The surface mounting was performed by a customer, and they selected the
profile.  I do not like the profile.

The assembly was a temp cycle test card.  All the components on the board
were the same, low mass, 0.8mm thick, leadless devices, so thermal mass was
constant.  The solder joint thickness was on the order of 75 micrometers
across a pad that measured approximately 0.25x0.3mm.  The hour glassing was
so severe on some joints that it measured about 75 micrometers, or more,
from both sides of the joint.  


Best regards,
Leo

Director of Applications Engineering
ASAT, Inc. 
3755 Capital of Texas Highway, Suite 100
Austin, Texas     78704

ph     512-383-4593
fx      512-383-1590
[log in to unmask]
www.asat.com


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-----Original Message-----
From: Leadfree [mailto:[log in to unmask]]On Behalf Of Werner Engelmaier
Sent: Saturday, October 15, 2005 10:38 AM
To: [log in to unmask]
Subject: Re: Leadfree Processing temp requirement


Hi Leo,
I do not like the tent-like profiles you used--they are o.k. if your 
assemblies have components of near-identical thermal masses. However, a
large 
variation of component sizes results in grossly diverging reflow profiles
for small 
components [and the PCB] and large components, and of course, the oven
recipe 
needs to be set according to the slowest heating SJs on the largest
component; 
thus, the smaller components and the PCB will see  much higher T's. The 
ramp-soak-spike profile results in much smaller T-differences and thus lower
T's for 
the smaller components and the PCB.
Your experience is puzzling, because it is contrary to my and other
peoople's 
experience. Perhaps, there is something else going on that is not obvious 
from your description. The ball-shaped SJs are not very good because they
have 
stress concentrations near thecomponet and PCB interfaces and that is where 
typical failures occur. Much better geometries are straight cylinders, and
better 
yet slight hour-glass geometries--no stress concentrations. So, you should 
have gotten a benefit of higher standoffs as well [the smaller] benefit of a

stress-concentration-free SJ geometry.

Werner

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