Here are some of the things that I have tried to reduce the tombstoning:
-I have run boards through the oven and rotated them 180° and run them again, and components continue to tombstone in the same direction with respect to the board.
-I have parts oriented both perpendicular to travel and parallel to travel, both of which exhibit tombstoning.
-I have looked at component placement, and some mis-centering occurs in all directions.
-I have increased placement pressure of the components to get good depression into the paste.
-Solder paste appears to be well aligned with the pads.
-I have tried running reflow without Nitrogen and it helped, although the joints looked significantly different.
I have not changed pad geometry yet (I believe I have a combination of too big a pad and too much paste) but that should not affect direction of tombstones, right? Obviously I will be happy if I can get rid of tombstones by changing one thing instead of determining all of the factors, but curiosity has taken control...
Ben
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dehoyos, Ramon
Sent: Wednesday, June 08, 2005 9:43 AM
To: [log in to unmask]
Subject: Re: [TN] mystery tombstone cause
Hi Ben:
Two of suggestions that have been made in the forum have been less solder paste and match pad size. Solder pulls the part over to one side when it melts one side faster than the other side due to surface tension. By placing less paste, the pull is less. Another recommendation was rounded far corner pads. Finally, solder mask between pads should be of less height than pads. If solder mask is taller than the pads, one end of the part is lifted from that pad, which will make it easier to tombstone.
Regards,
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Graham Collins
Sent: Wednesday, June 08, 2005 8:52 AM
To: [log in to unmask]
Subject: Re: [TN] mystery tombstone cause
Hi Ben
A couple of suggestions.
- is the solder paste well aligned with the board? If not, it would be consistent in direction.
- is the pick and place machine placing the parts evenly in the paste deposits?
If the part is more in one paste deposit compared to the other, it may lift from the other.
- have you looked at orientation going through the oven? E.G. if you reflow a board and always get tombstones lifted on the west side, and then run the next board rotated 180 degrees, do the tombstones match?
If so, can you run it rotated 90 degrees? Ideally you want both pads to reflow at the same time, so if the part is oriented perpendicular to the direction of travel you should get best results.
Pad geometry can also contribute. We have a board with 0603s where the pads are about 50% bigger than per IPC SM782, and the tombstoning rate is relatively high, whereas we have 0402s on properly proportioned pads and they never tombstone.
We also find that, with our particular paste (SnPb, RMA flux), running without nitrogen pretty much eliminates tombstones because it slows down the wetting.
Good luck.
regards,
Graham Collins
Process Engineer,
L-3 Communications Electronic Systems Inc.
Halifax
(902) 873-2000 ext 6215
>>> [log in to unmask] 06/08/05 09:09AM >>>
Technetters,
I have several SMT board styles which give me some trouble with 0306 capacitor tombstones, and through many attempts to reduce the number of these defects, I have found a condition that I can not seem to attribute a cause to. I have found that across a board, all the tombstones tend to lift in the same direction (for components in the same orientation).
I have looked at the build process, but different board styles result in tombstones in different directions, so I believe the cause to be board related. I have looked at solder mask registration (misregistration occurs in various directions), power/ground plane connections by pad (changes from location to location), and solder bumps due to HASL (wicked pads have similar results), and nothing has led me to a root cause.
Any ideas???
Thanks,
Ben Gumpert
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