T sub G

Hello, Our PWB supplier has requested that a change in the laminate material to reduce the incidence of pad lifting. The standard laminate materials (Polyclad PCL-FR-226 with a glass transition temperature of 135 C) have greater z-axis thermal expansion than high Tg laminate materials (Polyclad PCL-FR-370 with a glass transition temperature of 180 C). The combination of standard laminate materials and plated nickel's lower ductility than plated copper creates a higher incidence of pad lifting. This condition is typically found when the boards are thermally shocked. Is there any downside to having a higher Tg? Thanks, Al Knudson

reply »

| Hello, | Our PWB supplier has requested that a change in the laminate material to reduce the incidence of pad lifting. The standard laminate materials (Polyclad PCL-FR-226 with a glass transition temperature of 135 C) have greater z-axis thermal expansion than high Tg laminate materials (Polyclad PCL-FR-370 with a glass transition temperature of 180 C). The combination of standard laminate materials and plated nickel's lower ductility than plated copper creates a higher incidence of pad lifting. This condition is typically found when the boards are thermally shocked. | | Is there any downside to having a higher Tg? | Thanks, | Al Knudson Al, Glass transition temperature (Tg or T sub g) is a transition point where epoxy begins melting (in the case of epoxy glass resin systems). The fact is this often is confused with the Z axis expansion rate. This factor, for epoxy and polyimide resin systems, is between 50 and 60 ppm. Therefore, the Tg of a material does not always mean pad lifting of delamination will be effected during thermal stress or shock cycles. Many modified epoxy resin systems have a 180 degree C. Tg, but their Z axis expansion rate stays the same, as above. Often, the culprit in through hole destruction, as hole wall cracking, is too little ductility. As far as pad lifting is concerned, too much temperature applied too quickly or over time can cause pad lifting. There is no laminate bond strength number that will continue to bond a pad to a board's surface when too much temperature is applied or the material, or relamination process is not up to the task. This goes on and on. For a better picture, talk with laminate suppliers with a knowledge of what your boards are being subjected to. Polyimide, as an example, has a preferred Tg of 250 degrees Tg but still expands at a rate indicated above. It also has lesser bond strength, concerning pad or metal foil to the bonding resin, providing more ability for pad lifting. This is not too complicated. It is important. You must address the issue (s) from the standpoint of what your boards must be subjected to. However, the limits are constant, depending on the variables as resin type and systems. For more conversation about this subject, I will be happy to assist, Earl Moon

reply »