SMT lands formulas -- how are gull wings manufactured?

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.

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Mr.CRC
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Mr.CRC
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I don't understand what you mean by "land inner bound".

can you post an sketch or a drawing somewhere, please?
Reply to
John Fields

Thanks for the input John. I was afraid of this, the "a picture is worth 1000 words" problem. I'll try in ASCII, and if that doesn't help, I'll try to make a drawing when I have some time to play with web servers and all that...

Lets say the package center is at the left edge of the screen in a top view:

BBB represents the plastic body LLL represents the lead FFF represents the foot part of the lead Dashes, pipes, and pluses represent the land.

+---origin | v

BBBBBBBB BBBBBBBB +------+ BBBBBBBBLLLLLFFF | BBBBBBBBLLLLLFFF | BBBBBBBB +------+ BBBBBBBB ^ ^ | | LIB--------+ | | LOB---------------+

Hopefully this makes things clear.

The 1000 words problem again. I'm not sure what you mean.

Here's a glimpse of my reasoning for the present formulae:

Note that one of the main difficulties with calculating the LIB is that manufacturers aren't consistent about specifying the foot length. Some give only min or max values, and no nominal. Some have a huge difference between min and max, and an unspecified nominal.

Here's the LOB formula. This one is easy:

--------------------------------------------------------------------

  1. Determine land outer bound relative to the origin from the greater of:

(max lead length from org.) + 0.016~0.020" [0.40~0.51mm] OR (max lead length from org.) + (max lead thickness)

The consideration for land outer bound is simply to account for both the max lead length plus some room to apply a soldering tip for manual soldering, or for a minor fillet to form for reflow. For large lead thicknesses such as on power packages, then the extension beyond the max lead length is dictated by adequate fillet forming, thus the formula switches over to being based on the lead thickness itself.

--------------------------------------------------------------------

The LIB is the hard part:

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  1. Determine land inner boundary (LIB) from the greater of:

(nominal lead length from org.)-(nom foot length)- (nom lead thickness) OR (min body length from org.)-(nom lead thickness)

Justification:

There are two worst case possibilities for the LIB: A. min lead L - max foot L B. min body L (ie, lead is crushed against a minimally sized body)

#A is highly unlikely because it is unlikely that lead L and foot L are uncorrelated. Ie., if you have a short lead L, you probably also have a shorter than maximum foot L. The problem is that we don't know by how much to assume that the foot L is less than max. if the lead L is small.

Thus, we could wave hands and say it's probably a safe bet to modify the formula to: min lead Lo - nom foot L, because this is still likely to produce a LIB that isn't too far out for any likely tolerances. This is justified as well because this LIB is certain to be closer to the origin than the calculation resulting from purely nominal values.

#B is also highly unlikely because I doubt the lead forming operation allows for the lead to be pressed directly against the package, especially in the case where the body L is minimum. I think the lead forming die sets an inner bound for the foot bending corner distance from origin, and the body tolerance fluctuates independently of this due to injection molding variances. If this reasoning is correct, then the body dims really shouldn't factor into the formulas at all, except for a sanity check in some strange case where a value was computed less than the min body L. In fact, this would argue that a value less than the max body L would even be suspect!

Unless of course if my mental model of the process is incorrect.

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Thanks for comments.

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Mr.CRC
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Mr.CRC

I don't think that dim matters much.

John

Reply to
John Larkin

Actually, it might; the gull-wing at the inner contact point is curved upward, and the meniscus when solder is wet will pull upward on the pad at that point. If the force is large enough, the metal will lift from the PCB surface (and the scaling to ever-smaller pins INCREASES the force at the meniscus just as the surface area of adhesion is small).

Reply to
whit3rd

.PIN -->| |>> Also, I expect there is a tight coupling of the lead length and the

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I don't see where there's a problem as long as you make the length of
the pads longer than any combination of long and short lead length and
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Reply to
John Fields

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