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lousy wetting of film caps

#17332

lousy wetting of 0805 film caps | 23 August, 2001

So, we've already addressed the tombstoning on these guys by adopting the mfr's (Pana.) recommendations for pad size and spacing. Problem is, we have pretty bad wetting of some of these parts. The rest of the board we use them on looks great, nice and shiny (or as shiny as we expect to see with nc paste). I've monkeyed with the profile and only made everything else look worse, so back to square one. The wetting problem is fairly new, as far as I know, but consistent as of late.

Anyone else see this kind of problem with these parts? Obviously I'm concentrating on paste, profile, and part solderability as the 3 most likely causes, but has the layout been an issue for anyone? Based on my (occasionally suspect) math, the nominal pad exposure beyond the metallisation is only about .014", and under the part it's about .006". Seems like a smidge light to develop any kind of a fillet. Whatchya all think?

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Steve

#17334

lousy wetting of 0805 film caps | 23 August, 2001

You made the statement "The rest of the board we use them on looks great..." My question is, what's the difference between the assemblies? Mass? Solder? Oven?

Are the boards the same thickness, are other parts causing heat sinking, etc. The reflow may still be the answer. How many thermocouples do you use on the assembly? Maybe there is a differential across the board.

Steve

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#17336

lousy wetting of 0805 film caps | 23 August, 2001

Naw, the rest of the BOARD we use them on. Same assembly. Actually we use them on one other assembly and have the same problem. Same oven, same line, same paste, same problem. I've tried to get the production super. to move them to another line with a better printer to see if it's at all print related, but it's a huge setup for the other lines.

Stencil aps. reduced 10%, homeplate design. The board is really pretty low mass, nothing bigger than SOIC 8's, not particularly densely spaced, 15"x7" panels. The default profile I use for most jobs on that line (most boards on that line fit pretty much the same description) works better than the "ideal" profile I generated for it last night. Hmmmmm, could it be time to cal. the MOLE?

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#17340

lousy wetting of 0805 film caps | 23 August, 2001

Print paste on tempered glass, place the parts, and reflow. What's the story?

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Ryan Jennens

#17363

lousy wetting of 0805 film caps | 27 August, 2001

Steve-

We had the same trouble with 0805 Panasonic film caps. We had some tombstoning and open solder joints (non-wetting). We found two problems...like you, we changed our pad design to meet their recommendation. Also, they were able to supply us with reddesigned parts (new part numbers contained JX instead of JB). We ran a sample of fifty boards with no errors. WE have yet to do large volume testing, as the parts were just swapped out. Apparently, they say that they use a thin conductive epoxy fim to cover the copper end terminations before they finish with solder. This prevents the copper from oxidizing beneath a solder finish that may be too thin. The Panasonic engineer who came to our facility was actually able to show us where the solder finish was too thin when we showed him problem parts.

-Ryan Jennens

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#17395

lousy wetting of 0805 film caps | 28 August, 2001

Just talked to my component engineer. We use the B parts also, and apparently the B suffix indicates compatibility with wave or reflow applications, X is for reflow only. Since we only reflow these parts, sounds like that might be the best route for us as well. Nice tip Ryan, thanks.

Ya gotta wonder why they care how much oxidation there is on a part if the oxidation inhibitor isn't solderable, though. Am I missing something? Why don't they just make sure they get enough solderable metal on the flaky little *******s??

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