Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


tombstoning after wave?

Antonio

#2256

tombstoning after wave? | 8 December, 2000

Ok fellas, tell me how you'd fix this! I'm having melf connectors that are tombstoning in the wave, NOT the reflow, the wave. I've never seen this before. All other parameters are the same...solder, preheats, placements, the board, building humidity, flux, wave height, temps, and speed...every single thing is exactly the same, including pre-wave setups and ovens. But suddenly, the melfs on one side of the board (trailing edge) started standing up. I've temporarily fixed this by turning the board 180 degrees, the leading edge is now the trailing edge. This has fixed the problem. But, why did it occur in the first place? We've been running this particular assembly for literally years and it's never happened before. I've checked the components themselves and there is no contamination. The glue placement and size is good. Any thoughts?

reply »

#2257

Re: tombstoning after wave? | 8 December, 2000

Asymetry in heating or mechanical forces are the primary reasons for tombstoning regardless of the reflow method. [There's beaucoup of background in the fine SMTnet Archive on reflow tomstoning.]

Your situation is curious, but at least you know why it never happened before. I'd guess there is something about the design of the board that allows one side of the MELF to get hotter than the other.

Now in rereading your post, I'm getting more confused. These pups glued down, right? If so, that must be a poppa bear amount of force generated by this unbalance to snap the glue loose. Ya know I'm wondering if your gluing process wasn't defective this time around.

reply »

JohnW

#2258

Re: tombstoning after wave? | 11 December, 2000

I have to say I've never heard of anything tombstoning over wave, I've seen issues where the glue dot is too high or off to one side creating a poorly seated component that has too big a gap between pad and endcap(and hence an open), but never a tombstone. In term's of the component, MELF's....how are you putting them down? and what size are they (are yougetting the right amountof glue for the weight of the component?), if your using glue what's your dot pattern like?, double or single?...if you are gluing them then hat is the temperatur your getting to on the bottom side?, is it somehow allowing the epoxy to become elastic?

This is an intereting one, give us some more info....

I've done a few boards where turning it 180 deg has removed issues and problems, but unless you understand what it is your solved / cured it's not really a fix is it?..dave??

reply »

#2259

Re: tombstoning after wave? | 11 December, 2000

I'm with ya John.

I'm afraid I'm in brain lock on picturing the situation, mostly because of my bias against waving second side MELF. I know people do it, but we've never had good luck.

It would be good if we could have a clear process flow to help sort through this.

reply »

#2260

Re: tombstoning after wave? | 12 December, 2000

Another theory just developed in this very moment is that the glue did for what reason ever become "liquid" to a certain degree allowing gravity to do what gravity normally does but only to a certain point where this bubble-gum-like state stopped ( without loosing contact to the parts body) just holding the part now for let�s say may 45�.

a picture would be nice

Wolfgang

reply »

#2261

Re: tombstoning after wave? | 12 December, 2000

I�m with ya both.

I�m wondering why the hell those buggers didn�t just drop and started a new career mingling with the dross in the pot. If only a few for statistical reason and to confuse us all just showed this strange behavior and the majority decided to fall off the question would be why some of them behaved this strange.

But ....

Another theory just developed in this very moment is that the glue did for what reason ever become "liquid" to a certain degree allowing gravity to do what gravity normally does but only to a certain point where this bubble-gum-like state stopped ( without loosing contact to the parts body) just holding the part now for let�s say may 45�.

a picture would be nice

Wolfgang

reply »

jackofalltrades

#2262

Re: tombstoning after wave? | 13 December, 2000

We have seen this type of problem. After eliminating possibility after possibility we corrected the problem by throwing out the glue and starting fresh. It was not expired, just bad for whatever reason, we could not asertain what went wrong with it. Good Luck!

reply »

Reflow Oven

Used SMT machines