Previously in FRONT of HOUSE, we discussed some of the complications behind getting product certified — such as UL — for safety or electromagnetic emissions. That topic quickly devolves into alphabet soup, and can become rather expensive headache for companies designing product. It also drives tradeoffs beyond those involving the raw performance of a product.
This month, we approach RoHS (“Row Haas”), a topic that has similar ramifications for manufacturers, but is less sprawling in scope. RoHS, or Restriction of Hazardous Substances Directive, was created by the European Union in the early 2000’s to restrict manufacturers from using a variety of substances in the manufacture of their products for the benefit of the end-user and environment. This month, we will touch on the materials subject to RoHS, and the ramifications electronic components when companies can no longer use RoHS materials.
Environmental Gains
It is appropriate to lead with the manifold environmental and health gains from RoHS, despite the challenges to product design. It is our hope that the reader can appreciate that the substances subject to RoHS have clear, often preferred use in products. When we choose to not use these substances, tradeoffs and ingenuity are required. We choose to not use RoHS substances not only from the force of law, but also from the knowledge that their presence, especially as uncontrolled waste, could have long-term environmental implications.
Against this backdrop, the EU highlighted a number of substances that could no long be present in quantities more than 0.1% (or 0.01%) by weight across a broad selection of product classes. Items as diverse as toys, lighting and medical devices are covered within the confines of RoHS. Despite its EU origins, RoHS has grown towards de facto adoption around the world.
The manufacture of electronic components and assemblies contains a huge array of chemical elements and compounds. As an undergraduate, I remember disassembling and analyzing the chemical makeup of the components within a microwave oven. Our class found more than 70 elements from the periodic table — either in trace quantities or specifically present to serve a functional purpose. While some of these elements could be eliminated by improved purification of the starting materials, many were absolutely essential to the microwave’s operation.
And so it is with the RoHS substances. All six have clear engineering use cases, and all six would likely still be used without their respective environmental problems. We have chosen to place boundaries on our engineering for the sake of the environment.
Problem Children
What are these six problem substances (and some of their common uses), you might ask? They are:
Polybrominated biphenyls — Fire retardant additive
Mercury — Lamps and switches
Cadmium — Solar cells, electroplating and phosphors
Lead — Batteries, solder, solar and sensors
Hexavalent chromium — Corrosion resistant coatings
Polybrominated diphenyl ether — Another bromine-containing fire retardant.
We should note that RoHS specifically excludes batteries.
While cadmium and chromium are both used to provide corrosion resistance finishes on steel, there are other more affordable options to protect steel against moderate corrosion. For the pro audio industry, lead is by far the most important material on the RoHS list. Before RoHS, lead was a ubiquitous component in solder. To understand the wide reaching repercussions of restricting lead in pro audio gear, and electronics in general, requires understanding solder itself.
A Bit About Solder
Solder is the glue of electronics, the conductive stuff that lets electrons run from one bit of circuitry to another. But for modern surface mount electronics solder is more than just a path for current from point A to point B, it also forms the structural glue that keeps the electronic components together. Solder paste is stenciled on top of a printed circuit board (PCB) and then a precise robot places the electronic components atop each little section of solder left by the stencil. Then the board and all the electronics are placed in an oven to undergo a process called “reflow,” which melts the solder and permanently affixes the electronic components to the PCB.
Solder thus ends up encapsulating a great number of properties. When melted, it has to wet electronic parts, and then have sufficient surface tension to hold those parts in place through the reflow process. It has to form conductive joints with electronic components, but cannot be overly chemically reactive with those same parts. It must have acceptable mechanical properties for strength and flexure. Solder requires being compatible with parts that have different rates of thermal expansion. Solder must be stable at the temperatures common in electronics, but also has to melt at a low enough temperature to not exceed the “thermal margin” of the electronic components during reflow. And solder should be capable of all of this without excessive amounts of toxicity.
All About the Alloys
It is the quest for low melting temperature that caused the pioneers of electronics to the metal element tin (Sn). Other than the far more expensive and rare indium, tin is the lowest melting metal conductor that also exhibits acceptable chemical stability and mechanical properties. Tin also happened to be well known and well characterized by the dawn of the electronic age. Tin was the obvious choice for solder, but to reduce the melting point even further an alloy of tin and lead was used rather than pure tin. To understand how an alloy helps lower the melting temperature requires some chemistry.
Alloys are combinations of different elements, usually metals, together to make a new substance. Steel is one common alloy composed primarily of iron and carbon. The same free movement of electrons that give metals their conductivity also allows metal atoms to readily combine in different crystal arrangements. Just like you can only dissolve so much salt in water, these different crystal arrangements can accept only so much of one certain metal or another. This is called the solubility limit.
The roadmap to alloys and their crystal arrangements is called a phase diagram. On vertical axis of a phase diagram is temperature. The temperature is displayed high enough so that all elements in the alloy are melted to liquids. On the horizontal axis of a phase diagram the amount of a given element, usually express in percent by weight.
Fig. 1 shows a schematic phase diagram of a two-component alloy composed of “A” and “B.” At the top is the liquid region (L). The line bounding the liquid region touches either side of the diagram at the melting points of A and B. In the solid area there are two crystal arrangements represented by alpha (α) and beta (β). The alpha crystal arrangement contains mostly element A, and the beta crystal arrangement mostly element B, due to the solubility limit.
Below the pure liquid region are lower temperature regions where you have a mixture of liquid and α or liquid and β. Which arrangement you have depends on whether the alloy is mostly A or mostly B. Moving further down in temperature you hit a horizontal line. This line is called the “solidus.” Below the solidus, all parts of the alloy have frozen solid. Once you cross the solidus, your alloy is a mixture of alpha and beta crystal arrangements. The percentage of alpha and beta will depend on the overall alloy composition.
Notice there is a special point in the middle of the solidus line where a certain composition of the liquid dips down to the same temperature as the solids alpha and beta. This lowest melting point is called the eutectic composition. At the eutectic the both crystal arrangements melt simultaneously directly to the liquid. One way to think about this is that the disorder of the system changes most strongly when both crystal lattices decompose simultaneously, because the liquid has less structure to its atoms than either solid crystal. Since the change in disorder is what helps drive the melting process, you want both crystals to transition at the same temperature when seeking the lowest melting point. If you melt the system at point other than the eutectic composition, it will first partially melt one crystal arrangement or the other and then finally form a pure liquid.
Lead Tin Solder
This brings us to the real phase diagram for an alloy between lead and tin (Fig. 2). This diagram has all the same features as the schematic example in Fig. 1, but with the real world melting behavior of lead and tin. The eutectic composition, illustrated below the #1 on the diagram, is at approximately 37 weight percent (wt.%) lead, and the “eutectic temperature” is about 40 degrees lower than the melting point of pure tin. Thus by adding some lead, we have created a solder alloy with a meaningful reduction in the melting temperature. This allows solder to melt readily at a lower temperature, protecting electronic components.
For a long time, solders in electronics were made at compositions near the eutectic composition of lead and tin, typically at 37% lead or 40% lead by weight. These compositions exhibit a desirable mix of properties for electronics applications, but ultimately started with the key feature of low melting point afforded by the eutectic composition.
Enter RoHS
So when the European Union swoops in and bans the other low melting temperature metal (lead) that you were alloying with tin to make a low melting temperature solder then big complications have to be solved.
Among these are:
- Finding another low melting temperature alloy when the most obvious candidate has been eliminated;
- Improving the thermal margin of all other parts to accommodate a higher melting solder;
- Understanding how the replacement solder alloy(s) behave on PCBs; and
- Learning all of the idiosyncrasies of the new solder composition for mechanical properties, corrosion, inspection equipment, etc.
For the change away from lead/tin solders, common replacements are alloys at or near the eutectic in the tin-silver-copper alloy family. These alloys have a low melting point, but not as low as that of the lead tin alloys that they replaced. Typical melting temperatures are 10 to 20 degrees above their predecessors.
Finding eutectics becomes more complicated every time you add a new element to a chemical system. For a phase diagram in a system with three elements, there are three diagrams like the ones shown in the figure here, one for each of the binary combinations. The so called ternary eutectic is found within a 3D volume bounded by these three binary diagrams. For systems with more than three components, you can no longer readily visualize the phase diagrams as they exist in more than three dimensions. The allure of systems with more components is the chance to find eutectics at ever lower temperatures. Four component systems based around tin are currently being investigated as solders.
Conclusion
With the adoption of RoHS, the removal of lead from solder resulted in a titanic shift to the world of electronic manufacturing services (EMS). Whole new compositions for the very glue of electronic circuits had to be rolled out and implemented in a broad array of products. While pro audio electronics are a very small fraction of overall electronics manufacturing, they were swept along for the ride.
This process was not without incident, although I am not aware of any incidents directly affecting pro audio vendors. Saying that, I am sure someone will email me a horror story! One the biggest technical snafus widely suspected to be a consequence of the transition to lead free solders were the numerous technical problems had with the Microsoft XBOX360 video game console.
Pro audio should be thankful for the sheer scale of the larger EMS world. It was the collective resources of many players working worldwide to execute on a fundamental technical change to how electronics are assembled. These same industry tailwinds drive the ever-cheaper cost, and increased performance, of our mixing consoles, speaker processors, amplifiers, etc.
