Pin-in-Paste Calculations

Phil
What's today's question, Jim?

Jim
Well, Phil, this is a pertinent question because it falls right into your field of expertise, published research, and all the other wonderful things about you. This is about reflow of pin-in-paste, reflow of through-hole, intrusive soldering, whatever you want to call it. This is where we're going to try to create a through-hold solder joint by reflow soldering it. And the question is one of the most pertinent.

We are about to implement pin-in-paste technology. How can we insure proper plated hole fill during the screen printing process? Will the process require a different solder paste then the one we were using for SMT assemblies?

So now our resident R.O.T. or reflow of through-hole expert, Phil Z.

Phil
This is definitely one of my favorite subjects, has been for a long time; and although I do have to say that one of the terminologies used, intrusive soldering always sounded kind of rude and pushy, it can be an essential process.

I did a study of this many, many years ago back around very early '90s when I was at GSS/ARRAY.

While I was doing my study at GSS/ARRAY Technologies, our good friend Joe Belmonte was hard at work doing the same thing at Motorola/Codex. And about that same time, I believe Bob Willis was also working on this process in his lab in an undisclosed location in England. Long story short, we all, independently came up with essentially the same methodology and similar formulas.

Well, the idea of getting enough solder into the gap between the lead and the inside barrel of the hole was through over-printing. I think a lot of people freak out when they hear overprint because they think: "Oh man, whenever I have slight misprint of my solder I get solder balls". But this is the way to do it.

Jim
Perhaps we should be really basic with our audience out there and say that overprinting means using an aperture in your stencil that is larger than the pad on the circuit board. In this case, larger than the outer diameter of the annular ring or land around the plated through-hole.

Phil
And being that it's a continuous mass, that solder paste will flow together, with the capillary action pulling it into the barrel of the via. There are calculations that are done with regard to thickness of your stencil, how much overprint, to get enough solder to fill that gap between the outer diameter of the component lead and the inner barrel/diameter of hole.

Jim
Is there any place that people can go to get more specific information?

Phil
As a matter of fact, if you go the ITM website, you will see in the papers and articles section the paper that I did a number of years ago with the formulas.

Jim
Formulas, exact calculations, select estimate calculations.

Phil
Yeah, hard to believe - something like that coming from me. Remarkably, these are same ones, coincidentally, that Joe Belmonte developed as well. I believe you go to Bob Willis' site, he also has paper pertaining to the same.

One very important thing is that if you look at IPC-610 (Workmanship Standards), even going back to Rev C as well as Rev D, you'll find that it's not necessary to have a big positive fillet on both sides to comprise an acceptable through-hole solder joint.

That big ol' positive-fillet-on-both-sides-of-the-PCB-joint is what we are used to with regard to wave and hand soldering. Reflow is a different animal. So even getting something that's actually flat with a surface, even slightly concave is usually more than enough strength - that joint is not going anywhere.

Jim
Well certainly that's really great information, Phil; and I'm sure RL will be very happy. Obviously this is a very complex subject. There are many more parameters that are involved that we don't have time to get into at this point. But write us your comments.

If you're interested more about the specifics of pin-in-paste or reflow of through-hole or intrusive soldering, send us a line.

I think it will be a good idea to add solder thieve at the bottom side pad, so instead of dripping in the reflow, paste will go towards the thieve.

I have experienced this in a high volume automotive application. We worked with our paste supplier (Heraeus)on the flux chemistry to significantly reduce the problem.

Another issue is if the lead protrusion is too long (1.5mm+) the paste on the end of the lead will tend to drip off as the flux becomes active and the viscosity changes at the temperatures you mentioned. We tried to keep the lead protrusion to < 1.0mm.

I have this USB 4-pin connectors using pin 'n paste process in one of our high runner boards. I just noticed it today (after 1200 boards) that the solder paste being pushed-out the 4 pins at the bottom side is dripping and accumulating inside the oven mostly on the 2nd to 3rd zone (temp is 120deg.c to 140 deg. c) of our 12-zone Vitronics oven.

Question is, is this normal? Or what is the normal inside the oven for pin n' paste process. The protrusion of the 4 pins at the bottom side is 1mm.