Silanes improving mould compound adhesion
Silane is a surface treatment agent that helps epoxy molding compounds and plastic packages adhere firmly to metal lead frames, reducing delamination in semiconductor devices, especially after moisture testing and reflow soldering.
Delamination between the molded plastic package and the lead frame is one of the most common failure modes of packaged components. When moisture seeps into the delaminated gap, reflow soldering at 260°C causes the water to vaporize suddenly, cracking the package in what is commonly called the popcorn effect. Improving adhesion is the fundamental way to prevent this.
Silver-plated and nickel–palladium–gold plated lead frames have inert surfaces that are very difficult for molding compounds to bond to without treatment. These are also the component groups that automotive customers often require to meet moisture sensitivity level MSL 1.
How silane works
A silane molecule has two functional ends. One end is an alkoxy group, which after hydrolysis bonds chemically with hydroxyl groups on metal, oxide or glass surfaces. The other end is an organic group such as epoxy, amino or vinyl, which reacts with the molding compound. In this way, silane forms a bridge between two materials that are otherwise difficult to bond.
Choosing the right silane
- Epoxy silane: highly compatible with epoxy molding compounds, the most common in semiconductors.
- Amino silane: improves adhesion with many resins, used as a general-purpose primer.
- Mercapto silane: bonds well to silver-plated and gold-plated surfaces.
- Vinyl and methacryl silanes: used with silicone and acrylic resins.
In many cases, delamination is not caused by the adhesive but by the surface. A silane layer just a few nanometers thick, if applied correctly, can completely change moisture test results. — Application Chemist, AMS Vietnam
Treatment methods
Solution preparation
Silane is typically diluted to 0.5–2% in an alcohol and water mixture, with the pH adjusted to 4–5 using acetic acid for hydrolysis. The solution should be used the same day because silane self-condenses over time.
Coating and drying
Lead frames are dipped or sprayed, then dried at 100–120°C for 10–15 minutes to form bonds. A silane layer that is too thick actually reduces adhesion, so concentration and dipping time should be controlled.
Direct addition to resin
Some molding compound formulations already contain silane. For in-house formulated adhesives, 0.5–1% silane can be added to the mixture.
Evaluating effectiveness
- MSL moisture sensitivity testing per J-STD-020 before and after treatment.
- C-SAM ultrasonic scanning to detect delaminated areas.
- Measuring the shear strength of the molding compound on the lead frame.
Silane is a moisture-sensitive chemical; it must be stored in a sealed container, blanketed with nitrogen after each opening and used within the recommended shelf life.
AMS supplies silanes from Shin-Etsu and Dow to the semiconductor and electronics industries. Contact AMS Vietnam for advice.
