Hi Carl, I'm a little late in replying to your question, but anyways... BGA voids occur because the flux vehicle becomes trapped in the liquid solder. Remember of course that the flux vehicle is intermixed with the solder powder, and when the powder becomes liquid, it does not become liquid all at the same time. In fact, it may take several seconds for a paste deposit to become fully liquid. In the time between the powder changing from solid to liquid, the flux must be driven from the center of the paste deposit to the outside. Since the liquid will fall to the bottom of the paste deposit, and it is more dense than the flux, the flux will rise above the liquid solder. Surface tension will try to minimize the surface area of the solder that is liquid, helping to close any "tunnels" of flux that may exist within the liquid and drive the flux to the outside of the paste deposit. However, if the tunnel becomes closed, there will not be any forces to "push" the flux (and now the trapped volatiles) in any particular direction except straight up towards the BGA ball. This is the reason "flux only" BGA processing never has voids. As you can imagine, there are several factors that can help or hinder the entrapment of flux. First is the viscosity of the "liquid" flux in the reflow oven. (The viscosity of the flux during the dwell time has nothing to do with the viscosity of the paste during printing.) Reflow profile can certainly have an affect with the viscosity of the flux in the oven, however, each flux vehicle has its own unique way of responding. Some will gum up the longer it is heated, some will dissipate a little more, but most stay pretty consistent. This makes some solder pastes worse at voiding than others. Also, keep in mind that for many flux vehicles, as the flux removes oxidation, it turns into a gummy sludge. This is why nitrogen helps reduce voids; less oxidation to remove, less gummy flux residue. Another factor is how fast your spike to reflow occurs. As the solder powder changes from solid to liquid, it draws heat away from the surrounding powder, requiring additional heat to be input into the system. The faster you put heat into the system, the faster the solder deposit can go completely liquid. However, this also allows less time for the flux to escape to the outside of the solder deposit. Of course, once the flux is entrapped, it may still volatize and cause a small void to become large. This is why extended dwell above reflow has been known to increase the size of the void, and move them from being positioned at the PCB pad level to the BGA side. Another factor is the distance the flux must travel to get out of the solder deposit. Yet another factor is the volume of flux that must be displaced. Those two factors are the "size of pad and amount of solder paste" mentioned by your colleagues. Honestly, reflow profiles and paste deposit sizes are minor in controlling voids. The biggest impact on voids is the flux vehicle itself. If you have the ability to change solder paste models, that is where you will find the biggest change in void size. Ryan Grant -----Original Message----- From: Carlr Ray [mailto:[log in to unmask]] Sent: Wednesday, July 02, 2003 9:19 AM To: [log in to unmask] Subject: [TN] BGA Viods Hello All, I have a question (more like a poll I guess), concerning voiding of BGA devices. I have PBGA 480 IO paremeter grid device and I am seeing lots of voiding. Per the x-ray I am well within the IPC spec of 35% but I am still having failures at ICT (OPENS). Once I reviewed the profile I noticed we were not hitting the dewell time for flux activiation recommended by the solder paste vender (min 60 seconds) and we were seeing an extended time above 183c. We have changed the profile but some of my companions feel that the size of the pad and amount of solder paste is effecting the voiding. I disagree with them and feel that not activiating the fluxes and extended time above liquidous is a major contibutor to the voids. So now my question, what impact does the pads have with the voiding of the spheres on the BGA? Carl Ray Sr. Manufacturing Engineer Huntsville, AL 35807 Phone: 256-882-4800 ext. 8845 Cell: 256-990-1990 --------------------------------------------------- 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 -----------------------------------------------------