The recent (last two or three weeks) threads on this subject were started by myself and Werner 8 weeks ago, when I wrote a posting to let everyone know that we need better standards to ensure PWB resistance to delamination.
This came about because several of my clients were having troubles with meeting Pb-free process temperatures with pwbs that were made of superior materials, but still delaminated. There was also a rash of via reliability problems from Z-axis expansion at the higher temperatures in reflow.
In addition, because many different BGA packages were recently released to the industry with lead-free balls, and the BGAs were no longer available with 63/37 balls, many people reported on this forum that they were having reliability issues both with Pb-free balls and boards that were not intended to be used at the slightly higher reflow temperatures needed as a result. In other words, all of the other components on their assemblies had lead-free finishes that would solder up just fine with 63/37 solder paste, but they needed a higher temperature to solder the Pb-free BGAs. If they did not increase the temperature, the resulting solder joint was not homogenous, and had many lead-rich zones.
I proposed creating some type of standard at that time where the pwbs would be "graded" to meet certain time/temperature minimums, which Werner suggested would be better met with a STII, and I agreed.
The PWB vendors shot it down at an IPC meeting when Werner proposed it.
After I pointed out to the PWB vendors that it would protect them as well as their customers, more postings were issued, such as those from Valerie and others, that at least opened up discussion, but we learned that many others contributed to the ultimate ability of the PWB to withstand certain temperatures.
It is very clear from all of these postings that some type of rating system, or better methods of PWB fabrication were needed, so that PWBs can withstand slightly higher temperatures (220-240 deg. C) without delamination, blistering, and severe Z-axis expansion where 63/37 solder paste is used, but BGAs with Pb-free balls are present.
Much better fabrication procedures are definitely necessary where Pb-free solder paste is used in a pure Pb-free process at reflow temperatures in the 250-260 C range.
From all of the postings from Mansilla, et al, my understanding is that all of the slash sheets are useless in determining the ability of a given PWB to withstand delamination, blistering, and excessive Z-axis expansion.
Many of the PWB vendors stated that the end-user (customer) needed to work hand-in-hand with the fabricator to make sure the requirements were understood. Nobody argues with that, that is a given. But it often does not happen.
Others proposed using a coupon to run through the reflow to check for these issues, and others suggested using the solder float test. Many responded that neither one of those is an infallible test, and I think we all also agree with that.
That is where we are now.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Richard Kraszewski
Sent: Tuesday, December 27, 2005 5:15 PM
To: [log in to unmask]
Subject: Re: [TN] Lead Free Compatible Values
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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