Specialty Chemicals, Adhesives & Plastics
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Underfills are (mostly) epoxy based thermo set materials that contain (mainly) silica fillers and are designed to flow in the gap between the board and the component. They are typically applied after solder reflow and require heat cure.
Underfill materials need to have a low viscosity to allow them to flow under the components by capillary force. Substrate heating is also sometimes required to facilitate a good flow process and allow the capillary force to pull the underfill under the package.
Thermally, underfills increase the thermal package reliability by distributing the stress across the surface of the die or substrate instead of being concentrated in the solder balls especially during thermal cycling (TC). It is important to point out that the peripheral (outside) bumps are stressed the most.
Mechanically, it is used to increased mechanical shock resistance in various configurations such as button push, drop resistance and bending. Specifically for drop performance, the UF properties provide exquisite improvement.
Finally, underfills reduce moisture absorption and chemical contamination. Moisture can create dendrite formations that would result in electrical shorts and create bridges in places where traditional conformal coatings wouldn't be able to penetrate.
Underfills come in two types. Semiconductor (die) level and board level. The board level types are the ones applied between the finished IC package (ie BGA) and the PCB.
There are three main types of board level underfills:
Full Capillary Underfill
Partial Underfill - Corner Bond
Partial Underfill - Edge Bond
Capillary underfills have a pretty standard operational process. They are applied and cured post reflow so they don't have to withstand extremely high temperatures. The common steps one would have to follow are:
Before applying the underfill we need to prepare the substrate for the application. As a first step we need to clean the substrate with either solvent wash or plasma to improve the surface tension and reduce flux residues.
Afterwards we prebake the board to remover the moisture and any remaining solvents.. Normally this is not needed after reflow but it can be a necessary step when we had the boards stored for long, in order to remove the moisture trapped in the board/components that can create outgassing voids when curing.
Heat that is typically in a range between 60°C and 100°C can reduce the viscosity and the surface tension while improving the wetting properties of the substrate.
Of course it is not as simple as it sounds (is it ever?). In order to find the ideal underfill process there is a series or steps you need to take to optimize it.
To begin with we need to calculate the required volume: L x W x H (gap) in mm. Using this value, times the specific gravity (can be found in the TDS) we get the volume measured in mg. Granted this is a rough calculation, not taking into account the solder bumps, but it is good enough.
Let's say that the calculated volume is 10mg. In this case we start with passes of 1 mg. Dispense 1 pass and wait until the material is under the component, dispense second pass... proverbially rinse and repeat. When the underfill is creating a fillet on the opposite site than the component is finished. Now it is the time to check the underfill quality by flat and X sections.
Next step is to reduce the amount of passes to check the maximum possible speed. Flow out and top die contamination should be the main points of focus and concern . If needed, changing hardware can also be a solution. A small nozzle can result in an unstable dispense process. With jetting the flowrate is determined by the dot weight, so the machine will calculate the amount of dots needed for the programmed dispense pass. For needle dispensing the flowrate is calculated by the amount dispensed per second. The robot speed will be adjusted to ensure the right volume per pass is dispensed.