Herminia, Because the heel fillet is the only portion of the solder joint (typically) that is made to the finished (flat) portion of the component lead, it is the most critical. The toe and side fillets are formed to exposed copper. While it is desirable to have good toe and heel fillets, the heel is the most important. With that in mind, a decision was made to require a heel fillet at least half a lead thickness in height. Having sufficient amounts of solder paste certainly improves one's chances of obtaining a Class 3 fillet, but certainly does not gaurantee it. Just because you have proof that your solder thickness sufficient to form a good solder fillet isn't good enough. Solder does not always wet where it is supposed to, or this forum would never have existed. JEDEC standards exist for coplanarity of component leads. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Herminia Guevarra Sent: Wednesday, February 13, 2008 5:01 PM To: [log in to unmask] Subject: [TN] Minimum Heel Fillet Height Hi, folks, Can anybody please explain in details how the minimum heel fillet height criteria for Class 2 was computed and specified? F= solder thickness (G) + 50% lead thickness Can solder thickness be solder paste height prior reflow? What about if leads are not coplanar? What would be the spec for co planarity? How can they be measured? Need reply... Thanks, Herminia --------------------------------------------------- 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 -----------------------------------------------------