For environmental reasons, the world is adopting RoHS compliance for electronic circuit boards, although recommended materials may not be ideal for demanding applications.
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:
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:
It has been found that one or more of these three options can be effectively applied to most electronic components and thus that the majority of commercially available parts (read "compliant") can be made or converted to prohibited-material-free parts for military-aerospace and other high reliability applications.
A large number of compliant alternatives have been developed for many of the company's products, with new part numbers assigned to all items converted to compliant versions. Current parts produced with noncompliant materials will not be changed and will be offered as long as there is a viable market.
Conversion of parts containing "prohibited materials" is possible in many cases, especially for parts produced with 100-percent tin. The tin can be stripped and replated with tin-lead or alloyed with tin-lead (for example, solder dipped) to ensure the control of whisker growth. Many M/A-COM products contain no "prohibited materials" but have always met RoHS compliance criteria. Finally, the company is introducing new compliant parts that incorporate fully approved alternate materials.
Coatings or films of hexavalent chromium (Hex-Cr) have been used for decades as corrosion inhibitors and as adhesion promoters for paints and other coatings. Hex-Cr usage increased dramatically over time as testing showed it would protect more and more different metallic surfaces including steels, aluminum, zinc castings, and others. In addition, Hex-Cr coatings are self-healing; the coating material migrates to exposed metal areas in scratches and effectively "re-seals" the surface. As a result, Hex-Cr is the subject of many standards and specifications. Thinner coatings are clear while thicker applications yield colors from yellow through olive to black. Thicker coatings also provide protection from harsh environments for longer periods of time. See, for example, Table X2.1 in ASTM B201-806 for expected and accepted results. Some additional very positive aspects of HexCr coatings and coating processes are: long history of effectiveness, a wealth of application knowledge, numerous vendors capable of applying Hex-Cr effectively and good surface conductivity.
Unfortunately, hexavalent chromium compounds in these coatings and in plating baths used to apply metallic chrome platings are carcinogenic and otherwise hazardous. There are no direct "drop-in" replacements for Hex-Cr coatings with the above properties on all different surfaces. However, there are replacement coatings for specific surfaces and applications that have been shown to provide equivalent or better protection and good adhesion in testing such as ASTM B117-03 Salt Spray7 and other testing regimens. Finally, customers often specify and require a HexCr coating be applied for various reasons. Where so required, it is M/A-COM's intention to continue using Hex-Cr until specifications are changed or until proven, accepted replacements exist.
A number of replacement coatings based on trivalent chromium, permanganates, molybdates and organic materials are available. Some are conductive (one milliohm surface resistivity and less) while others, including most of the organics, are insulators. It is beyond the scope of this report to list all these replacements and the substrates on which they are effective. Many end users including vehicle manufacturers list in their specifications and on their websites what replacement coatings they have tested and found acceptable as replacements. In one case, the military (US Navy) has developed a trivalent chromium coating called TCP for protection of aluminum. This coating system meets the requirements of MIL–C–55418 on aluminum and is effective on other surfaces. Several commercial plating chemical manufacturers have been licensed to produce TCP solutions.
At M/A-COM, tests have been performed on the same coating on zinc-plated steel where it passed over 200 hours of salt-fog exposure (Fig. 1). Other coatings have also met the requirements of this specification. The costs for both the replacement solutions and for the coating application vary considerably and will change with time based on usage and performance.
Availability and capability are two important factors with respect to replacement coatings. Since each chemistry is different, one must qualify vendors who supply that coating chemistry, have the proper baths and equipment, and are capable of applying it correctly to the desired substrates. Capability includes pre-cleaning, masking, and post processing. A qualification process based on both vendor capability and the coating itself should be performed to ensure performance, reliability, and repeatability.
LEAD-FREE SOLDER
Tin-lead solder has been in use for thousands of years as an adhesive, and within recent history for electronics. From empirical and theoretical analyses, there is little that is not known about its performance, reliability, and use for electronic interconnections. Since tin-lead solder has been used in electronics for over a century, most parts, circuitry, reliability tests, and specifications have all been developed for tin-lead solder assembly. The solder composition most often used is the eutectic (liquid solidifies into a two-phase metal structure at a single temperature) Sn63-Pb37 (weight percent) that melts at +183°C.
The lead-free solders most highly recommended to replace tin-lead are the tin-silver-copper solders. These alloys all melt in the +215 to +221°C temperature range and have compositions from 95 to 96 percent tin by weight. The two alloys most often recommended are SAC 305 (3.0 weight percent silver, 0.5 weight percent copper, and the remainder tin) and SAC 387 (3.8 percent silver, 0.7 percent copper, and the remainder tin) although other compositions also exist with equivalent properties. Although other solders with melting temperatures in the +183°C range exist, they either contain RoHS restricted materials or have other problems such as a much higher tendency for the solder joint to oxidize with time.
Soldering is a process that uses a molten metal to wet and alloy with solid surfaces by forming an intermetallic compound (IMC), an ordered structure that is easily formed by dissolving the surface of the solid and solidifying as this occurs. IMCs are formed in all solder joining processes but are not necessarily the most optimum materials for an electronic connection. Good electronic solder joints all contain IMCs but not in such quantities or thick, continuous layers as to cause electrical or reliability problems.
The most common IMCs formed in most electronic applications are the tincopper compounds Cu3Sn and Cu6Sn5. These IMCs are brittle with fairly high electrical resistivity. Solder melts at a low temperature so the atomic structure is fairly open and copper atoms can easily diffuse into the solder at room temperature. Even at ambient temperatures, the intermetallic structures continue to grow (Fig. 2).9 Among the concerns with the higher melting alloys— especially those with high tin content—is that the higher temperatures and increased tin levels will create more tin-copper IMC.
Although studies have shown the reliability of solder joints produced using lead-free materials can be very high, the following points must be considered:
Via process changes and training for inspection of lead-free solder joints, the first three conditions can be controlled to an acceptable point. The last two, however, result in concerns in the design, cost, and reliability areas. There are a number of thermal issues associated with a higher temperature soldering process. The first concerns material degradation. Polymers (plastics) currently in use in circuit boards, over-molded plastic IC packages, and in connectors may not be able to withstand the new reflow and other soldering conditions such as rework. Assemblers and electronics producers are asking that parts now be able to withstand three SMT reflows at temperatures to +250 or +260°C for 30 seconds rather than the tin-lead standard of temperatures to +235°C for 10 seconds without damage or loss of reliability (lifetime). With some plastic materials, this extra time and temperature have resulted in thermal damage to materials and in moisture and/or thermal expansion induced damage to plastic packages and circuit boards.
Most polymers have a degradation temperature (Td) above which the material rapidly deteriorates. At somewhat lower temperatures, longer exposure times can create the same damage. Dimensional changes, sagging, and loss of strengths can occur in some standard plastic materials after exposure to these new reflow, wave-soldering, and rework temperatures. All polymers absorb moisture to some degree. Plastic packages all have a Moisture Sensitivity Level (MSL) rating based on materials, size of the parts, and moisture exposure. For higher MSL values, more care is required to minimize the ingress of moisture into these parts. If the amount of moisture absorbed becomes too high, rapid release on heating to temperatures well in excess of the boiling point of water can cause package delamination (Fig. 4).11 Failure of the conductive epoxy die attach bonding the semiconductor die to the package lead-frames, circuit-board delaminations between the polymer and the woven glass reinforcement, and blistering of polymer surfaces also can occur. Lead-free processing thus adds +20°C or more degrees of heat to the process for 20 seconds longer time. MSL levels have been shown to rise by 1 or 2 for the new processes. Parts thus can no longer be stored in the open but must be sealed and kept dry by various approved means or reliability may suffer. In circuit boards, damage can occur due to higher internal stresses created by thermal expansion mismatch. This can cause cracking of copper interconnects between layers (Fig. 5).12
Figure 6 shows a Conductive Anodic Filament (CAF) that has shorted out two copper layers.13 After degradation caused loss of adhesion between the polymer and glass fibers, moisture ingress provided a transit path for copper electromigration between two differently biased circuit traces. A short circuit resulted due to formation of copper filaments. Since this damage can occur over a period of time, reliability may be an issue.
Some of the existing higher temperature materials including the PTFEbased RF circuit materials, have been shown to not have these degradation issues. Less-expensive FR-4 materials have too low a degradation temperature so new materials with higher thermal ratings are being developed. The new laminates are more expensive and have different dielectric properties. These value changes will often require a redesign for higher frequency circuits because widths of 50-ohm lines and other impedance-dependent parameters will change.
The final issue of lead-free reliability, new solderability finishes, can result in three problems: loss of solderability, circuit board warp/distortion, and growth of whiskers. The first two are related to temperature. The most inexpensive solderability finish on circuit cards is organic solder preservative (OSP). In lead-free reflow of a double-sided board, the second-side finish must now withstand higher temperatures, often in air, and still be solderable to prevent poor joints and defects. Other finishes may also oxidize in an air atmosphere reflow process and become less solderable. Further testing is needed.
The second, circuit-board distortion, can occur in the other common solderability preservative application, hot air solder leveling (HASL). The final steps in producing boards with HASL finishes are to flux the exposed copper areas on the board, pass the board through a molten solder bath to wet the copper pads with solder, and blow the excess molten solder off with focused hot air. This step has one of the lowest yields due to distortion resulting from the thermal gradients the circuit card experiences with this process. The new solders most often used for this are tincopper based and melt in excess of +225°C versus +183°C for tin-lead solder, thus increasing the gradients and the chances for warp or other damage. Tincopper intermetallic structures may also grow more rapidly under these higher-temperature processing conditions. If exposed, IMCs may absorb water that vaporizes during reflow creating an unwanted condition known as dewet that may adversely affect solder joint integrity.
Alternate board finishes that are hazardous material free and that do not exhibit these problems are being used. The three most common and well characterized are 100-percent immersion tin (Im-Sn), immersion silver (Im-Ag), and electroless nickel immersion gold (ENIG). But 100-percent tin can grow whiskers under some conditions of application and storage, and both the tin and the silver have shelf-life problems due to oxide growth and tarnish. At one time, electroless nickel platings had a bad reputation due to the formation of black pad, an oxidized, non-solderable surface hidden under the final gold plating. In the attachment process, the gold would dissolve into the molten solder with very little attachment to the nickel oxide underneath. Parts would actually just fall off the board in some cases. This cause is now well understood and board manufacturers have this under control. The feeling at M/A-COM is that ENIG is the best currently available finish for RF circuit traces with good solderability, a wide processing window for high yields, and good RF performance in both tinlead and lead-free soldering applications. The other approaches will also perform well with proper handling and solder reflow conditions. If the Im-Sn melts during reflow, many studies show the tendency to form whiskers is drastically reduced since stress is removed. Note that tin whiskers have been shown to grow and emerge through solder mask and conformal coatings (Fig. 7).14
Multiple studies have shown that additions of a little as 3 percent lead in tin platings drastically reduces the probability that whiskers will form under most all conditions. Since the RoHS directive limits the permissible amount of lead to 0.1 percent, the most cost-effective replacement finish is considered to be 100 percent tin. Tin grows whiskers. In some applications these whiskers have been shown to grow and either bridge gaps causing short circuits or break and move freely causing short circuits elsewhere. Such failures can have serious consequences.
Whiskers are commonly observed in highly stressed, bright tin, electroplated coatings. In recent years, a number of matte tin coating processes designed to produce a lower-stress, lead-free tin plating that grows much smaller, and far fewer whiskers have been developed. Stresses are one cause of whisker formation. The term that applies to these finishes is "whisker inhibited"—NOT whisker free—100 percent matte tin coating. Under many conditions, these coatings have been shown to grow fewer and smaller or no whiskers when compared to many bright tin platings. Studies at iNEMI15, Soldertec16, and elsewhere over the last few years have shown that factors enhancing whisker formation and growth are much more complex. Therefore, under the wrong conditions of application and use, these "inhibited" coatings can grow whiskers–as can nearly any plated metal (Fig. 8).17 The images shown in Fig. 8 demonstrate that under the right conditions, many types of plating can grow whiskers. Fortunately, most materials do not normally grow whiskers but care must be taken to ensure platings are optimized against such growths.
Test data and analyses by iNEMI, Soldertec, and other consortia have defined some conditions capable of accelerating tin whisker growth and some recommended methodologies for measuring and mitigating whisker initiation and growth. Acceleration conditions include thermal cycling and, more effectively, temperature and humidity storage. Standards are being developed around these tests and recommendations by JEDEC18, IEC19, GEIA20 and other organizations.
Mitigation procedures include depositing the matte tin over a nickel underplate. The nickel layer prevents diffusion of copper into the tin and the continued growth of CuSn intermetallics. IMCs have been shown to enhance whisker growth. Unfortunately, many plastic packaging houses do not have nickel-plating capability. The alternative is to heat the matte tin plated part to +150°C for an hour shortly after plating. While this process grows a bit more CuSn IMC, it also allows the tin to diffuse and lower stresses, reducing the whisker growth tendency. Ultimately, there is not enough data to define all the conditions necessary to preclude long whisker growth in 100percent tin and the reliability is not fully proven for these recommendations. At one time it was thought that solder reflowed leads would not grow whiskers even in bright tin, but that has been shown not to be the case21 since post reflow stresses will grow whiskers even in Sn-Pb solder.22
Another alternative finish that has not been shown to exhibit whisker growth is nickel-palladium-gold. A number of references show that this is a robust lead finish and there has been no significant evidence of whisker formation on parts plated with this finish. Again, not all packaging contractors can furnish this finish. M/A-COM has specified this finish for a number of "no-prohibited-material" applications being produced with excellent performance and solderability results. Ni-Pd-Au is an approved alternative to tin with three-plus percent lead. Gold over nickel has been used for years on ceramic packaged parts with almost no evidence of whiskers (Fig. 8). In addition to cost, full gold coverage is required to prevent the nickel surface that is ultimately bonded from oxidizing and not wetting. If too much gold is used, dissolution of the gold in the solder will raise the melting temperature and freeze-out the solder, preventing complete wetting (Fig. 9).23
Since the demand for RoHS-compliant parts is not universal, many plastic packaging vendors are maintaining a tin-lead lead finish capability. Where it makes sense, M/A-COM will maintain parts with the more-than-three-percent Pb in Sn finish and issue new part numbers for changes to RoHS-compliant 100-percent matte tin and other finishes. Documentation will always show what finish is used. The company has identified and qualified suppliers who can strip the 100-percent tin finish and refinish parts with lead-bearing tin. The costs for retinning (so far) have not been prohibitive. This will be done where required to meet specifications on parts only available with 100-percent tin finish.
M/A-COM has a large number of parts that are both RoHS compliant and do not contain military-aerospace prohibited materials. Figure 10 shows several such parts including hermetic ceramic, weld-sealed, metal-lidded units, and plastic packages. The ceramic packages and lidded units use a gold-over-nickel solderability finish and meet many high reliability test specifications. The plastic packages are in prototype development and use an epoxy mold compound that meets UL 94V-0 flammability standard and high thermal conductivity die attach materials that contain no banned, brominated flame-retardants. The solderability finish on the lead-frames is nickel-palladium-gold, a very stable long-life finish that has many approvals and no known whisker growth issues. These plastic packages also far exceed ASTM E595-9324 space application outgassing limits by factors of 10 or more (Table 2). M/A-COM packages of this type have been tested to MSL Level 1.25
In summary, there are a number of concerns associated with the advent of RoHS-compliant part requirements for military-aerospace and other high-reliability electronic applications. While the RoHS compliant replacement materials and processes have been shown to meet many reliability criteria, the levels of testing, understanding, and "comfort" have not reached to point where they can be used without uncertainty. This is especially true of the new solders. Their higher soldering temperatures coupled with their less understood mechanical behavior have not been fully
studied. In addition, the cost-effective use of "whisker-inhibited" 100-percent matte tin still warrants concern based on a number of studies shown whiskers can still grow and attain sizes large enough to short electronics under some conditions.
ACKNOWLEDGEMENT
The author would like to thank Carole Sundius of M/ACOM Research & Development for insightful discussions and suggestions for this article series.
REFERENCES
