In a message dated 4/21/06 17:47:28, [log in to unmask] writes:
> Just a quick question.  What are you speaking of below when you suggest 
> eliminating solder fillets on the end caps?  Aren't those fillets necessary for 
> reasons of reliability?
> 
Hi Leland,
This is a common misconception--and goes way back to ceramic leadless 
castellated chip carriers. What the fillets do is pull the component down to the PCB 
soldering pad because of solder surface tension, thus minimizing the solder 
joint height. Increased solder joint height reduces the strains experienced by 
the solder from a given thermal expansion displacement. Thus, in chip 
components as in leadless chip carriers, the SJ underneath the component fractures 
relatively early [much earlier for the larger chip carriers]; what the fillets do 
provide is solder volume through which the crack more or less slowly 
propagates--but this really is a Band-Aid approach. Much more reliability can be 
achieved with the proper SJ height; see BGAs and CGAs neither of which has solder 
fillets.
Reducing the PCB pad size allows the chip component to swim up on the liquid 
solder [now surface tension works to your benefit]—better yet is gluing the 
components down and let the high CTE of the adhesive lift the components away 
from the PCB during the soldering process thus increasing the SJ height, and 
thus increasing fatigue reliability. 

Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com