We utilized LCCs many years ago, tempcycled up to 1,000 changes without specific problems. Difficulties to clean 100% underneath was reason to abandon them. Think well balanced boards and fine graded and contaminant free solder joints are an necessity to get longliving products. Precise and repeted solder geometry is also of crucial importance, a fact that was hard to hammer into people's mind. Not to mention the thickness of the solder line. With up to 200 LCCs on a board and several thousands of castellations, you need good control over mentioned factors.
Ingemar Hernefjord
Ericsson Microwave Systems
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Terry Kozlyk
Sent: den 18 oktober 2004 17:10
To: [log in to unmask]
Subject: Re: [TN] LCC process and manufacturing recommendations
If I can add....
One military contractor I worked for years ago told me of their results on
cycling PCB's over the full mil temp range. My eyes opened when he said they
found the leads of ic's were pulling the copper pads off the PCB substrate
material after approx. 50 cycles !! They were rather surprised also.
So, just because a PCB cycles well several times & passes, doesn't mean it
is going to work forever.
We know that in the mil(itary) environment, it doesn't take long for a PCB
assembly to be cycled 50 times ( Plane lands on deck, open cockpit, temp
falls to outside temp, canopy closes, back into heat & sunlight, etc).
Also, the change in temp is very rapid. No time for this gradual temp change
!
I was told that having leaded ic's is a saving grace as the leads themselves
are compliant & flex enough to absorb the CTE differences between PCB
substrate material & IC packaging material.
But having CLCC for mil temps ranges is asking for trouble. I guess it is
best to spec (& use) leaded ic's
Interested in hearing other people's comments.....
Regards
TDK
-----Original Message-----
From: Werner Engelmaier [mailto:[log in to unmask]]
Sent: Sunday, October 17, 2004 9:00 AM
To: [log in to unmask]
Subject: Re: [TN] LCC process and manufacturing recommendations
Hi Jorge and all,
CLCCs have a large CTE mismatch with resin-based PCBs which translate to
large expansion mismatches for larger-sized CLCCs and larger delta-T's. This
does
not create higher stresses in solder joints--those are limited by the yield
strength of solder--but larger displacement strains in the SJs and thus
larger
plastic deformation. On a cyclic basis, these larger strains lead to much
earlier creep-fatigue failure.
Having thicker solder joints [balls, columns] or compliant leads [QFPs]
reduces the strains in the solder for a given displacement.
PLCCs are quite different, because they lack the large CTE-mismatch with
PCBs.
If you need more information, get a copy of IPC-D-279 and/or IPC-SM-785--or
you could take one of my workshops.
If CLCCs are indeed coming back, people simply have forgotten history, will
be condemned to repeat it, and my consulting business will increase.
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
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