Certainly underfills were first developed to help increase the reliability of devices where there is a significant TCE (aka CTE) mismatch between the package (a flip chip) and the substrate. These types of systems are filled premixed and frozen (-40 deg C) epoxies. The filler drives the CTE downward to closer match the midpoint between the silicon, FR4, and solder bumps. Since then things have gotten a bit more complicated. We now have silicon dies that are quite large and the requirements are a bit different, driving the CTE as low as possible may stop solder joint fatigue cracking, but then you may run into other issues (e.g. fillet cracking). In those cases a blend of properties is needed (a blend between the low CTE and low modulus - the filler that lowers the CTE typically raises the modulus). Now you add the CSP and BGA devices that need some enhanced reliability, but they have flexible interposers (substrates in the package), now CTE is definitely not the driver. These tend to need more increased reliability with respect to shock, drop, and vibrations, plus some extended thermal cycling. On top of this you add some "features", such as: fluxing (no-flow), reworkable, snap-cure, refrigerator storage. There are quite a number of choices. Knowing the size of the die or package, the gap between the package and the substrate, the pitch between the bumps, the bump and paste metallurgy (Sn63 or Pb-free), final reliability requirements (does it need to meet JEDEC, etc.), and knowing what "features" are needed or nice to have, will help in making the best product selection. Brian J. Toleno, Ph.D. Sr. Applications Chemist Loctite Electronics 15051 East Don Julian Rd Industry, CA 91746 Ph: 626-968-6511 x304 Cell: 626-215-0879 ********************************************************************** This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please notify the system manager. This footnote also confirms that this email message has been swept by MIMEsweeper for the presence of computer viruses. http://www.loctite.com/ ********************************************************************** --------------------------------------------------- Technet Mail List provided as a 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/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 -----------------------------------------------------