Bryan

As Dave mentioned these cracks may have their origin in the solidification process of the solder.
Solder alloys with high tin content solidify dendritic with some eutectic solution between the branches of the dendrites. These dendrites form a solid weblike structure with the eutectic solidifying a bit later than the dendrites. The solidification process starts from a cool spot. This can be a massive component or a solder land connected to large copper traces on the PCB surface without thermal compensation stars not heating up during soldering and ends in the coldest area of the solder joint. 
When the solidification front meets a solid surface it can happen that these two surfaces don't join and upon shrinkage of the dendrites during cool down a gap opens along the solidification front. This can occur in the joint if two fronts coming from the component and the solder land meet, it can happen along the solder land if this is the hottest place (fillet lifting) or it can happen along the pin if this is the area where the solidification occurs the latest.
We see this problem frequently with companies making their first steps with lead free soldering and in many cases it is a design problem (through holes and solder lands connected to high masses without thermal compensation stars). 
From the process side you can only get higher with the preheating to bring the components closer to the solder temperature and slower cooling. However, you now the drawbacks: some components don't like higher preheating, your fluxer doesn't like it either and your boss doesn't like the reduced throughput.
Regarding the reliability it is difficult to say what these shrinkage gaps mean. I don't want the mail become too long. But at temperatures above 20°C the material is very ductile and thus the effect of stress concentration at ta crack tip doesn't apply. Below -20°C the material becomes always stiffer and the stress concentration becomes more and more the governing driving force for the degradation. 
Looking at IPC 610 you have two interpretations. Firstly evidence of cracking is a nonconforming defect, secondly is partial filling not necessarily nonconforming. From the point of metallurgy I'd say that in this case the filling requirement holds. From the customer side it is well possible that the fracture criteria is to be applied. Hmmm, not much of help isn't it?



EMPA
Swiss Federal Laboratories  for Materials Testing and Research
Centre for Reliability
Guenter Grossmann,  Senior Engineer

8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :     xx41 1 823 4054
mail:     [log in to unmask]

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
-----------------------------------------------------