One thing we just completed was a pilot where we put apertures in our topside SMT stencil and pasted over all vias to prevent Conformal Coat leaking through the board. We opened the apertures +10% and got good solder fill this also prevented corrosion or having trapped mask inside filled vias possible "volcano" during reflow. Dave Chapman Manufacturing Engineer Circuit Service Inc. -----Original Message----- From: Willie Duersch [mailto:[log in to unmask]] Sent: Friday, September 27, 2002 2:01 PM To: [log in to unmask] Subject: [TN] PCB design with mask over vias. Hello Peers: We are a CM who recently fabricated a PCBA which had the vias covered by solder mask film design on the PCB. This is not the first time we have seen this done in PCBA. However, in this example, the PCBA ended up inside a refrigerator. This customer adds few notes with the PCB and nothing was specified associated with the fab on plugging or capping of the vias. The PCBA did not require wave solder. Aqueous clean paste was used and PCBA washed after SMT. The PCBA was supplied to the customer who then selectively applied Conformal coating over the top side components on the board with a brush, but did not coat other through holes or vias on the component side or any area of the back side. Over a period of time, The vias on the back side showed evidence of corrosion and subsequent cross section analysis showed copper corrosion from the pad part way into the barrel even under soldermask. The cross section also showed that the LPI solder mask did not plug and did have pin holes in the mask with bare copper in the air pockets that occur during LPI screen deposition. 1. We feel the customer is responsible to specify the PCB and PCBA performance criteria. Comments? 2. Is there an IPC document that covers solder mask covered vias and guidelines associated with the practice? If not, why not? 3. Is it the PCB shop or the CM who should question, and if not specified...should either question the practice? 4. What are the issues related to designing with solder mask film design covered vias? In this case, the vias were .010-.014 diameter and space constraints were not an issue. There were other holes on the PCB that were larger diameter and were left open to coat with solder from the HASL operation...and not coated with conformal coat. 5. What would you tell a customer to correct the issue? There are some obvious issues and answers and I have had 12 years of experience with PCB's. However, I really would like to have some comments from my peers. Best regards, Willie ---------------------------------------------------------------------------- ----- Technet Mail List provided as a free 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/html/forum.htm for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------- ----- --------------------------------------------------------------------------------- Technet Mail List provided as a free 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/html/forum.htm for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------------