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When To Use Adhesive To Bond SMT Components



When To Use Adhesive To Bond SMT Components
When should we use adhesive to bond SMT components to the bottom-side of a double-sided PWA before going through reflow soldering?
Board Talk

Transcript


Phil
And welcome to Board Talk with Jim Hall and Phil Zarrow of ITM Consulting, the Assembly Brothers pick and place. Today we are coming to you from the ITM reflow laboratory and information center.

But we are here to talk about electronic assembly, materials, equipment, components, practices and procedures, among other things. I think we have an interesting question today. Don't we, Jim?

Jim

Yes, we do. All of our questions are interesting. Because we are interesting guys. This comes from D.M.

When should we use adhesive to bond SMT components to the bottom-side of a double-sided PWA before going through reflow soldering, component size, weight, etc. First off, I want to acknowledge that D.M. uses the same terminology as I do.

He describes the bottom side of this double-sided assembly as the one that ends up on the bottom during the second reflow. The one which is reflowed first.

We all know there is no standard terminology, unfortunately. So we have to be really careful how we describe this. But Phil, you are the world's expert in components falling off.

Phil
What do you mean by that, Jim? You know, it is interesting a number of us over the years have done process development work with intrusive reflow. Somehow part of that led into reflowing parts and how it is related.

A number of us did various experiments over the years. What kind of component will fall off when it is being re-reflowed on the other side.

Bob Willis has done work. I have done it, Joe Belmonte, a lot of other people.

There was a fellow, whose name slips my mind, who originally did this work at Motorola a long time ago. He derived a formula. Based on the numbers I have done, and again this is something, try this at home.

Basically, we calculate the surface area of the lead of the component or the ball to the area when it is collapsed that is in contact with the solder paste. In other words, tangent.

And for each of the inter-connects, leads, balls, whatever. Or in the case of a QFN, we haven't done that experiment yet. Excuse me, terminated leadless components.

I have to stay in good graces with Jim. Always use the right terminology.

Jim
Very good, Phil.

Phil
Yeah, I'm trying. The idea is you calculate, and you may need the help of your design or component people with this. For each of the interconnects what the actual contact area, the surface area, is. You multiply that by the number of interconnects that you actually have.

Then you divide that into the weight of the component. Now the way I remember, if you have anything less than or equal to 44 grams per square inch the part will not fall off. You know what that translates to.

In case you just want to take a shortcut and not bother. That is almost every component these days.

Unless you have something really heavy duty ceramic, like a big ceramic BGA or some leaded ceramic part, or a big heavy duty ceramic capacitor. I would do the calculations.

By the way on that capacitor you have two surface areas, one for each of the connections. Generally, we found most of these parts won't fall off.

Again fine-pitch, even more so the more leads, of course there is more interface area. The other thing to be sure of is that you're not interrupting the process with any external forces. For example, make sure that your reflow oven conveyor is well-maintained.

It is smooth and lubricated. That there is no rumble or vibration introduced.
v That could help coax a part hanging on there on the bottom-side. Jim, anything to add to that.

Jim
Well, I sort of disagree with you a little bit on your ceramic BGA. Ceramic BGAs tend to have a lot of balls on them. Although the component is heavy, you typically have a lot of area to hold them on.

My way thinking is, big components with only a couple of leads. And you hit the biggest one, capacitors. They tend to be big and heavy, and only have two leads on them.

Switches, connectors, other things that have heavy bodies but not too many leads. Those are the things to look for. Because you don't want to put on any more glue than you have to.

If you have to repair something that increases the degree of difficulty of removing the defective component.

Phil
Right. Plus, the whole gluing is another process step and material. You want to streamline as much as possible.

Jim
But, I am under the impression that there are reliable adhesives that you can put on and cure during your reflow cycle of your first side, of your bottom-side when you are doing the first reflow. Would you agree with that, my brother?

Phil
Yes, I would. We are in agreement. Well, very good.

Thank you for listening to Board Talk. And remember 9 out of 10 Board Talk listeners are current, the other one just impedes.

And when you are reflowing, whether or not you are using glue to attach components, or for life-enhancing effect, whatever you do, please don't solder like my brother.

Jim
And don't solder like my brother.

Comments

Question what is the best way to keep from components from being knocked off during assembly. I'm assembling a watch, very tight spaces. Underfill? Glue attach (a dot on ea corner?, or conformal coat process, like jetting.
Mike Mariscal
We have noticed that applying adhesives to the underside of components on the bottom side (before first reflow) has resulted in the adhesive impeding on the formation of a target solder joint. The adhesive used is designed to cure before reflow temperature, and it has a "expansion ratio" of up to 10% in diameter. Because of this, it keeps the component on top of the solder thus preventing it from "bedding" itself flush with the PCB. We have changed the application of the adhesive to AFTER the first reflow, and applied it to the side of the component.

What are your thoughts on where and at which process point to apply the adhesive?
Stephan Smeda, Cobham Satcom Cape Town

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