A question on the PCB Land Patterns for the TM4C1290CPDT. Being a TQFP part and also being there is almost NO documentation on the web concerning TQFP pad dimensioning - a ton on the QFN! The little info I found doesn't make sense to me. This part in our design is a .4MM pitch 128 pin part. What does make sense to me is to dimension the PCB pads to guard against the part moving during reflow. In other words, the only info I've seen on an all equivalent .4mm PQFP 128 pin part (from Microchip) says to dimension the lead length of the pads to 1.45MM, with the foot length of the part (the part that actually contacts the PCB) being .6mm nominal. This would allow the part to move .85MM in any direction, which is much larger than the pad to pad spacing creating shorts!
Is there something I'm missing about this? We all know that parts move during reflow. Why would I give the opportunity to a part to move that much as opposed to limiting the pad length to say approx. 5mil - 9mil clearance ?
I've looked on T.I.'s web for land pattern specifically on this TQFP part and haven't found it.
Thanks Much.
Dan Ross
