Properties that can affect conductive die attach film selection
Die mounting temperature
Supplier recommendations for bonding temperatures are a good place to start. The stipulated temperatures show the strongest bonding strength for specified materials. However, because the surfaces to be bonded can vary (roughness, type of material and level of contamination), the die attach process will need to be tested for each new product before scaling production.
Therefore, optimization is necessary to determine the bonding force, temperature, and bonding time that will achieve the strongest bond possible with minimum bondline thickness and negligible voids.
Tg, viscosity, and low stress
The stress characteristics of packages with film adhesives are a function of CTE, elastic modulus, and Tg to varying degrees.
Low modulus DAF can absorb a large amount of stress before fracture occurs. However, because the modulus for thin films at high temperatures is difficult to determine, adhesion at high temperatures is an important parameter when selecting a DAF variant. A DAF with high Tg is recommended at high temperatures. The Tg is the point (range) where the film changes from glassy solid to rubbery. As such, operating close to Tg means that the reliability of an assembly can become compromised. Testing the elastic modulus of a DAF at different temperatures is important.
Dicing
Optimizing the dicing parameters ensures dicing occurs with minimal chipping, cracking, or flying die. Die fly when the adhesion between DAF and dicing tape isn't strong enough. On the other hand, too strong an adhesion will lead to difficulties for die pick up. Appropriate film selection makes a trade-off between these competing challenges.
Good wetting
For handling thin wafers and large die sizes, films characterized with excellent wetting are recommended to enable complete and robust adhesion to substrates. This removes the need to apply excessive pressure that may damage the wafer. On the other hand, poor adhesion in large die can lead to warping and other problems.
Compatibility
For wafers, a film that is compatible with the various wafer processing stages will add a higher level of flexibility and performance to the manufacturing process. For other applications, the type of resin a film is composed of should agree with the substrate, whether metal, glass, ceramics or organics and also the end-use device. The manufacturer's guidelines will prove useful here.
Curing
With every application of heat, the film gets partly cured. This is not a significant issue for processes with few heating steps. However, for multichip packages, the total heat applied for heat treatment at wafer lamination, die attach and wire bonding needs to take into account the curing required for molding. There's a risk of delamination when attempting to mold packaging with fully cured DAF.
Certain films need heat and pressure to be applied simultaneously during curing, e.g., anisotropic films (so that metal filler particles become aligned) or where plastic substrates are involved (so that trapped air and volatiles can be pushed out). Techniques for preventing trapped air or moisture bubbles include curing under vacuum and vacuum baking surfaces before adhesion.
Storage:
DAF contains UV-sensitive components. Therefore, exposure to ambient light for periods of time can initiate curing. Rolled film can be stored from 6 months to a year (depending on the manufacturer) in a dark place before it loses its effectiveness.
Ideally, based on their storage time and minimum order quantity, DAF is suited to large scale manufacturing where these requirements will be matched by production volume to prevent wastage.