unless I absolutely had to. Bridging between the
underside of the BGA and board is exactly what I want to avoid as the
modulus of the standard acrylic increases greatly below about 10 degC.
Regards
Ian
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Graham Collins
Sent: 20 July 2005 11:49
To: [log in to unmask]
Subject: Re: [TN] Coating under BGA's
Hi Ian
In our applications we have used one of three options depending on what
the product is and where it is going:
1) underfill the BGA with a purpose made material, not with conformal
coating.
2) parylene coat the assembly (which, by virtue of it being a vacuum
deposition process, will coat under the BGA)
3) spray coating on the board and accept that the BGA underside is
shadowed.
#3 isn't without precedent, we don't typically worry much about old
style boards with pin grid array components...
I would not underfill a BGA with a regular coating material. This has
been discussed in the past on TechNet, one of the concerns is the
expansion with temperature putting stress on the joints.
Good luck!
regards,
Graham Collins
Process Engineer,
L-3 Communications Electronic Systems Inc.
Halifax
(902) 873-2000 ext 6215
>>> [log in to unmask] 07/19/05 01:23PM >>>
Fellow techies, a BGA related question. We're just starting to design
BGA's into some new designs which are class 3 high rel assemblies and I
am asking myself questions about the ability to adequately conformally
coat under the packages. Our standard coating is a solvent based acrylic
and we are using a 456PBGA device and a 429CBGA device, both on 1.27mm
pitch, as the maximum sizes so far and I'd be interested to hear what
other class 3 builders have experienced when applying the coating.
Regards
Ian Fox
Goodrich ECS
Birmingham
UK
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e To
unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or
(re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing
per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
Search the archives of previous posts at:
http://listserv.ipc.org/archives Please visit IPC web site
http://www.ipc.org/contentpage.asp?Pageid=4.3.16
for additional information, or contact Keach Sasamori at [log in to unmask]
or 847-615-7100 ext.2815
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e To
unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or
(re-start) delivery of Technet send e-mail to [log in to unmask]: SET
Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the
posts: send e-mail to [log in to unmask]: SET Technet Digest Search the
archives of previous posts at: http://listserv.ipc.org/archives Please
visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for
additional information, or contact Keach Sasamori at [log in to unmask] or
847-615-7100
ext.2815
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e To
unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or
(re-start) delivery of Technet send e-mail to [log in to unmask]: SET
Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the
posts: send e-mail to [log in to unmask]: SET Technet Digest Search the
archives of previous posts at: http://listserv.ipc.org/archives Please
visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for
additional information, or contact Keach Sasamori at [log in to unmask] or
847-615-7100 ext.2815
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
|
