Hi, Paul, I have personally reflowed (literally, hands-on) thousands and thousands of BGAs, CSPs, and flip chips, all sizes and types, soldermask-defined and non-soldermask defined, without using solder paste. I have done this as part of developing rework processes, as part of a "normal" production assembly and reflow process, and as part of a laser ball attachment process using tacky flux only. I have done this on flex, on rigid PWBs, and rigid-flex. I have done it on medical products, military products, disc drive preamps, communications products, and automotive products. However, this should not be done with liquid flux, only a flux in paste form (commonly called tacky flux). I know also that there are tacky fluxes, and then there are tacky fluxes. Some are much, much better than others. I have never experienced a single premature failure. Not one. I know from firsthand experience that this is considered by many companies to be a standard, preferred process. Can you expand on why you had to rework a lot of BGAs soldered with this method? Or can you tell us why more solder paste would be required? More volume does not equal increased reliability, in fact the opposite is usually true. With the additional (unneeded) volume that the paste provides, there is also an increased tendency for bridging. Also, increasing the volume of solder simply gives you a larger and more rigid set of "cheese loaves" between your BGA substrate and the PWB. This does not improve your modulus of elasticity of the BGA solder joints; it reduces their ability to accommodate the delta CTE of the BGA substrate (interposer) and the PWB itself. I find using a thin veneer of tacky flux on the PWB pads improves ball wetting to the pads tremendously as opposed to using a brick of solder paste. Likewise when performing ball attachment to the pads on the BGA. I also find it much easier to clean away the tacky flux residues (at least the ones that I use) than it is to clean the flux residues left behind from the solder paste. The tacky flux can even spill over and be on the bare PWB, and I have firsthand experience that certain tacky fluxes actually protect the PWB from excessive heat, even from laser damage. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Edwards Sent: Wednesday, June 03, 2009 11:43 AM To: [log in to unmask] Subject: Re: [TN] BGA-113 package Steve, I would not recommend you only use flux to reflow the BGA... I have had to rework a lot of BGAs because someone thought that that would be a reliable process for rigid PCBs... Paul Paul Edwards Surface Art Engineering -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Kelly Sent: Wednesday, June 03, 2009 9:10 AM To: [log in to unmask] Subject: [TN] BGA-113 package Hi All, We have a rigid flex that has a BGA-113 (microstar BGA) package to be mounted to it. The routing in this area on the top layer leaves me with .0005 of soldermask clearance from the edge of the BGA pad and .003 distance from the edge of the soldermask to the nearest track- in other words I have .0035 of clearance from pad edge to next track assuming a perfect etch and perfect soldermask registration. Having rooted around Texas Instruments web site I believe that it may be possible to reflow this BGA without adding solder paste to the pad - just letting the ball collapse do the attachment. If my scenario is correct I could open up the soldermask and not have too many concerns about bridging between the BGA pads and the tracks. Any thoughts or comments would be appreciated. Thanks Steve Kelly Steve Kelly (PLEASE NOTE NEW E-MAIL ADDRESS) (416) 750-8433 (work) (416) 750-0016 (fax) (416) 577-8433 (cell) --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 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 ----------------------------------------------------- ======================================================================== ====== This email and any attachments thereto may contain private, confidential, and privileged material for the sole use of the intended recipient. Any review, copying, or distribution of this email (or any attachments thereto) by others is strictly prohibited. If you are not the intended recipient, please contact [log in to unmask] immediately and permanently delete the original and any copies of this email and any attachments thereto. ======================================================================== ====== --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 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 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 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 -----------------------------------------------------