John Haddy wrote:
I would always ensure that there is some heel fillet as this is often the strongest external part of the joint. In many cases the toe of a lead can solder quite poorly (it is not tinned due having been cropped after the lead was plated). A sound heel filet prevents a weak solder joint lifting due to crack propagation from the thermal expansion of the package.I'm not familiar with any current work reflecting this. (Doesn't mean that I'm up to the minute with my reading though...)
IPC-SM-782a suggests (section 3.3.3.1) fillets in the range of:
Toe: 0.4mm - 0.6mm Heel: 0.0mm - 0.2mm Side: -0.02mm - 0.02mm
I've always been wary of excess heel fillet since, to my way of thinking, extra metal in this region must necessarily restrict the ability of a lead to flex in accommodation of thermal stresses. Note that this is my assumption rather than having a basis in any published work.
I note, though, that in the example land patterns in the back of IPC-SM-782a the QFP toe and heel fillets are generally about equal.
John Haddy
if you have no heal fillet then any expansion forces are concentrated at the pad/foot interface with the peel action concentrated by the small size of the interface normal to the direction of the force.
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