Royce, Here are a few suggestions...

Tombstoning Mitigation Techniques: 
.	Use a larger width and area of metallization under the chip
component 
.	Use adequate spacing between the two pads of the chips 
.	Use a proper extension of the solder pad beyond the chip ends.
Circular pads appear to be more promising than rectangular or square pads
.	Reduce the width of the solder pads
.	Minimize the uneven distribution of thermal mass, including the
connection of pads with heat sinks
.	Minimize the shadow effect through adequate design of PCB and
selection of reflow methods
.	Use organic solderability preservatives or nickel/gold coating or Sn
coating instead of Sn/Pb coating on copper pads
.	Reduce the contamination or oxidation level of the component
termination metallization or PCB pad metallization 
.	Use a thinner paste print thickness
.	Improve component placement accuracy 
.	Use a milder heating rate at reflow
.	Avoid using vapor phase reflow method
.	Pre-dry the paste before reflow or use a profile with a long soaking
zone to reduce the Outgassing rate of the fluxes 
.	Use solder paste with retarded melting temperature or with a wide
pasty range (Indalloy #100: Anti-Tombstoning Alloy offered by Indium
Corporation) 
.	Use a profile with a very slow ramp rate across the melting
temperature of the solder


Christopher J. Nash 
Indium Corporation of America 


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Royce Taylor
Sent: Monday, January 15, 2007 2:04 PM
To: [log in to unmask]
Subject: [TN] tombstoning of 0508 ceramic capacitors

I have a question for the "Adhesive Dispense"  OR the "0508 chip" experts 
out in Technet land.

We at times struggle with 0508 capacitors tombstoning (yes, 0508 not 
0805). We do run Nitrogen and turning it off helps at times but there are 
many times we need to run nitrogen for other reasons. Most times we try to 
dispense a dot of adhesive (Loctite 3609) but we are having occasional 
issues with the adhesive getting into the solderpaste, the gap between the 
pads measures ONLY 0.020" so it is difficult to prevent this from 
happening.  Does anyone out there run into the same issues?? Any 
thoughts?? We run a Camalot Gemini and a Fuji GL dispenser and use a 25 GA 
needle. Also we use Indium SMQ92J No Clean solderpaste so I don't know if 
this paste is contributing to the tombstoning.

A re-layout of the boards is not an option. We have tried both a "straight 
ramp"  and a "ramp-soak-ramp" profile with little or no difference.

Thanks for your thoughts and advice

Royce
*************************************************
Royce Taylor
Raytheon
350 Lowell St.
Andover, MA  01810
(978) 470-7431

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