Silicon
Silicon is a semi-conductor material. Silicon lets electrical current pass under specific conditions. This is like the light switch and copper wire in a house. This can be useful, for example: If we were to take 10,000 light bulbs arranged in a 100X100 square and each light bulb had a switch then we can make pictures or words by turning lights on and off.
Silicon switches or “Gates” form the basis of modern digital circuitry.
The Wafer
Making the kind of Silicon that works for modern circuits is a difficult process costing Billions in capital equipment, millions in raw materials and thousands of labor hours. The process begins with making a Silicon Ingot. The ingot is a large single crystal made of pure Silicon in a large furnace. The ingot can be as tall as a person and diameters as large as a foot.
The single crystal Silicon ingot is sliced to round wafers like a bread loaf. Each wafer looks like a disk made of a shiny metal.
The FAB
At this point, the Silicon wafer is processed in a “Fab”. The circuitry is painted on the wafer in layers as it moves down the “Line” in the fab. This is similar to printing a picture with a spray paint and stencil. The number of layers depends on the intended function. At the end of this process, the wafer is diced to square pieces.
The Die
These squares are called the “chip” or the “die” interchangeably. Each die has a piece of Silicon with circuitry printed on it. The circuitry closest to the Silicon is called the “Front End Of Line” or FEOL and it is usually very delicate. The layers on top are called the “Back End Of Line” or BEOL. The last layer has exposed Aluminum pads with a Silicon Nitride cover called “Passivation”.
All the layers are a combination of a conductor metal and non-conducting material (Usually a ceramic)
Happy die, Sad die
All of this wonderful electric wizardry we can make from Silicon is great, but the mechanics are difficult. Silicon, mechanically, behaves like a ceramic. If you bend it, it breaks like glass, or kitchen tiles.
Silicon, like most materials, expands when heated and contracts when cooled. The coefficient of thermal expansion of Silicon is around 3 ppm/°C (Parts per million per degree C). In plain words this means that a one meter long rod of Silicon will expand by three micro-meters over one degree Celsius increase in temperature. That is low compared to Copper at 17 ppm/°C or 23 for Aluminum.
The temperature is very high (1,000-300 °C) when the fab deposits layers on the wafer. As the wafer is cooled to room temperature, the different materials contract with different CTE and the wafer can be warped. After the die dicing and singulation, each die will also have some small warpage.
The CTE problems continue…
Each individual die is connected to the rest of the electric circuit using a Solder. The rest of the circuit can be a PCB (Printed Circuit Board) or a “Substrate” (11-20 ppm/°C). Solder is a catch-all term for any metal that connects two pieces by having a lower melting temperature. Any material can be a solder, but Tin based metal alloys are the most common in electronics. The most common solders are a combination of Tin, Silver, Copper and Lead with a CTE of 20-25ppm/°C.
The soldering process
A Solder paste is placed on the PCB surface. The Die is placed on the paste and the whole thing goes in to an oven. This is called a “reflow” process. The temperature profile depends on the solder material but generally, the temperature goes up, stays above the solder melting temperature for a few minutes and the cools down to room temperature. At the end of this process, we have the “bottom” of the die, solidly welded to the PCB or substrate. (A substrate is just a nickname for a miniature PCB that holds one or multiple dies with optional additional components. A substrate, with a die is a component.). The “top” of the die still has the exposed Aluminum pads. A metal wire (usually Gold) is thermo-sonically bonded to the Al pad and a copper pad on the PCB. That is a fancy name for the mechanical process that heats up a the tip of a gold wire and pushes down on the pads and creates “wirebonds”.
The gold wires are very small and delicate, to protect them, epoxy is applied on top of the whole assembly. The plastic “overmold” encapsulates the die, solder and wires.
Assuming that the assembly survives this whole process without any cracks, the circuit is now ready for normal operation.
CTE mismatch
The CTE of each material causes stresses in the whole assembly as it heats up and cools. The die, PCB, Solder and overmold bend with each temperature cycle. The softest and worst mechanical material in the assembly is the solder. There is a reason why we don’t make bridges out of Tin. The repetitive stresses in the Solder cause it to crack and eventually fail due to fatigue.
This problem is so prevalent that there almost every electronic design cycle includes a thermal cycling test.
Copyright Gil Sharon April 30, 2025. All rights reserved.