December, 2019
www.us-
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Increasing Signal Integrity in HDI, 5G, High-Frequency PCBs
Continued from page 54
metal). A surface finish satisfying these requirements could reasonably replace ENIG in high-frequency applications, without a loss of fea- tures that ENIG provides in lower- frequency circuits. An alternative to ENIG for
high-frequency applications is Ni- less ENIG-Premium. This is a nickel- free solution with all of the benefits of ENIG-Premium, in which a barri- er layer is deposited on bare copper in place of nickel in ENIG and gold is deposited on the barrier. Ni-less ENIG-Premium includes a nano- engineered barrier layer instead of the Ni-P layer in ENIG. Since there is no nickel in this approach, there are no adverse effects of insertion loss for high-frequency circuits. The reliability of the gold final finish is still upheld.
Comparing Finishes A comparison was conducted to
see how a sample with the barrier layer (Ni-less ENIG-Premium) com- pared with samples without any bar- rier (DIG) and a barrier layer in a different surface finish. In this case, it was 100 nm of palladium in EPIG/EPAG surface finished to intermetallic growth. Despite the harsher sample
preparation for the Ni-less surface finish samples, the results of the experiment show that the Ni-less surface finish has consistently small- er intermetallics than the DIG and EPIG/EPAG surface finishes. Since the DIG and EPIG/EPAG
intermetallics are larger than the Ni- less surface finish intermetallics, the solder balls that are on the Ni-less surface finish should be stronger than those on DIG and EPIG/EPAG surface finishes. During pull and shear tests on
solder balls on the Ni-less surface finish, it was shown that the only types of failures were ductile solder ball failure and pad lifting, which are issues with the solder and laminate and not with the surface finish. Both solder ball pull and shear
tests showed no failures related to the surface finish, so the strong intermetallics predicted by smaller intermetallics are proven true. Since there are no brittle intermetallic fail- ures, the Ni-less surface finish can be said to enable a stronger solder joint that ENIG. The insertion loss of the Ni-less
surface finish is almost exactly iden- tical to the insertion loss of bare cop- per over the 0 to 100 GHz frequency range. This indicates that the Ni-free surface finish is able to be used in high-frequency applications, with hardly any insertion loss increase compared with bare copper. ENIG has been shown to have
an insertion loss of around 2.75 dB/in. at 50 GHz, while the Ni-less surface finish exhibits about 1.25 dB/in. of insertion loss at 50 GHz. This is more than a 50 percent decrease in insertion loss. Since the Ni-less surface finish
has such a small difference in inser- tion loss when compared with bare copper, especially when contrasted with ENIG insertion loss, the Ni-less surface finish is a good surface finish for high-frequency applications.
The Ni-less ENIG-Premium is
cyanide-free immersion gold plated onto a nano-engineered barrier layer
on top of copper, which offers a viable solution for HDI, 5G, high-frequency PCB applications.
The surface finish was tested for
intermetallic growth, solder ball brit- tleness failures and insertion loss. The results showed that this
surface finish performs better than other currently available Ni-free surface finishes, such as DIG and EPIG/EPAG, due to its smaller intermetallics, lack of brittle solder joint failures and extremely low insertion loss, when compared with bare copper. Contact: LiloTree, 17371 N.E.
Figure 2: example of Ni-less surface finish intermetallics after six
reflow cycles and 1,000 hours of aging (left). The same finish after six cycles and 500 hours of aging (right).
67th Court, Suite 209, Redmond, WA 98053 % 425-780-2732 E-mail:
kunal.shah@
lilotree.com Web:
www.lilotree.com r
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