Tin Whiskers IV: Mitigation


In the last post on tin whiskers, we discussed detection. In this post, we will cover mitigation. Since compressive stresses are a primary cause of tin whiskers, minimizing these stresses will help to mitigate tin whisker formation. There are several approaches to accomplish this compressive stress reduction. The first is to establish a process that produces a matte finish as opposed to a bright tin finish. Experience has shown that a satin or matte tin finish, which has larger grain sizes, has lower internal compressive stresses than a bright tin finish. Studies have shown that avoiding a bright tin finish alone can reduce tin whisker formation by more than a factor of ten. Thicker tin layers will often reduce compressive stresses.

Since a major source of the compressive stresses in tin is due to copper diffusion into the tin, minimizing this diffusion will significantly reduce tin whisker formation. One proven approach to minimizing copper diffusion is to have a flash of nickel between the copper and the tin. Since nickel does not readily diffuse into the tin after initial intermetallic formation, tin whisker formation can be all but eliminated in many cases.

Adding bismuth to the tin, in small amounts, can also reduce tin whisker formation. The bismuth solid solution strengthens the tin. This strengthening will often reduce tin whisker formation.

Another mitigation approach is the use of coatings. Acrylics, epoxies, urethanes, alkali silicate glasses and parylene C have been used. Parylene C appears to be the most promising.

Often a tin whisker will penetrate the coating as seen in Figure 1. However, to be a reliability risk, it must penetrate a second coating.

Figure 1. A tin whisker about to penetrate a polymer coating. Source: Dr. Chris Hunt, NPL.

This situation is almost impossible as the tin whisker is fragile and will bend as it tries to penetrate the second layer of coating. See Figure 2. So, coatings can be a very effective tin whisker mitigation approach.

Figure 2. To be a reliability concern, a tin whisker must penetrate two protective coatings.

The next and last tin whisker post will be on using FMEA (failure modes and effects analysis) to develop a tin whisker reduction strategy.


Dr. Ron

Removal of Conformal Coating with Small Sandblasters

Development of conformal coating technology was driven to a large degree by the military and aerospace industries. While conformal coatings are mostly used on populated, printed wiring boards (PWBs), they are also used to protect components such as transistors, diodes, rectifiers, resistors, integrated circuits (ICs) and hybrid circuits including multichip modules (MCMs) and chip-on-board (COB).

Recent environmental regulations and concerns have had a significant impact on both coating materials and application methods, particularly with regard to control of volatile organic compounds and chlorofluorocarbon compounds. VOCs and CFCs have been extensively used as solvent carriers. Manufacturers and suppliers of conformal coating materials have responded by developing non-solvent based coatings and environmentally acceptable methods of application, curing and removal.

It is important to consider how the choice of a conformal coating material affects the rework and repair issues. The need for rework or repair of a conformal coating can occur any time after completion of an assembly due to a variety of process or product requirements and component replacement issues.

A number of methods are available for rework of conformal coatings. These include thermal, chemical, mechanical, plasma and laser-based systems and small sandblasters or “micro abrasive blasters,” which will be the focus of this column.

Micro-abrasive blasters used for conformal coating removal are small sandblasting systems that are commonly used for metal deburring and etching as well as surface preparation. The cutting media is introduced into a compressed air stream and is ejected through a hand piece utilizing tips as small as 0.026″. This is directed at a component or surface area on PCB where the conformal coating has to be removed. This system can remove conformal coating from a single test node, an axial leaded component, a through-hole IC, an SMT component or an entire PCB without any modification to the system for a variety of coating materials. This method provides the most practical and environmentally friendly means for removing conformal coating from PCB assemblies.

Although these small Micro Abrasive Blasters provide the most practical and environmentally friendly means of removal, they also pose a problem. Micro Abrasive Blasters can generate static electricity as the high velocity air and particles impinge on the PWB surface. The ESD voltage generated at the point of contact can cause damage to components and electrical circuits on an assembly.

Equipment manufacturers have used several different approaches to solving the ESD problem. These are: 1) the installation of AC or DC pulsed ionizer bars in the chamber results in a rapid decay of ESD voltages in the work cell and tubing 2) the installation of a point ionizer at the end of the nozzle to dissipate any static charge built-up in the media stream at the point of contact 3) the use of an inline, auto balanced ionizer where the air source is split, one side flowing to the media and the other side flowing to the inline ionizer. This ionized air is then injected into the media stream just before it leaves the nozzle, eliminating the static charge buildup in the media chamber. The ionized air is also pumped into the work chamber. With this type of system, ESD levels are reportedly in the +10V range.

Patent Case Has Heads Swiveling

PVA’s shot across the bow of the conformal coating industry is sure to raise the hackles of some of the equipment manufacturers, but it’s hard to see that they have much choice but to go along.

To recap: In October 2000 and September 2002, respectively, the US PTO issued PVA a pair of patents 6,132,809 (the ‘809 patent) and 6,447,847 (the ‘847 patent). The ’809 patent covers the utilization of multiple nozzles on a conformal coating applicator. In short, this would cover any robotic conformal coater with more than one application head. The ‘847 patent covers the rotation around at least a single axis. In other words, if the head rotates, it is covered by the ‘847 patent.

I’m no patent attorney, but those patents appear very broad to my untrained eye. But I do know that an issued patent is a valid patent. PVA is in a strong position here.

This week, PVA issued a cease-and-desist letter to Specialty Coating Systems , citing 33 potential infractions of the two patents. Other than a press release informing the same, the two sides are, as far as I can tell, now letting their lawyers to the talking.

But for those who follow the conformal coating industry, allegations of infringement shouldn’t stop with SCS. The use of a rotating application head is hardly unique among conformal coating equipment. PVA’s complaint may start with SCS, but I would almost guarantee it won’t end there. Readers should talk to their conformal coating equipment suppliers about what this might mean for them.