-----Original Message----- From: Jim Mathis [SMTP:[log in to unmask]] Sent: Monday, February 16, 1998 3:53 PM To: [log in to unmask] Subject: [TN] PCB warpage What causes PCBs to warp? How can we prevent this from happening? (Thanks to all responses in advance!) In general warpage is caused by unrelieved stresses built up in the fabrication process. The most common cause for these stresses is an imbalance of copper from layer to layer (i.e. a single plane layer on the third layer of a 4 layer PWB.) Other things that can contribute to this are an odd number of layers, and imbalanced stacking of dielectric between layers. To control this you need to do three things: 1) design PWBs with an even number of layers, 2) make the stack-up of the PWB symmetrical about the center of the boards thickness, and 3) balance the copper to etch from layer to layer. Following these guide lines should allow your fabricator to supply you with PWB that meet level C requirements for Bow and Twist as outlined in 5.2.5.1 of IPC-D-275. Jeffrey McGlaughlin [log in to unmask] Columbus Ohio ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ################################################################