Hi:
I have a document which I obsessively revise that contains my formulas for SMT lands. There is one formula that I can never settle on. It is for what I call the "land inner bound" (LIB) which is the distance from the origin on one axis to where the land begins. The land outer bound (LOB) I have no trouble with. The inner bound is tricky. I base the LIB and LOB on the lead and/or body dims and tolerances to compute the land size and location. I use my own formulas mainly because I hand solder many prototypes, and prefer my own land designs to IPC and manufacturers recommendations.
In order to come up with a logical argument for how to compute the LIB, I need to understand more about the manufacturing process for these packages.
I imagine that the "lead frame" is flat until just before or after the injection molding process, at which point it is slammed against some die which bends the leads into the wing shape, and simultaneously snips the excess length.
The main issue is this: If there is a die, then I expect the tolerances on the foot sizes to be rather independent of the tolerances of the plastic body dimensions. In this case, any formula for the LIB should be based solely on dims. and tolerances for the leads and feet.
Alternately there is a coupling of the dim. tolerances of the feet and the body dims. I think this is unlikely.
Also, I expect there is a tight coupling of the lead length and the foot length according to this manufacturing process. Ie., you would expect that if the lead length is toward its max. tolerance, that the foot length would be as well, and vice/versa. You would not expect that the foot length was at its max tol. and the the lead length was minimum.
However, if there was a correlation between body tolerances and the lead bending process, then the above paragraph might be incorrect.
Perhaps if someone can explain the chip packaging and lead forming process, we will all be better informed and I might be able to make the final revision of my LIB formula!
Thanks for comments.