Let IPC-4761 be your 'guide'... -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of James Verrette Sent: Monday, November 06, 2006 2:06 PM To: [log in to unmask] Subject: [TN] Chemistry in Vias How long would it take for flux from a HASL finish process trapped in a via to cause a defect? The PCB is .050 thick with 8 mil vias. Regular FR4 material for SnPb solder process. 8 mil vias, 4 layer, 1/2 oz all layers with 1 oz plate up on outer layers. the board is fabricated to IPC Class 2. The board is all SMT components on the top side only. We had been tenting both sides of the board with primary Solder mask, but have been having problems with shorts due to in complete coverage of the via. We tried a Post HASL plug process, but have had problems with bumps under a 64 pin QFN package. The reason for the problem is that sometimes the vias are filled with HASL other times open, which makes the caps inconsistent causing assembly issues. So we are considering going back to tenting the bottom and pre-HASL capping the vias that are causing shorts. Only the vias under the QFN need to be fully covered. The PCB Quality Engineer at our CM tells us that tenting both sides prior to HASL would not be a big concern, but since 35 vias are in test points on the bottom, both sides on these vias can't be covered before HASL and covering only one side can create a situation where flux from the HASL process which contains corrosive stuff is trapped in the via by the HASL. The product has a shelf life of about 2 years with 3V applied (if it stays on the shelf for that long) and only needs to operate for 3 days. Do I really need to be concerned about this? When the PCBs go through a reflow oven with the HASL side of these vias facing down, wouldn't this reflow the HASL and allow gravity to work in our favor? The other option is to do 100% non-conductive fill and re-planerization after plating, but this a more expensive process. Also, changing board finishes would make the post finish cap process more predictable, but that is not an option right now. Jim Verrette Electrical Engineer --------------------------------------------------- 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 -----------------------------------------------------