Hi Ioan, What we have found is the FR-4 stiffener is a) often not flat enough so when you reflow the BGA you can have issues b) being so thin it does not provide enough support in assembly and/or end use of the product. Often we wind up using stainless steel stiffeners and we pick a stainless steel type which considers all the CTE's in the build plus the BGA. Usually the BGA rules because flex being so thin the thermal mass is not the overriding factor. We often have the stainless steel stiffeners built by wire EDM to hold a very tight flatness which is obviously not inexpensive. You also may have to consider the CTE's of the adhesive used to bond the stiffener to the flex- sometimes we use a polyimide, sometimes an acrylic sometimes even a lo-flow pre-preg(polyimide or FR-4). Finally the new IPC-6013 specification tightened up the stiffener requirements a lot. Before a stiffener was a mechanical support only- now there are all kinds of requirements for voids etc. How one determines this for a stiffener you can't see through I have no idea so for my company we wrote most of our critical customers and took exception to 3.3.2.13 or we sample - but doing a x-section in one area of a stiffener quite frankly tells me zilch and how we apply this spec to stiffeners put on with pressure sensitive adhesives is beyond me ( forgive the rant on the last part). So in summary if we use FR-4 the BGA is usually small but we will probably wind up doing some kind of under fill or corner fill unless in the final package everything is well supported, and for high end applications we use metal. Good luck. Regards Steve Kelly -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Ioan Tempea Sent: February-04-10 9:21 AM To: [log in to unmask] Subject: Re: [TN] Laser soldering of BGA Technos, There are 2 main ideas I can gather from your answers: 1. Why do I bother with laser when I could reflow? Well, as I said, reflow seems to warp either the component or the PCB and having no idea about laser soldering myself, I thought it would be a good idea to inquire. Since it seems to have no benefit, I believe I better get back to reflow. But not before asking another one: the BGA is soldered onto flex that is backed by a 12 mil rigid something; are there lasers that could solder from the bottom, through the substrate, rather than through the part, thus minimizing the heat seen by the assembly? 2. Steve K, could you please elaborate regarding the metal stiffener vs. FR-4. To my surprise, it seems that brass stiffeners yield better results than FR-4 indeed; shouldn't they give a bigger CTE mismatch? Thanks, Ioan Tempea, ing. Ingénieur Principal de Fabrication / Senior Manufacturing Engineer T | 450.967.7100 ext.244 E | [log in to unmask] W | www.digico.cc N'imprimer que si nécessaire - Print only if you must --------------------------------------------------- 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 -----------------------------------------------------