On 17 December, Ron wuz' wonderin':

>>   I would greatly appreciate any help I can get in reference to tombstoning 
>> of plastic film caps. My pad design for each terminal is .060" X .050". The 
>> .060" dimension is for width of the cap.  Each terminal is centered on the  
>> .050", leaving .025" of the pad exposed on each side of the cap. We have to 
>> run the parts perpendicular to the direction of travel through the reflow   
>> because it is critical to the design. Please reply if anyone has experience 
>> with this.  Thanks.


Ron,

   Ain't that weird how them parts just wind-up standin' on their ends all by 
themselves? It's almost like there's some gremlins inside yer' oven with little 
gremlin tweezers standing yer' caps on their ends just trying ta' drive ya' 
crazy huh?

   Looking at yer' pad geometry's it don't seem like there's any big problem 
there...the gap between the pads don't seem excessive, and yer' sayin' that the 
part is centered-up on the pads prior to reflow...hmmmmm, there's a bunch of 
other things that might be causing this. Lemme' ask ya' a few more questions... 
I'm sure the other "Techie's" may have some of the same questions:

1. First thing, is the part okay? I mean as far as solderability...

2. What kinda' oven ya' got? Straight I/R, or Convection?

3. What's the profile set-points?

4. What else ya' got on the board? A lotta' IC's? or a bunch more passives?

5. How big is the board? How thick is it? Is there a lot of copper in the inner 
   layers? (eg: heavy ground or power planes?)

6. How's the solder print? How thick are ya' printin' the paste? Are both pads 
   equal and uniform in the paste deposit?

For tombstoning to happen, something is causing the wetting forces acting on the
component to become seriously unbalanced...it could be component solderabilty, 
or the timing is off when each pad becomes liquidous due to the profile or 
design variables in the PCB itself, or the solder deposition could be a factor 
too. 

I think the first place I would look though, is at yer' reflow profile. It's 
been my experience that most of the time that's what it's turned out to be. Do a
plot and see that everything is in the window for an acceptable profile for the 
paste yer' using. Ya' know;
  
                            
                              "YER' BASIC PCB COOKIN' RECIPE"
                                  (For Medium-well done)

Degrees C.

  230
  220
  210                                     _-_<----Max peak 210-215 degrees C.
  200                                    /   -_
                                        /      \
                                      _-        \
                                    _-           \
  183-SN63 Solder melts           _-<--Dwell at-->\
                                _-     Liquidous   \
  170                         _-                    \
                _-------------                       |
  150         ,-                                      |
             / <--Activators begin to react            |
  120       /                                           |
           /                                             |
  100     / (Keep temperature ramp rates between 2-4     \
         /  degrees per second to avoid package cracking, \
   50   /   solder balls, improper flux activity, or the   | 
       /    dreaded "tombstones"!)                         \
   20 /_____________________________________________________\
    | Phase-1 |  Phase-2  | Phase-3 |  Phase-4   |  Phase-5  |
    |         |           |         |            |           |
Time:1-minute |      2-3 minutes    | 30-seconds | 1-minute  |
                                          to
                                       1-minute

Let us know what ya' find out....


                                 
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