Closest that I see anyone coming to trying to define a composite formula on this topic is Werner Engelmaier. The formula seems very reasonable and in-line with my thoughts on the subject. There were some earlier TechNet discussions on this topic. To save you the bother of searching the archives on this topic, listed below were his comments:
"Tg is not the whole story, you also need to look at Td [decomposition
temerature] as well as the CTE. I recommend PCB materials with a STII [Soldering temperature impact index] of at least 215. The STII is defined as
STII= [Tg + Td]/2 - [% thermal expansion from 50 to 260 C]x10
You may be interested in my workshop on PCB reliability at NPL on October 13.
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"
Rich K / KEDS
260.925.8719
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Baski Devre
Sent: Monday, December 26, 2005 11:10 AM
To: [log in to unmask]
Subject: [TN] Lead Free Compatible Values
Dear Technetter
We have collected RoHS declaration of our copper clad laminate supplier.In RoHS declaration they assure that their PCBs
do not include or limited six restricted elements Cd, Pb,Hg,Pb, Cr VI,.PBB and PBDE. With these RoHS
declaration nobody can claim that these CCLs are lead free compatible.Our customers and directly we have already started
to produce RoHS compatible PCBs. As you know all that RoHS compatible PCBs do not mean that these PCBs can withstand
high temperature of lead free proccess steps. These PCBs naturally their CCLs are subjected to higher thermal applications
which can cause some problem during the soldering or assembly such as expansion, bow and twist blister or delamination. So
to let us decide If any CCL can withstand high temperature or lead free process we and our customer wants to have
appropriate Tg, TMA, Td and CTE values. These value can help us to determine whether our CCL raw materials they supply us
are appropriate for high temperatrure lead free proccess or not. In these view we would appreciate that If you would
issue new declarations in which they assure their CCLs are completely RoHS compatible and can endure lead free process
temperature at each stage.
To sum up we all need to have the Tg, TMA, Td and CTE values. Do you know that is there any exact or specified value
which can be used as limititation or what are the minimum value of these
I wish you all merry and happy christmas
Best Regards
--
Yusuf GÖMEÇ
Product Quality Engineer
BASKI DEVRE SAN ve TIC LTD STI
Ceyhan Sk. No:10 34896 Pendik-Istanbul-Turkey
Tel: 00 90 216 3901036 (4 Hat/Line)-173(Extention)
00 90 216 4836560 (4 Hat/Line)-173(Extention)
Fax: 00 90 216 3544941
00 90 216 4913269
Gsm: 00 90 533 6579897
00 90 533 7480014
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