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0402 tombstoning



0402 tombstoning | 11 April, 2002

We're having some problems with 0402's tombstoning. On a board with about 300 parts we're getting about 12 to 20 tombstones per board. The caps seem to do it more often than the res.

The pads appear to be in accordance with IPC SM 782 amendment 1, SMD land pattern design, dated Oct 1996.

The pads agree with 782 and are .035 long and .028 wide.

The stencil is .005 with trap aps.

It's worth mentioning that a newer IPC 7525, Stencil Design Guideline, has pad geometries that don't agree with 782. This spec states 0402 pads should be .0256 long and .0197 wide. It's also worth mentioning that other specs such as Philips EMT guidelines recommend sizes closer to 7525 with .0197 long and .0236 wide.

Sine the pads are .010 longer that the 7525 spec and .016 longer than the Philips spec, we believe the problem is due to pad length.

However, I can't help to think maybe the profile is a factor. The board has a very nice ramp to spike profile with a max temp of 205 to 210 C over a 4 or 4.5 minute period.

Any suggestions would be appreciated as well as comments on the pad sizes.

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0402 tombstoning | 11 April, 2002

Profile would be the first place I look. Check your rise and fall rates for temp. Should be no more than 2 deg/sec for rise and 4-6 deg/sec for cooling.


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0402 tombstoning | 11 April, 2002

Messy. Messy. Messy.

Consider gluing these components, until you control your process.

It is unclear to us that a single factor drives these defects, but multiple factors that need to be tuned to work well together.

PAD DIMENSIONS * Some SM-782 pads are a bit archaic. 7525, Philips, and Topline pads are more in touch. Suppliers are the best source of information. Go there first. Everyone else is sheep. Baaaa. * At some point of development, the gap between pads is probably more critical that the size of the pads. Pads that are closer are better than those further apart. Something like 18 thou is better than the more common 25 thou.

REFLOW RECIPE: Have not tested different solder recipes, but be careful with fast ramp up.

SOLDER PASTE: Solder paste with same commercial characteristics ( Type 3, 63/37, No-clean ) gave different results.

IMBALANCES: Generally, tomb-stoning is caused by inequalities between the two pads, terminations, profile, etc. Search the fine SMTnet Archives for background. Beyond those general points, other things to consider are: * Evaluating effects of poor component termination plating. * Reviewing pad size relationships, since greater solder volume applied gets more tombstone results. * Reducing the purity of your N2. N2 increases the wetting action and decreases the surface tension of your solder, creating unbalanced forces on the component. * Checking placements not only for skew, but for exact alignment prior to reflow. * Assessing inner-wiring density of the board to determine if one side of the component may be getting hotter than the other, if this is a reoccurring problem. * Thermocoupling problem components and checking for a temperature differential, if all else fails.

FINALLY: I believe that with 0402, you have to be more concerned with placement accuracy than with other fly shit [well except of course 0201, ugh].

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0402 tombstoning | 12 April, 2002

IMHO it's caused by three things: 1) placement 2) placement 3) placement

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