The morning rush hour is in full swing and the metro station is crammed with people. I reach for my mobile phone to call work and announce a possible late arrival, but at that moment someone bumps into me and my mobile slips out of my hand. It makes a near perfect somersault, hops down a few stairs and comes to rest at platform level. After having survived numerous such drops this final one proved lethal. I pick up its remains, the plastic shell, the broken display and the battery.
The loss of a mobile phone or any other electronic gadget may be annoying, but there are far more serious situations, such as on aeroplanes or in medical equipment, where the malfunction of electronic equipment can be much more severe, even life-threatening. In our technology-driven world we have become reliant on machines and their proper functioning. In an electronic device, components are soldered onto the printed circuit board to provide electrical (and mechanical) contact. A typical computer is “held together” by several thousands of such solder joints. But these joints are vulnerable to frequent mechanical shocks, vibrations, abrupt temperature changes – in fact, to all the conditions we face in daily life.
Before I pack away the pieces that once were a working mobile phone, one particular piece catches my eye, a square chip with an array of solder balls attached to it. Components of this kind – so-called ball-grid arrays – are frequently used to pack a large number of electrical connections into a tight space. The solder balls provide the electrical and mechanical contact between the chip and the printed circuit board.
Ball-Grid Array mounted on a printed circuit board
The ball grid array is soldered to pads on the printed circuit board. These pads are made of an amorphous alloy of nickel and phosphorus. Phosphorous is a physiologically important element found in many biological molecules, but in its pure form it is highly flammable and can easily be evaporated. Though its appearance in electronics seems strange at first glance, the industry likes this type of alloy, because it solders well and can easily be applied to copper surfaces – such as connectors on a circuit board – by reacting a nickel salt solution using sodium hypophosphite.
In my now-defunct mobile phone the ball grid array has broken off the board, somewhere at the interfaces between the solder balls and the nickel-phosphorous pad. Solders are ductile tin-based materials, but during soldering they chemically react with the solid metals being soldered – such as the nickel-phosphorous alloy – to form intermetallic compounds. These compounds are brittle and so may break during an impact.
Diagram showing the influence of the Phase Diagram on the whole materials design process
As the mobile phone incident shows, the selection of a material requires compromises between desirable physical and mechanical properties, processing ease, behaviour during operational life and, increasingly, consideration of options for end-of-life re-use, re-cycling or disposal. The development of alloys and other materials for use in electronics is the focus of the Institute of Inorganic Chemistry / Materials Chemistry at the University of Vienna. Compilation of the phase diagram is the basis of research into alloys. Each diagram, specific to the material, acts like a map. Instead of rivers, roads and towns, it shows melting temperatures, compounds, transformations and many other things that are important for materials design. With the help of the nickel-phosphorous-tin phase diagram, which is currently under investigation in our laboratory, it is possible to understand the processes that occur when a tin-based solder is used to join nickel-phosphorous surfaces. The compounds formed during soldering are related to the phase diagram and so understanding this diagram must be the basis for any further developments in its practical application.
Phosphorous is difficult to work with. Preparing samples of nickel-phosphorous-tin alloys in the laboratory requires that those components be heated together in a sealed crucible. The phosphorous evaporates easily, causing pressure to build up and, sometimes, the crucible to rupture. For this reason, our work has concentrated on more nickel-rich alloys. Even in this restricted range of compositions, a very complex variety of different substances has been found to co-exist, which possibly can form in a solder joint and determine its properties.
Our understanding of the nickel-phosphorous-tin system, and consequently of this type of solder joint, still has a long way to develop. Most of the formed compounds are brittle, which therefore determines the strength of solder joints. In the future, more complete knowledge of the phase diagram and the properties of the compounds it describes will allow us not only to understand the behaviour of a solder joint (or any material) but also to predict that behaviour. Improvements to the soldering process and to the finished product based on this knowledge will enhance the durability and reliability of electronic devices. It is likely that in a few years my mobile phone would have survived being dropped down the staircase.
I finally arrive in work and set my broken mobile phone on my desk. Switching on my computer, I realize that the thousands of components and solder joints coming to life also contain ball grid arrays and phosphorus. Contrary to my mobile phone – and not having been dropped down a flight of stairs – the computer is working fine. A quick check of the most recently acquired research data leaves me satisfied and, after all, it is still not a bad day.
