Our team are passionate about all things CFD and love sharing their findings. In this issue, Product Marketing Manager and prolific blogger, Robin Bornoff describes how to bake a games console in an oven!
Video games consoles appeared in the early ‘70s and since then have grown to be a major industry now worth approximately $30bn per year. Enjoyed mostly by the young, it has not stopped the young at heart from continuing gaming well into middle age and often into the early hours. This includes my colleague Steve Hanslow (FloTHERM software development project manager) who, together with his love of coding, heavy metal and guitars is every inch a geek. When his games console stopped working it wasn’t long before he had determined the likely cause, got his screwdrivers out and embarked on a particularly novel method of repair.
It was estimated in 2007 that such consoles account for 75% of the entire world’s general purpose computational power. Such power comes in the form of components that are soldered onto a printed circuit board (PCB). The components are packaged integrated circuits, small wafers of (commonly) silicon encased in a package that allows the micro circuit to be connected to others on the board. It is these wafers that are actually referred to as ‘chips’. Modern day CPUs and GPUs are packaged such that an array of solder balls are used to connect that component to the PCB. The PCB manufacturing process sees the component placed on the surface of the PCB, the PCB assembly put through a hot air oven that causes the solder balls to melt such that they form a solid contact with the board, the so-called solder reflow process. The PCB then cools, the solder solidifies, the PCB is then put into a box and the sold product.
In operation the components on the PCB dissipate power and get hot as a consequence. Extreme temperature variations in the component and PCB lead to stress and strain build up so that it can ultimately result in cracks appearing in the solder balls. This break in the electrical connectivity results in the failure of the system to function properly. Gamers will be well aware of RROD and YLOD indication of such failures (if you know what those acronyms stand for without resorting to Google then you too have achieved geek status) and it was this that Steve encountered.
Commercially available reflow ovens run into the $100ks but the humble domestic oven can reach the temperatures necessary to reflow the solder, reconnecting the component to the PCB and thus restore electrical function.
To do this Steve removed the console PCB from its housing, used bolts to ensure an air gap under the PCB (Figure 1) and then placed this on the middle shelf of the oven (Figure 2) and turned it on.
Central to achieving a good reflow process is control of the rate at which the PCB heats up, the time the PCB experiences sufficient temperature to melt the solder and finally the rate at which the PCB cools. The key is to make sure all the components experience solder melt temperature but do not get so hot in the oven so as to break them. If all components heated up at the same rate then this would not be an issue. However, larger components heat up more slowly and smaller ones more quickly leading to temperature variations during the reflow process. Lead free solder melt temperature is ~217degC and commonly the maximum temperature a component can withstand during reflow is ~250degC. This ‘reflow window’ is therefore about 35degC.
Thermal simulation using tools such as FloTHERM or FloTHERM PCB allow for such transient thermal responses to be predicted. Figure 3 shows the temperature variation over the PCB when the CPU component (the one that has cracked solder ball connections) reaches solder melt.
One way of controlling this temperature variation is to shield those smaller components that would otherwise exceed their maximum temperature. In keeping with the DIY nature of this repair Steve used bluetack shielded by foil to cover those components, minimising their temperature rise thus reducing the temperature variation at the point of reflow (Figure 4).
The rate at which the PCB assembly cools after reflow is also important. A fast cooling rate creates a fine grain structure within the solder that improves subsequent mechanical stability. Such a rate is not possible to achieve simply by turning the oven off, nor by also opening the oven door. To achieve the required 4 degC/s cooling rate Steve placed a large fan next to the open oven door to help induce convection of cooler air into the oven, accelerating the cooling rate (Figure 5).
The location and orientation of the fan will control how much cool air is pushed into the oven. Simulation is well placed to determine this. In this case, having the fan blow parallel to the open oven door induces a recirculation in the oven that pulls in just enough cool air to achieve the desired cooling rate (Figure 6).
Suffice to say that after Steve reassembled the PCB in the console chassis he was not at all surprised to find that it was working properly again. Gaming has continued unabated ever since.