Bob, Those voids are too big. Since many of the voids are near the edges of the spheres, you may get some of them out will a little more time in reflow. (However, other voids will become larger when two or more small voids combine). Most voids will never come out of the BGA sphere because it has nowhere to go but up to the pads on the BGA side. As a general rule, voids are bad. The small voids in your X-ray photo have been shown by a Motorola study (SMI 96 Proceedings) to be better for joint reliability. But as Gabriela said, there is a limit to the size the void can be. The best explanation I have found for the source of voids is Dr Lee's flux exclusion rate theory. You can view his paper that was presented in SMT magazine (Jan 96) at Indium's web site. If you don't want to read the paper, the two sentence summary is: voids are caused by entrapped flux, and flux residue. (Note that entrapped flux will still volatize). Since paste does not transform from solidus to liquidus instantaneously, (much like melting ice cubes), the liquid solder will drive the flux and flux residue out of the solid solder powder until all of the solder powder is liquid. The less viscous the flux and residue is, the less likely it will get entrapped by the liquefying solder. If you have the luxury of using Nitrogen, that will help control voids immensely. This is because it reduces the amount of oxidation that occurs, thus reducing the amount of gummy residue that is generated. (Gummy residue is more viscous, so it does not move out of the paste as easily.) Certain paste will control voids as well. There is an aqueous paste that is advertised as reducing voids in BGA balls. (They advertise in SMT and Circuits Assembly). I have used this paste and it does work to control voids. As far as the reflow profile, I think it is still somewhat of a black art. (even with the flux exclusion rate theory.) You will just have to play around with it a bit. I have not found anyone who has explained the dynamics required to reduce BGA voids with the profile, and I haven't had time to try out my own theories. i.e.. reducing the time to reflow and slowing down the reflow spike. Good luck. (I would be interested in hearing what you end up doing). Ryan Grant > -----Original Message----- > From: Bob Perkins [SMTP:[log in to unmask]] > Sent: Monday, March 20, 2000 2:01 PM > To: [log in to unmask] > Subject: [TN] Voids in BGA Balls after reflow > > Hello Technetters > > Recently with the help of a high end piece of x-ray equipment we > have noticed voids in the spheres of the bga after reflow, "see > attachment-file". Has anyone seen this before, or has any idea how the > voids are created? Also does anyone know if this is a bad or good thing? > Does anyone have any recommendations to the profile? > > > Thanks > Bob Perkins > Automation Technician/Manufacturing Engineer > Aimtronics > [log in to unmask] > > > <<wboard6.jpg>> << File: wboard6.jpg >> ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. If you need assistance - contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ##############################################################