In my experience a stiffener is the way to go. 1. If you don't use a stiffener you'll risk a lot of failures in the outer row of balls due to handing. With no stiffener any bending or shear loads are taken by only the outer row of balls. You've also got the problem of maintaining coplanarity between BGA and PWB pads during soldering. 2. Interesting point and I can see the logic. An insubstantial sliver of polyimide will subject the joints to little in the way of loading when subjected to temperature cycling. However If the particular package is qualified for mounting on a rigid PWB then you should be able to stiffen the polyimide with a piece of FR4 and achieve the same performance. Choice of material depends upon the application. However I would start by considering ~0.50mm thick FR4..... > Let's say you're mounting a BGA to a flex..... > > 1. Will the use of a local stiffener behind the BGA increase yields > through reflow or improve the solderabilty of the BGA to the flex by > preventing the substrate from warping? > > 2. What would be the effect of this stiffener on the BGA joint > reliability over thermal cycling, thermal soaking, etc? CTE differential > is > obviously a concern here. One Technetter that I already talked to said > that > in his experience the use of stiffeners for BGA's on flex decreased the > mean time to failure significantly. Does anyone else have experience > with > this? What type of stiffener do you recommend? > > After looking through the archives and IPC-7095, I wasn't able to find > anything that addressed this issue directly. Eric Christison Mechanical Engineer STMicrolectronics 33 Pinkhill Edinburgh EH12 7BF UK Tel 44 (0) 131 336 6165 Fax 44 (0) 131 336 6001 --------------------------------------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8d 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 delivery of Technet send the following message: SET Technet NOMAIL Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail 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 ---------------------------------------------------------------------------------