Problems with Pin-In-Paste

Phil
Welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers, pick and place.

Jim
Also known as mild-mannered consultants for ITM Consulting.

Phil
And probably known as a few other things too, but, you know, this is a family program. We're coming to you from high atop Mt. Rialto, Board Talk Headquarters in Durham, New Hampshire. And what area of question do we have today?

Jim
This is about reflow of through hole, or if you like, pin-in-paste, or pin-in-hole, or intrusive soldering. Whatever you wish to call this technique. And the question comes from Madolfo in Canada.

I have this USB four pin connector using a pin-in-paste process in one of our high runner boards." High runner means large volume manufacturing I assume. I just noticed it today after 1200 boards that the solder paste being pushed out of the four pins at the bottom side is dripping and accumulating inside our oven.

Mostly on the second and third zones whose temperatures are operating at 120 and 140C respectively in a 12 zone oven. The question is, Is this normal, or what is normal inside the oven for pin-in-paste process? The protrusions of the four pins at the bottom side is 1 millimeter or 40 mils.

Phil
The first question, "What's normal?" I don't know what's normal. But we won't get philosophical here. I think one of the key problems here is the protrusion of your leads below the board being .040" is probably excessive. If it's at all possible, it would be best to acquire the connectors or pre-trim them to a maximum of .030" to alleviate the dripping problem.

The other question to ask yourself, assuming you put up with the tidiness question here with regard to the dripping solder, "What do the joints look like?" Are you losing solder volume?

Jim
What is your hole fill ratio?

Phil
That's right. Are you filling the hole? What do the fillets look like? Knowing that, you don't necessarily have to have a positive fillet on either side. That's been proven. But are you getting antiquate joints? So, if you can live with the sloppiness factor I guess you could say, that's fine, but ideally you want to limit that protrusion to between .020" and .030" below the lower surface of the board.

Jim
But, do understand, it is perfectly normal to have some amount of dripping solder paste falling off the bottom of the leads, because when you print over the hole with a paste process, then insert your part, you're going to knock some small volume of paste onto the pin, and some of that is bound to fall off. You just need to make sure that it's not degrading the integrity of your final joints.

Phil
The only other thing I want to add is this applies to both reflow ovens and vapor phase. Beyond that, this is Phil Zarrow and Jim Hall.

Jim
Saying whatever you do, don't solder like my brother.

Phil
Right. And, again, regardless of whether you're using wave, reflow, vapor phase or hand soldering, don't solder like my brother either.

It all boils down to the rheology of the paste. Paste should be able to hold itself during the preheating process and be able to have good capillary effect to pull itself back to position if it is being dislodge during pin insertion. You should seek advise from your paste vendor's for the right paste chemistry.

Would one not be better off making the aperture like a pcb thermal, putting paste only close to the edges in the holes. Block off maybe 65% or more of the hole area for paste. Maybe the paste area could be made somewhat wider than pads where possible, to provide more solder to "pull in" when solder flows. Have not done this, just a suggestion!

I try to avoid printing in the holes due to the mess and loss of solder. The best solution is to increase your print thickness with a larger aperture or a step stencil. It will require clearance in your design but provides a cleaner and more consistent volume of paste. I use a string and dot through the aperture to block the hole. Make sure you touch the annular ring. You may have to open your solder mask a little. The solder will coalesce from a surprising distance. I have printed almost .3 from the hole and had it pull in. In extreme cases, we have used solder fortifications which are solder blocks formed into a chip size package and taped for pick and place. These can be expensive but will provide a lot of solder to the joints without the excess flux of deep prints.

For a pin-in-paste process you should control mainly:

- Diameter of the hole. It depends of the pin diameter. For example for a 0.64mm pin would be about 1.1mm

- Quantity of solder paste. A good aperture design is needed. For a 1.1 pin hole would be a good design about 2mm aperture diameter, and try to not exceed the thickness of the panel with the solder paste at the bottom of the panel

- Length of the pin. Not to exceed more than 1mm at the bottom of the panel

It's not uncommon to have paste dripping but PIHR is a simple process for tin/lead and lead free that works very well and produces joints that are just as reliable as any other reflow process. Its very cost effective to implement just have to look at the detail like any other NPI project My FREE text book Pin In Hole Intrusive Reflow Technology which can be downloaded at http://pihrtechnology.com/

This can also be caused by the flux in the solder paste. Different flux chemistries will drip more than others. You may want to talk to your paste supplier.

I would say that always when you use a pin in paste process, you could face issues related to soldering dripping, it would depend on the paste you use, the amount you print and the hole diameter. You pay a price for using this process, some times the price can be cheap, some times the price can be high, remember that pin in paste process is a kind of hybrid process. It is not a SMT process and not a PTH process. Try to avoid this kind of processes. I consider the pin in paste process as a patched process and patched things are not the best to choose.

You should (almost) never design a "pin-in-Paste" process with paste in the through hole. The volume of paste in the through hole is almost impossible to control (printer set-up, environmental conditions, paste type, hole diameter, etc.) and therefore the paste in the hole should not be used to determine the ultimate solder joint volume which is usually ~ 1/2 the volume of paste deposited.

Also if paste is dripping into the oven, you not only have a mess in the oven, but you potentially have a starved or open solder joint. For precision solder joint volume with the "Pin-in-Paste" process you should count on volume from "over-printing" around the through hole and adding a solid alloy preform to the paste deposit if required. Also, with significant solder volume requirements you will end-up with a tremendous amount of flux residue if you try to use paste alone.

For longer pins, it is preferrable to block or partially block the hole with the stencil. Use a solder preform from tape and reel packaging to make up any additional solder volume you need.

Pin length is critical in pin-in-paste processessing. A protrusion through the board of 1 mm is ideal. Much longer and the pin will push the paste too far out of the hole.