Low-Moisture Polymer Prevents Popcorning
By J. Scott Moore
Delamination of polymeric packaging materials due to trapped moisture can now be virtually eliminated for many types of packaging, thanks to a new die attach adhesive developed by Johnson Matthey Electronics (JME; San Diego, CA).
The delamination, usually termed "popcorning," is caused by moisture, often introduced by die attach adhesives, absorbed by polymer packaging. High temperatures (approaching 240*C) during solder reflow vaporize the trapped water, and the consequent steam pressure causes an explosive delamination. This phenomenon ruins the package's heat dissipation and other performance characteristics. Popcorning, which destroys up to 15 - 20% of polymer packages, is particularly troublesome because it occurs not at the chip producer's site, but during surface mount at the customer's location. Generally, the entire board is lost and must be re-worked.
When the trapped moisture concentration is more than 0.1% by weight, the hygrothermal stresses exceed the package's interfacial toughness and cracking occurs. Most available die attach pastes have 1 - 2% moisture content, and epoxy-based molding compounds have around 0.3% moisture.
Normally, die attach pastes form a one-dimensional crosslinked network. The JME engineers chose a modified cycloolefin thermoset (MCOT) polymer for die attach. On application of heat, the liquid prepolymer molecules combine to form a tight, three-dimensional multimolecular network. The resulting high crosslink density prevents penetration of moisture, and gives the material a high tolerance to stress with very little weight loss during cure. The MCOT absorbs less than 0.1% moisture.
Compared to epoxies, the JM7100 MCOT has 25% lower adhesive strength. The tradeoff between strength and moisture-induced stress is essentially unavoidable: removing a hydroxyl group from the monomer, which is the most important factor in enhancing moisture resistance, also lowers adhesive strength. Overall, the paste has good mechanical strength, and adhesive strength is still twice that required for die bonding.
The MCOT adhesive can be made both thermally and electrically conducting. Besides preventing popcorning, it eliminates the need for laminate prebaking, reduces package warpage, and lowers cure cycle temperature to 100*C. The MCOT also reduces the cure cycle from several hours to as little as five minutes. According to JME, the MCOT die attach paste will completely prevent popcorning in plastic BGA, TQFP and similar packages under normal semiconductor packaging conditions.
A modified form of the MCOT, named JM7204, is used for flip chip underfill and die attach on flexible circuit substrates, tape BGA and chip scale packaging. The modified material can also function as a thermal interface adhesive. Unlike the more common thermal grease, the thermal performance of the olefin adhesive will not degrade after thermal or power cycling.
For more information: Bernard Ho, Johnson Matthey Electronics, 10080 Willow Creek Rd., San Diego, CA 92131. Tel: 619-536-4787.
Dr. J. Scott Moore is a consultant based in New York. Having worked many years as a scientist in the computer industry, Dr. Moore now writes technology based articles for several publications, and provides business and market intelligence services to a range of clients. His recent work includes conference preparation and a series of in-depth reports on emerging technologies, emphasizing semiconductor and electronics applications.