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Conny Thomasson columns | January 16, 2006

Use of Lead-Free HAL/HASL as surface finish on PCBs increases

To choose surface finish for your PCBs when we are entering the Lead-Free era is not allways that easy. There are a number of surfce finishes to choose between.
Chem-Sn, Chem-Ag and OSP has been used with mixed success during the last years. ENIG (Chemical nickel/gold) is considered as a well-established surface finish. For about three years ago even HASL/HAL with Lead-Free alloys started to be used. Within these three years the number of users have increased significantly. Today there are over 100 installations of HASL/HAL using tin/copper doped with nickel as surface finish. Even in Asia they have discovered the benefits of HAL/HASL as surface finish. Despite the fact that OSP is well-used here there are 22 facilities using HASL/HAL with tin/copper/nickel here as well.

Benefits of Lead-Free HAL/HASL with tin/copper/nickel SN 100 C

- Over a year in storage time. The intermetal formed during the HAL process offers an effective protection against migration of copper into the tin. That process is slown down significantly.

- The surface finish endures several heating cycles without any negative impact on the solderability.

- There is already 2-15 microns of solderable alloy available that helps to moisture all the surfaces. Especially during hand soldering this will help.

- During the HAL process only 1 micron is etched off from the holes and the copper surfaces on the PCB.

- The surface is as smoth as you would obtain with Sn/Pb

- A more levelled surface than with Sn/Pb

- "Whisker-free" coating. The risk of whiskers is considered to be the same as for Sn/Pb, since an intermetal is formed during the HAL process such as in the Sn/Pb process, and the solderwire hardens without any tensions. None of the tests made have shown any larger risk for whiskers than for Sn/Pb.

- Tin/copper/nickel can be used as surface finish both in Sn/Pb and Lead-Free processes.

Then what do the opponents to Lead-Free HAL say?

- Higher temperature results in twisted PCBs.

Until now there have been no signs for that provided that you haven't built in any tensions in the PCB with the wrong kind of glass fibre for example. Any bigger difference compared to Sn/PB couldn't be found.

- CAF (Conductive Anodic Filament)
This is a phenomenon that started to be discussed already in the 70's. Through better and improved harts systems and glass treatment CAF hasn't been much of a problem. Smaller and smaller isolation range between hole - hole, inner layer - hole, layer - layer and wire - wire at the same layer increses the risks for CAF. A lot of research has been made about this.

Those who doubt HAL claim that the higher temperatures brings a higher risk of CAF. During the Lead-Free HAL process the temperature is raised by 15-20° C degrees compared to Sn/Pb. Just like in the Lead-Free wave soldering process the process temperature is 265° C.

Until now we haven't seen any problems with CAF in HAL or wave soldering with "normal" boards where FR4 tg 135° C is used. Of course CAF not only depends on the temperature. The PCB process also matters. Rough scratches in the hole walls increases the risk of moist absorbtion. Moisture is one condition for CAF to occur.

When designing new products you surely have to consider which material to use. Then it is preferable to look at the new materials which are specially designed for Lead-Free processes. These materials also have a high t(d) value, (digestive temperature) which further reducing the risk of CAF.

A discussion with the supplier of the materials is recommended by the choice of material. Polyclad has a matrix where you can pick material by thickness, temp cycles etc.


Conny Thomasson
Candor Sweden AB

This chronicle is translated by evertiq. The original chronicle is published on www.evertiq.se

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