On Thu, 10 May 2001 07:59:58 -0500, Jason Gregory <[log in to unmask]> wrote: >OK, guys, here's one. >Why is it so important for local fiducials to be placed near diagonal corners of BGA's. The keyword here is "SO". I've performed studies in the past detailing the natural tendency of BGA's to self-align during reflow. It's even lead to me to work on a patent concerning a BGA rework preform. What I'm wondering is....ssop's, tssops, qfp's, etc. have greater impact on placement quality when misplaced. And they do not self-align!!! I do, however, understand that BGA's balls are not easily seen once placed (you can only see the edges). But my point that I'm getting to is.....wouldn't you want local fiducials more at the parts that will not "self-correct" themselves during reflow than at the parts are more forgiving? Just wondering. > > >Jason Gregory >Software Specialist - NPI Group >SCI Systems/Plant 2 >13000 S. Memorial Pkwy. >Huntsville, AL. 35803 >(256) 882-4107 x3728 >[log in to unmask] > >--------------------------------------------------------------------------- ------ >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 >--------------------------------------------------------------------------- ------ Jason, Global and local fiducials are a requirement of the machines that place them. You need to design/layout fiducials for ALL the machinery in all the locations and countries that your PCB will be loaded from prototype to full production. Global fiducials will have a error tolerance per length so any distance over 12 inches should have another global fiducial inserted between them including fiducials that are diagonally opposite. Global fiducials are also used on the stencil for solder printing. On Fuji machines a fiducial is good for 2 inches in the X and Y direction so every 4 inches is recommended for local-to-local-to global fiducials. Chip placement machines do not refer to local fiducials as often as IC placing machines. But Fuji does not make all placement machines. Every time a placement machine resets itself it is to a fiducial and if it has to go to the global ones then it is sometimes a longer distance than to a local one. The convenience of giving every fine pitch part and/or BGA its own fiducials is in the layout library hence no thinking and you cannot forget to do it. It is definitely not necessary to have two fiducials diagonally opposite for every part with modern equipment butn we all don't have modern equipment. Dorothy Lush Manufacturing Engineer --------------------------------------------------------------------------------- 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 ---------------------------------------------------------------------------------