STENCIL THICKNESS: After the squeegee has filled the apertures in the stencil, it gets lifted up and 'hopefully' all the paste leaves the apertures and stays on the pads of the board. The battle is between the forces of the 'suction' by the pad and the friction along the side walls of the aperture on the solder brick. Experiments have shown that for a typical laser-cut stencil the bottom area of the aperture has to be greater than 0.7 of the wall area. After applying a little geometry and math a simple rule of thumb can be derived: for round and square apertures the diameter (or width) has to be greater than 2.5 times the stencil thickness; for rectangles, where the length is several times the width, the width has to be greater than 1.5 times the stencil thickness. The above means that for your 5 mil stencil a square aperture should be at least 12.5 mil wide. So you are working with apertures that are close to the 'rule of thumb'. Given a good laser-cut stencil it is likely that you will be successful. But a stencil with questionably cut apertures may give you some trouble. To make life easier, go to larger apertures. You are not yet close to the other rule-of-thumb that says make the apertures not wider than 50% of the pitch. This means anything under 16 mil is ok, but with BGA-s you may want to go a little smaller anyway. To give yourself a little wider process window (fewer print errors) you can choose a finer mesh solder paste. Have fun, Ahne. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of bbarr Sent: Wednesday, May 29, 2002 08:40 To: [log in to unmask] Subject: [TN] FPBGA Area Ratio Question On a fine pitch BGA (.8mm pitch, 384 balls, .5mm ball diameter) I am using a .014" diameter land per TI's recommendation. I was thinking then of a .013" square stencil aperture with .003" radiused corners on a 5 mil plate. This seemed reasonable until I did an area ratio calculation per IPC-7525. The result was .592, which fails compared to the acceptable value of >.66. The aspect ratio is a solid 2.6, although IPC says I should not consider aspect ratio in this case, only area ratio. If I reduce down to a .012" square aperture I can get to an area ratio of .642. Still fails, but close. But, reducing the aperture 2 mils compared to the pad seems extreme and violates the recommendation in para. 3.2.2.4. So my question is, what do you guys and gals use for an aperture design on a component like this? Do you go by the IPC guidelines? Thanks. Bob ======================= Robert Barr Manufacturing Engineering Formation, Inc. Voice: 856-234-5020 x3035 Fax: 856-234-6679 ---------------------------------------------------------------------------- ----- 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 e-mail to [log in to unmask]: SET Technet NOMAIL 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/html/forum.htm for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------- ----- --------------------------------------------------------------------------------- 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 e-mail to [log in to unmask]: SET Technet NOMAIL 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/html/forum.htm for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------------