Solder balls

[Henrik Kronborg]

We're having some trouble with solderballs in our production. Does one of you guys have had trouble with this?
I believe it's because the chips ( the problem is mainly on chip-compenets ) are covering to much of the pads, but perhaps i'm wrong. I have just made an 2012 (0805 ) chip landpattern with the calculator, and there it looks like it will bring trouble. The pads are half covered.
How thick stencils do you use? Or is it an issue with our reflowprocess?

[Tom Frayda]

We had some trouble with this in the past. You are correct in that the problem is directly proportional to the amount of solder on the pads, particularly under the inside corners of the component metallization.

Our Manufacturing Engineers found it beneficial to reduce the stencil thickness from 8 mils to 6 mils and to have the stencil manufacturer change the shape of all 0805 and 1206 apertures to a "home plate" shape. This reduces the volume of solder located in the region where solder balls form.

Sort of resembles this:

[> <]


Keep in mind, this is something the stencil manufacturer can do and it does not require you to modify the paste mask apertures in your Gerber data.

There is a great article on the subject in the November 2001 issue of SMT magazine.

http://smt.pennnet.com/Articles/Article_Display.cfm?Section=Archives&Subsection=Display&ARTICLE_ID=125633

HTH


+Tom

[Tom]

Henrik,

One of the solutions is to have the Paste Mask Stencil Maker change the rectangle pad shapes and round all the corners. I've seen this done at very large (high volume) manufacturing facilities like Xerox Corporation. But I would always consult with an assembly shop that has experienced the same type of problem and how they solved it. Also, a thinner stencil will definitely help. When it comes to solder, sometimes less is better. Solder Balls are the result of excess solder.