CTE for FR4 is only around 7 below the Tg. Get above the Tg and it rises to around 13/14 on both the x and y axis. Ceramic will stay at around the low figure throughout the whole thermal excursion. Z axis, however, will go from 50/70 to around 270/290 above Tg. This may or may not have an effect of the cracking effect dependant on pad design and area attached.
Colin W.T. McVean M.Inst.C.T
Production Manager
Artetch Circuits Ltd
Direct Dial 01903 712926
Switch Board 01903 725365
www.artetch.co.uk
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Neifer, Gebhard
Sent: 19 October 2006 16:20
To: [log in to unmask]
Subject: [TN] AW: [TN] Tombstoning and nitrogen
Richard,
my understanding is that ceramic chip capacitors are very sensitive to bending and tensile stresses, not to compressive loads. Since the CTE of such a chip component is 9 ... 12 [ppm/K] and the CTE for FR4 is 17 ... 18 [ppm/K] I do not really understand why cracks should occur during cooling after reflow. Can you please explain, thanks, Gebhard
-----Ursprüngliche Nachricht-----
Von: TechNet [mailto:[log in to unmask]] Im Auftrag von Stadem, Richard D.
Gesendet: Donnerstag, 19. Oktober 2006 16:44
An: [log in to unmask]
Betreff: Re: [TN] Tombstoning and nitrogen
Yes,
Two different companies called me in the past few weeks where they were
seeing an increase in MLCCC chip caps cracking. The first one had the
problem with a new design, but they did not associate it with the fact
that the process had changed simultaneously to a Pb-free process. The
second company, they simply changed an existing power supply design over
to a Pb-free process.
They thought it was a fault of their board layout, but I explained to
them that with the hotter reflow temperatures there is a longer cooldown
from solidus to room temperature. Once at solidus the caps are locked
in. But as the board continues to cool it imparts a major stress. Being
that ceramic caps often have a much different CTE than the new pwb
materials, they crack. The cracks are sometimes visible under 100X
magnification, but are present immediately after reflow. They may fail
electrical test immediately, or they will fail later.
In both cases the same exact lots of capacitors did not exhibit this
problem with standard 63/37 reflow profiles on other pwbs that (I
suspect) are more forgiving. I had not thought of the lesser elasticity
in the SAC alloy until you mentioned it, but all of these things work
together.
The more I see of the reliability issues related to Pb-free, the more it
is impressed upon me that all of these failures are seldom the result of
a single root cause, but rather a combination of different factors.
So, another fallout from RoHS.
________________________________
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Thursday, October 19, 2006 8:58 AM
To: Stadem, Richard D.; [log in to unmask]
Subject: Re: [TN] Tombstoning and nitrogen
Hi Richard,
You are absolutely correct. Large fillets also cause chip cracking,
particularly with the stiffer LF-solders.
Werner
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