History repeats itself in the form of matte tin
07/01/2001
BY BANCE HOM AND SANDRA WINKLER
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Perhaps the trendiest topic in the electronics industry is how we're going to reinvent the "smaller, faster, cheaper" paradigm to incorporate "environmentally responsible." In particular, the focus has been on the reduction or elimination of lead and harmful bromide-based flame retardants in the manufacture of components and printed wiring boards (PWBs). There seems to be consensus that "green" or "lead-free" is inevitable for our industry, but we can't seem to agree or understand how we're going to get there.
Plating Finish, Toxicity and Expense
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The key to an environmental movement in electronics is the place where the most hazards rear their heads - this "place" is the plating finish used on components and leads. Lead traditionally has been used in lead frame finishes but is particularly harmful because it's extremely mobile when used in plating. All individuals who work in a plating facility are exposed to harmful levels of lead through both direct contact and the inhalation of fumes. Because of its effects on the development and function of many organs, and its devastation of the central nervous system, lead has been classified as a probable human carcinogen class B2 by the EPA. The medical dangers posed by lead plating processes, by themselves, warrant an effort to reduce or remove lead from this step in the manufacture of electronics.
In addition to its toxicity to humans, lead plating poses economic threats to the industry. Lead frames that are plated with any concentration of lead must be thoroughly rinsed to prevent ionic contamination on the printed circuit board (PCB). The residual ionic lead accumulates in the rinse water and must be removed continuously (also known as precipitating or flocculating). To do this, it is necessary to add chemicals that will form a sludge that can be easily removed. Toxic lead sludge must be handled with extreme caution and taken to a regulated disposal site.
These added materials and processes add up significantly in terms of time and money. And, in an industry where time to market can be the difference between success and failure, time is money. The need to deliver products both quickly and safely to the market requires a replacement for the dangerous and time-consuming lead plating processes that are currently so popular. That replacement is matte tin.
But What About Tin Whiskers?
Whiskering, in this era of fine-pitch devices and microvia PCBs, is a serious concern. Whiskering occurs as a result of stresses within the plated deposit, and can pose a danger when the whiskers become long enough to bridge across leads. While tin whiskering might always be a concern, it is important to note that reflowing can deter such occurrences. The high temperatures during board assembly can alleviate stress in the finish and, therefore, eliminate the potential for whiskering.
A widespread misconception is that lead or bismuth (a byproduct of lead mining) can reduce the incidence of tin whiskers, but all forms of tin and tin alloys have the potential to whisker, including tin/lead, tin/bismuth and tin/copper alloys. The key to reducing or eliminating whiskers is not the addition of lead but a balanced combination of proper plating additives, good material selection, and stringent process controls and operating parameters.
From Tin to Lead and Back to Tin Again
As early as the 1950s, bright tin was the component finish of choice. But, believe it or not, it was popular not because it was effective or safe or economical - it was popular because it was pretty. Unfortunately, the same organics that made the plating bright and shiny also caused serious outgassing problems during the soldering process. Such outgassing results in a porous and cratered finish that can lead to electrolytic corrosion and poor solderability of the leads.
Problems with bright tin were compounded when the U.S. military, in the late 1970s, initiated accelerated burn-in tests to reduce premature electrical failures before board assembly. The elevated burn-in time and temperature created solderability failures in the bright tin finish because of excessive organic outgassing. It was this requirement, and resulting dilemma, that drove the IC packaging industry to adopt a generic low-organic matte tin for plating purposes.
As with all movements, there was some opposition. Many thought that solder dipping was more compatible for soldering onto PWBs because the component solder-dip process used the same eutectic 63/37 solder as the boards. Unfortunately, pre-solder-dipped components could not survive high-temperature burn-in because the eutectic melting point was too low; many of these devices became permanently soldered into the test sockets. Nonetheless, solder dipping components with tin/lead and plating with pure matte tin became the two prevalent and competing lead-frame finishes.
Studies have shown that unreflowed matte tin is superior to bright tin and tin/lead in terms of hours to first failure, and pull strength.1 The conclusions of these studies initially and temporarily delayed the military from changing its specifications, and matte tin plating without reflow remained the component finish of choice. Yet, in 1985, MIL-M-38510 required that leadframe components contain a minimum of five percent lead, or that they be reflowed. This requirement made tin/lead as a solder dip the predominate component lead finish.
After a decade of solder dipping, however, the industry reverted back to plating because dipping was not viable for high-I/O-count, fine-pitch devices. Thus, tin/lead plating became the standard.
The Pure Tin Plate Solution
Alternative lead-free plating finishes are being developed, including tin/bismuth, tin/silver/copper, tin/silver, tin/silver/bismuth and tin/copper. These alternatives all exhibit drawbacks, such as high melting points, toxicity, low availability, high cost and complex process controls. Matte tin, on the other hand, has a melting point of 232°C, is readily available, has low toxicity and is easily controlled. Matte tin also does not contain the radioactive daughter elements of uranium and thorium that come from lead and that decay to form problematic alpha particles.
Matte tin plate is a logical solution for several reasons, including that a single-element finish is easy to control. Also, many matte tin plating baths are commercially available and are ready for use with lead-free solder paste. Matte tin plate is compatible with all existing lead-alloy and lead-free solders, and therefore covers the spectrum of requirements as the industry transitions to lead free.
The Bandwagon
The return to matte tin plate is not just a prospect, as many industry-leading companies, such as Philips Semiconductors, On Semiconductor, Carsem, STATS and ASAT, currently maintain matte tin capabilities for their customers. Others, such as STMicroelectronics, IBM, Maxim, Vishay-Dale, Vishay-Siliconix, Advanced Interconnect Technologies and Amkor, are in development of lead-free solutions that include matte tin processes.
Matte tin, the lead-free comeback kid, shows promise for a bright (though matte) future. History has already shown matte tin to be inexpensive, easy to control, reliable and environmentally responsible. It is now our responsibility to learn from this past and foster a complete return to matte tin as a plating finish. AP
Reference
- Rome Air Development Center study, 1982 (IBM, Texas Instruments, Fairchild, Naval Avionics, RADC), coordinated by Bance Hom.
Bance Hom is a consultant in semiconductor, fiber optics, wireless and other high-tech fields. Sandra Winkler is a senior analyst at Electronic Trend Publications, www.electronictrendpubs.com. For more information, contact Bance Hom at Consultech; 510-743-1956; E-mail:[email protected].