Good point(s), Guy.
Slow and cautios pp, not a violent shooter.
Thermal damage...you mean high losses=heated caps...no, used as transient protection on DC lines 24V.
Cleaning..hardly..no water jets
Handling..not likely. Not stacked.
But you have an interesting point there, pressfit the large 6 row backplane connectors is done after soldering caps. Good point, because the force from the pressfit machine is enormous. Will check that.
Thanks
Inge
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: den 12 september 2008 03:26
To: [log in to unmask]
Subject: Re: [TN] exploding cercaps
Assembly method?
Mechanical damage from placement?
Thermal damage?
Cleaning damage, mechanical from high pressure water jets?
Handling damage? Stacked boards in the factory?
Are the press fit pins installed after the caps? Board flex!
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge
Sent: Thursday, September 11, 2008 2:34 PM
To: [log in to unmask]
Subject: [TN] exploding cercaps
Hi all,
I have solved numbers of cercap issues, some easy tasks, others very qualified. Now, in order to find the right track asap, I thought ; why not utilize TN expertise? Alzo, my question:
What is the failure mode most likely to be?
FR4 backplanes, 4 mm thick, lots of pressfit connectors. Transient protection by means of ceramic capacitors, size approx 8x6x1mm (LxWxT). SMT 60Sn. The catastrophic failures ALWAYS start at the end of the cap, from small microfumaroles to half ceramic body vaporized. From tiny spots of soot on the FR4 to something that reminds of what happens when you are careless with your gas torch welder! The board is warped, about 2 mm along 200 mm.
The board is torqued down flat with several bolts. May not seem to result in a lot of tension, but ceramic capacitors are very sensitive to axial/bend forces. I have cut out samples about 25x100 mm, populated with 4-8 caps, then polished them until only half thickness remains, then bent the samples slightly ('simulate' torque down). Out of a hundred, some caps developed a obvious crack CLOSE to the end metallisation (Ag;Ni). The cracks are always in parallel with the end metallisation and across whole width of cap body.
EDS on the solder joints gives that the solder surface is irregular and grainy, and contains small amounts of both Silver and Nickel. The end metallisation is unusually thick with a 60-80 um Silver layer. On outside of this is some 2-5 um Nickel. The meniscus is normal, which means very 'fat'
in my opinion (most of us don't realise yet, the benefit with meagre fillets). The failure occurs stochastically in field use. Forgot to mention that the BaTiO is beige and it's a 100k MLC. One and the same manufacturer.
My speculations are:
A. Mechanical force from curved/straightened board is root cause B Anomalous metallisation give local current issues which create hotspots and finally sparks etc C. Solder metallurgy faulty, or fatigued, giving too high serial resistance which gives local hotspots etc D. Shorts or delaminations not likely, as the failues are always at the end.
E. Bad Silver adhesion not very common
F. Nickel with bad pretinning = high resistivity or loss of galvanic contact not very likely either.
I know some of you have had many working days without sleep, because of puffing cercaps.
Thanks in advance
Inge
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 15.0 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 15.0 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 15.0
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
-----------------------------------------------------
|
