Hi David,
I wrote the following in my White Paper Report: 'Recommendations for PCB FAB
Notes and Specifications in Printed Circuit Board Drawings for SnPb and
Lead-Free Soldering Assemblies, the Qualification of PCB Shops and Activities to
Assure Continued Quality,: "Also, in order to reduce the z-axis thermal
expansion, various fillers are added. This has in some cases reduced the cohesive
strength of the material and has led to cohesive delamination failures. Unlike
an adhesive failure resulting from a delamination at the copper-to-resin
interface, cohesive delamination is totally within the dielectric material, as
shown in Figure 1.
Specifically, IS410 [STII=225]has been variously implicated as being prone
to delamination at high soldering temperatures, particularly at power and
ground planes, even though it meets the properties for ‘Advanced Lead-Free
Compatible Laminates’ in Table I. The indication is that there may be an
incompatibility with the phenolic material and the oxide coatings.
Improving some properties individually can easily be achieved but may be at
the expense of other properties; thus, it is imperative that a proper balance
of properties meeting the needs of the product be achieved. It is easy to
understand, that these needs will vary significantly with (1) the end use, such
as high-end servers, telecommunications, high-end servers, avionics, military,
medical or automotive, (2) PCB design and construction, such as thin PCBs with
few layers vs. thick complex many-layered PCBs, and (3) the type and number
of lead-free soldering steps, such as HASL, wave, reflow, vapor phase, repair
and rework.
Therefore, PCBs of new constructions and new materials and destined for
lead-free solder assembly need not only be properly selected according to their
data sheet information, but need to have coupons tested in the product
configuration."
I am also receiving reports on Nelco 4000-13 [STII=247] with similar
problems.
For best results it seems that non-dicy/non-phenolic materials wouls be best;
they yield STII values in excess of 250, whereas dicy-dured seem to top out
at 210-220 and phenolic-cured at about 230-245.
While the STII-value is a useful pre-selection indicator, the quoted last
paragraph needs to be heeded.
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, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com
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