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Component solderability problems TO263 package

Charles stringer

#3391

Component solderability problems TO263 package | 4 August, 2000

I am experiencing a problem when soldering down a TO263 package. The device has a large solderable tab (approx 8mm square)on the back and 5 leads. Sometimes the leads are not soldered to the board properly. I am using a metal mask with 9 "windows" of paste under the large tab to prevent paste scavanging. We use IR reflow. Any suggestions as to how to reduce the defect rate?

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#3392

Re: Component solderability problems TO263 package | 9 August, 2000

Hey Charles!!!

Where in earthly heaven did you get a TO263 with "large solderable tab (approx 8mm square) on the back and 5 leads"? All the TO263s that�ve reached-up and smacked me have been "TO220s" with a modified tab and three leads that are formed for surface mounting. Altho, ever so offin you cetch one style o� them TO263s with the middle lead all shrunk-up an stubbie-like ... but I digress.

Anywho, what do you mean by saying that "the leads are not soldered to the board properly"? Please help us understand the situation (ie, problem, frequency, location, process, paste type, reflow profile, board type, pad solderability preservative, board mask type, component lead finish, etc).

Tell us more about how your metal stencil "with 9 �windows� of paste under the large tab" that is aimed at preventing paste scavenging. How would it preventing paste scavenging? What is the tab scavenging solder from? Are some of the leads connected to the tab? Does it work? It must be doin sumpin!!! The "not soldered to the board properly" problem is with the leads. Right?

Assuming you have good solder, solderable components and boards, and a stable oven; the three most important things you can do are: profile, profile, profile.

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Charles

#3393

Re: Component solderability problems TO263 package | 10 August, 2000

It looks like a giant dpak device but with 5 leads. The problem is that on approx 3% of our boards one more pins are open circuit. It appears that the pins are not in contact with the pads. Board is HASL finish IR oven Paste Koki 953i. If you have a large opening in a metal foil then it is possible for the squeegee to deform into the opening and "scavange the printed paste out" leaving the correct thickness of solder at the edge of the print and almost none in the middle. Hence we break up the large opening into a series of smaller panes by and this ensures you get more paste down even though some of the pad is masked. The device in question is a voltage regulator and the connection on the back of the body is an electrical and thermal connection. Thanks to Wayne (who replied off newsgroup) it looks like we may have got to the bottom of the problem. We used the manufacturers reccomended footprint and this features a large solderable pad that covers the whole of the back of the device. However at the pin end of the device there is a small fillet of plastic. You've guessed it....... we have been printing solder paste in this area and it has been tilting the device up. After a few trials wiping the paste from this area prior to placement we had a visually better product, we have ordered a new screen with no solder paste printed in this area and long term we will modify the board as well.

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#3394

Re: Component solderability problems TO263 package | 10 August, 2000

Charles: Thank you for closure. It's nice to learn the answer.

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