Pressures to eliminate materials deemed hazardous throughout the world have led to uncertainties about the long-term reliability of replacement materials, especially in hazardous environments. Initiatives such as the Restriction of Hazardous Substances¹ (RoHS) by the European Economic Union (EU) in particular have targeted six materials for removal from electronic manufacturing. Although studies have shown replacement materials to be effective in many applications, long-term reliability data is still being gathered and assessed. And there is concern that some alternative materials may not be suitable in applications where unfailing operation and performance under long-term or extreme conditions is required. What follows is the opening installment of a multipart article on these existing and their replacement materials and the methods being used to ensure continued availability of proven materials where high-reliability is critical.

The six materials designated as hazardous and banned by the EU are lead (Pb); cadmium (Cd); mercury (Hg); hexavalent chromium (Hex-Cr); polybrominated biphenyls (PBBs); and polybrominated diphenyl ethers (PBDEs). The majority of electronic parts available today contain one or more of these banned substances. Cadmium is found in batteries and pigments. Mercury is used in fluorescent lighting, switches, and as a constituent of the II-VI compounds used in infrared detectors. The PBB and PBDE families of organic compounds are used as flame-retardants for plastics. Even more widespread is the use of Hex-Cr for inhibiting corrosion and promoting adhesion on metal housings and chasses and the use of lead in electrical solder joints. Cadmium is banned in concentrations exceeding 0.01 percent by weight, while the other five substances are allowed in concentrations of up to 0.1 percent (Table 1).

Military, aerospace, and other high reliability electronics are exempted from this legislation and there are numerous other specific exemptions as well. Still, the majority of standard commercial and commercial–off–the–shelf (COTS) electronic equipment and many other components will be available only with restricted substances removed, i.e., as RoHS-compliant parts.

In addition to the RoHS directive, the EU has approved the following resolutions:

1. End of life vehicles (ELV²) restricting many of the RoHS materials in automobiles but allowing the use of lead in solders.
2. Waste Electrical and Electronic Equipment (WEEE³) requiring manufacturer/seller take-back of consumer/commercial electronics and removal of hazardous substances, including the six banned by RoHS, prior to discarding the unit.
3. Registration, Evaluation, and Authorization of Chemicals (REACH⁴) requiring testing of any imported chemicals with unknown health effects, authorization by the legislature to use chemicals deemed a health risk, and replacement with safer materials through development of alternates and production methods eliminating the hazardous material(s).

This article focuses the RoHS directive because it has the largest impact thus far on restricting materials for electronics. Special emphasis is given to Hex-Cr and Pb-in-Sn solders and solderability finish replacements.

Through experimentation and testing the electronics industry has arrived at several "recommended" replacement materials for the RoHS banned substances. Reliability testing is in progress and various replacements have approval for specific applications. Still the volume and depth of documentation necessary to guarantee reliable, repeatable performance does not yet exist. This should not be taken to mean that compliant materials are necessarily less reliable than their banned counterparts; only that evidence for reliability in all applications has not been fully gathered and assessed.

Replacements for Hex-Cr are numerous and varied and are qualified for applications specific to the substrate on which they are applied. Failures and/or reduced protection may occur with improper application or use on substrates other than those for which the replacement was developed and approved.

Principal Pb-free solder candidates include those in the tin-silver-copper (Sn-Ag-Cu or SAC) alloy family. Virtually all SAC solders melt at temperatures exceeding +215°C. By comparison, tinlead (Sn-Pb) eutectic melts at +183°C. SAC solder joints are also different in appearance and wetting behavior from those of Sn-Pb. Failure mechanisms for solder joints include fatigue (cyclical reversal of stress) and/or creep (continuous, low-level stress). Reliability for the SAC solder joints is not necessarily worse or better but the failure mechanisms for creep and fatigue are different. Test methods developed specifically for Sn-Pb solder joints may not be optimal for SAC alloys and, in fact, SAC performance in these tests is variable. Better understanding of the effects of stress on SAC solders is needed to develop reliability test methods designed to target failure modes most common in SAC solders.

The most commonly available, low-cost, Pb-free, solderability finish for leads on electronic parts is a plating of 100-percent matte tin. Pure bright tin, especially electroplated tin that contains high compressive stresses, can and will grow tin whiskers long enough to cause electrical shorts by bridging gaps between adjacent leads and traces. This can occur either at the whisker nucleation site or after a whisker breaks loose and moves via shock or vibration to a different site. Matte tin platings contain stress-reducing additives that were developed and have been shown to inhibit the growth and size of whiskers under the application and test conditions employed. Unfortunately, some matte tin finishes have grown unacceptable whiskers when applied differently, exposed to certain conditions or when stressed as in compliant-pin insertions. On the other hand, years of analyses have shown that the tendency for whisker growth in properly applied tin-lead alloy platings containing three or more percent lead is negligible.

In short, there is insufficient performance and reliability data on replacement materials. As a result, some of the replacement coatings, solders and 100percent matte tin solderability finish along with zinc and cadmium platings have been declared unacceptable or even given "prohibited" status⁵; they are not to be used in many military-aerospace and other high reliability, harsh environment applications. Removal and replacement of the banned flame-retardants is also an issue, one of safety. Replacements show promise but most are not fully tested in some applications. Pb, Cd, Hg, and Hex-Cr appear in numerous forms throughout electronics and many exemptions have been granted because there is no viable replacement for these specific applications. The remainder of this article focuses on HexCr replacements and Pb-in-tin solder and solderability coating replacements.

With significant business in both commercial and military-aerospace sectors, M/A-COM is a supplier and a user of compliant and non-compliant parts. As a manufacturer, the company must meet specifications and price/delivery targets. Some products must be RoHS compliant, yet the parts available to meet performance requirements are non-compliant. Some products must be free of substances prohibited for military-aerospace use but, due to availability, cost and delivery targets can best be met with RoHS-compliant parts. To be successful in both the commercial and military-aerospace sectors, the company must:

1. Make and use similar parts that are either RoHS compliant or are non-compliant using proven-reliable, but now RoHS restricted, materials.
2. Find methods of converting parts meeting one application to parts acceptable for the other.
3. Develop and use parts that are both RoHS compliant and do not contain military-aerospace prohibited substances.