Ask Helena & Leo

Question: We install D-Paks on a number of boards (8.3.14 in IPC-A-610). These parts have a tendency to float during reflow. We have done quite a bit of process work to minimize the floating, but parts do still occasionally float and are difficult to rework because of the thermal plane. Where we get into trouble is when the part floats straight back and the heel of the gull wing leads are no longer on pad. All other criteria meet IPC specifications(adequate side joint length, not violating minimum electrical clearance, etc.). The IPC does not stipulate that the heel needs to be on pad. I checked the NASA specs, and it does require the heel to be on pad. This could come up as a question for a number of leaded SMT parts. Has the IPC ever considered adding heel overhang as a criteria?

Answer: I’ve had this question asked many times and the question is always about the absence of the heel fillets.

When using IPC 610 documents, especially on Page 8-94 Section 8.3.14, the criteria for heel fillets is the same as the 8.3.5 requirements, so by using these two sections for the acceptability requirements, there is no way the condition you describe is acceptable. Once it is decided by the appropriate personnel, the reject disposition can be to rework and realign the component on the pads, however; with the size of the pads and the amount of heat which will be required to reflow the component, the decision could be “use as is”. The decision must be based upon the reliability of the component and solder joint design, which in this case is a lap solder joint, to determine the condition in relation to its end product application.

I believe the problem is the component footprint being used could be wrong for the particular component. The spacing between the thermal pad and the pad for the leads is spaced too far apart. What happens is as the solder melts, the larger volume of solder beneath the component has more surface energy, thereby causing surface tension where it then pulls the component towards the larger mass of solder and pulls the leads off the pads, resulting in no heel fillets. The component lead land areas need to be longer to accept this movement of the component on the large thermal pad.

Check the following website for information on footprint design:

TR0144 IPC Batch Footprint Generator Reference

It provides some information on a way to design the component pad library to eliminate this condition. Other information on pad design can be found in IPC-7351B, “Generic Requirements for Surface Mount Design and Land Pattern Standard”, which can be ordered directly from us.