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Soldering Relays Intrusively in Lead Free Process

Soldering Relays Intrusively in Lead Free Process
We use intrusive reflow soldering on relays using a lead process. We are now challenged to also solder relays intrusively in a lead-free process. Our design has relays, bunched very close with gaps of only 4 mm. Phil Zarrow and Jim Hall, The Assembly Brothers, share their experiences.
Board Talk



And welcome to Board Talk with Phil Zarrow and Jim Hall, the Assembly Brothers, who by day go as ITM Consulting, where they charge for their services. But here, we are here to give you free information.

Today’s process question has to do with one of our favorite subjects, intrusive reflow other otherwise known as reflow of through-hole, pin in paste. Whatever you want to call it. He’s got a problem. This is from P.K. P.K. says, good day currently we use intrusive reflow soldering on relays using a lead process. With the move to lead-free, we are now challenged to also solder relays intrusively in a lead-free process. Our designers have set a design with seven relays, bunched very close together with gaps of only 4 mm between relays.

They expect these to be soldered using a lead-free process. Early trials show that a gap of 4 mm between relays is not sufficient as the closeness is not allowing the heat down to solder the leads. If we bump up the heating zones too much then the smaller package types are going above 260 degrees C. Is there a formula or recommendation for minimum distances between relays for lead-free intrusive reflow? Trials also show that when we leave a gap between relays 25 mm that soldering is okay. The designers are not happy as extra space is taken up. If you could give me any further information on this topic, it would be appreciated.


This is probably one of the most classic problems that we have in our industry. Everybody wants to make their products small, so therefore they want to put the components close together. Makes sense. The problem is the closer that you put things together, the more difficult it is for assembly operation especially through-hole soldering. You have to get heat down in these joints to get, hopefully, wetting all the way to the top, at least 50 percent or 75 percent. When things are really close together, as P.K. has indicated, that is difficult.

You are trying to reflow the solder paste which is now under the body of the relay. This is pin in paste process, so we have printed solder paste. We put this relay on top of it, so that is shielding it. He talks about masks affecting it.

Being an oven designer, it is more like the physical blocking of the convection gas flows. We are all using these convection ovens. We are getting these little vertical impingment jets that are all over the whole board. When you put a relay on top of it, the air has to go around that. You put seven of these bunched together and the air has very little opportunity to get down there to the bottom of the board immediately adjacent to them so that it can heat and then conduct under the body of the component to reflow the paste.

The other heat is coming up from the bottom. Now you have an empty through hole with a lead in it, so there is limited conduction through that. As he is observing here, when they are too close together, you can’t get enough heat down there without overheating the other packages.

He talks about raising his own temperatures. The other possibility is to slow down the heating, slow down your belt speed, maybe reduce some of your temperatures to allow more time for that heat to conduct horizontally from the perimeter of the relays underneath to melt the solder paste. My recommendation, we always tell the designers, give us as much space as possible because we have to get these soldering operations done.

When you restrict us, it just makes it that much harder as P.K. is experiencing here. Unfortunately, I don’t have any formulas. The actual spacing that is going to be acceptable is going to be determined by the size of the relays, the board, how much copper is in the board and all of the other specific things to this location on this product.

If you want to find minimum spacing, you are going to have to experiment yourself to determine that. You have the boundaries already, you know that 4 mm is too small, 25 is okay. What in between there, I couldn’t speculate.


Also, we don’t know the actual mass, the size of the relays in question here. Jim, what do you think about in P.K.’s experimentation, boosting the lower temperatures to a certain degree?


Some convection ovens will let you do that. They’ll let you set the set point on your bottom cell in a vertical zone a little bit hotter than the top zone. Some ovens will stabilize that way, others won’t. You will have to experiment with your oven or talk to your oven manufacturer. We have had success with complex BGA packages setting the bottom temperature in the zone a little bit higher, 5 degrees maybe 8 degrees whatever your oven will tolerate, to drive the heat up through the bottom of the board.


Another example of design people having no idea about the various laws of physics that we have to contend with in the assembly area of the things. We can cite lots of other examples but a lot of them come down to physical and thermal dynamics. Their race for space, say in micro-passives like an 0201, it is a whole different story when we start talking about these relays here.


But if you think about all of the assembly operations, placing things very close together becomes a problem because of the tolerance. Placement and component sizes and everything else. People want to stack BTC packages edge to edge on a board. Good luck. Find me a placement machine that has 0 tolerance and has 100 percent absolute repeatability. It doesn’t exist.


It is funny that you bring that up. That was the original intent of the motivation of going with the BTCs, was the idea that they would be side by side. I can’t recall running into any of these applications where somebody is actually doing that. That just shows the difference, the economy between design and the actual application.

Gee P.K., I hope we answered your question. We wish you the best in that endeavor. I am sure we will get some input from our readers. As Jim mentioned, everything is so application specific. What can I say, happy profiling. On that note, regardless of what kind of components you are putting down and how well you are spacing them, when you go to solder them please don’t solder them like my brother.


Don’t solder like my brother.


PK does not mention how many zones his oven has. 8 zones may be tough to get a balance, 10 may be easier, and 12 would probably get them there with a much better Delta.
Alan Woodford, NeoTech

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