Arnaud,
We probably did not reply because we don't know the details. How thick is the PWB, and how far apart are the two vias? I did not understand what you meant by "strapping" in your earlier post, but now I think you mean jumpering? All kinds of potential issues come to mind. Is the CCA conformal coated? Do you have a picture Steve can post?
It's hard to tell a PCB/PCBA Technology Expert what he should do for modifications on his own flight control assembly if it is intended to be flightworthy.
If it was mine, I would bring it to a contract house with a good laser soldering system, along with a scrap board. The jumpers can be pre-tinned with extra solder and then laser soldered to the vias. This process precludes sending the BGA solder balls into reflow. Use the scrap board to develop the process and perhaps microsection it to qualify it, then perform the process on the flightworthy CCA and x-ray it. Re-inspect and re-test the CCA completely when finished. Coat the entire jumper with a good two part epoxy after inspection, along with a witness sample cured in the same oven at the same time, and test the hardness with a Durometer and record it on a mix record. Perform a fit analysis to ensure the finished jumper will not cause any fit issues. Make sure the round jumper wire will carry the current required by the original design, or by the modification. Refer to the crossover table in IPC 7721 for the current carrying capacity of flat copper traces and different gauge wires. Make sure you have supporting documentation from the customer in the form of an ECN or waiver. Perform ESS test and inspection to make sure there are no CTE issues that could weaken the jumper solder joints.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of GRIVON Arnaud
Sent: Wednesday, June 29, 2011 10:20 AM
To: [log in to unmask]
Subject: Re: [TN] [COM] Strapping Adjacent BGA Vias
TechNetters,
It looks like my earlier query on the best practice to use for strapping two adjacent vias at the bottom of a BGA has been partcularly unsuccessful with no reply so far, which is quite unusual for the TechNet forum...
This message is only a reminder or a second chance...
Thanks for sharing your views on this topic.
Best regards,
Arnaud Grivon
PCB/PCBA Technology Expert
Thales Corporate Services - EPM
18, avenue du Maréchal Juin
Bât. C - Bureau C2-135
92366 Meudon-la-Forêt Cedex
France
Tél : (+33) (0)1 70 28 23 88
-----Message d'origine-----
De : TechNet [mailto:[log in to unmask]] De la part de GRIVON Arnaud
Envoyé : mercredi 22 juin 2011 17:46
À : [log in to unmask]
Objet : [TN] [COM] Strapping Adjacent BGA Vias
Dear all,
What would the best repair process to use for strapping two adjacent vias of a 1mm-pitch BGA?
I did not find this situation in the IPC-7711/7721 standard.
Soldering a jumper wire in the two vias at the BGA bottom side looks risky because of the possibility of partially reflowing the BGA joints, but this may not be a real concern for a small wire.
Soldering a chip could be another option, but I am not sure this is an acceptable practice.
Insights welcome. Assume worst case, i.e. requirements to ensure high-reliability and harsh environment.
Thanks in advance.
A. Grivon
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