Improving Flip Chips With Underfills

Interview conducted by G.P. ThomasApr 25 2013

Dr. Walter Brenner, Technical Director of Master Bond Inc., talks to AZoM about the future of flip chip packaging and advances in underfill formulation and processing.

What does a typical flip chip package involve?

The flip chip package involves the construction of tiny bumps of solder that allows for higher interconnect densities coupled with shorter interconnect lengths. This ultimately helps in attaining greater speed. These tiny bumps of solder are placed on the chip’s working surface during fabrication, and the chip is mounted facedown onto a substrate. These flip chips offered many advantages over wire bonded chips, including cost savings and increased reliability.

Why were epoxies first used with flip chips?

Flip chips may be attached directly to a circuit board or placed into a ball grid array (BGA) or other package. To prevent damage to the solder bumps, packaging and product assembly engineers began using epoxies to fill in the gap between the chip (usually, silicon chip) and the substrate. Epoxies help provide mechanical support, protect against moisture and lessen the strain on solder joints.

What were some of the challenges facing design engineers with the early flip chips?

Due to the mechanics of their construction, flip chip assemblies are more sensitive to the effects of temperature excursions than their wire bonded counterparts. The stiff solder bumps in flip chips are firmly sandwiched between the chip and the substrate, and are not free to flex. During thermal excursions, the silicon chip expands and contracts at a different rate from that of the substrate. This induces stress, which is concentrated in the solder joints. Just how much each material expands or contracts depends on its length and its coefficient of thermal expansion (CTE), or the ratio of the change in length per degree temperature change to the initial length. (ppm/C)

The differences between the CTE of the silicon die and the substrates used in early flip chip assemblies induced enough stress to cause solder fatigue during thermal cycling, primarily in the outermost solder bump interconnects.

An underfill material is typically used to help couple the chip and the substrate effectively to act as a thermo-mechanical bridge. This distributes some of the stress especially during some of the thermal excursions thereby preserving the integrity of the solder joints. Moreover, the substrate’s motion is constrained to a certain extent by the motion of the silicon chip, and most of the stress is dissipated non-destructively in the substrate.

What are the different types of underfills?

Flow and non-flow underfills are the two broad types of underfills. Flow underfills are typically used for flip chip applications. Non-flow underfills are dispensed prior to the attachment process, whereas flow underfills are dispensed in a fixed volume post attachment.

How did underfills improve flip chip performance and reliability?

By distributing thermally induced stress, underfills made it possible for silicon chips to be directly attached to some low cost organic materials that present an even greater CTE mismatch than ceramic materials. As a result lower cost flip chip packaging technologies, including direct chip attach (DCA), wafer level chip scale package (CSP), and plastic BGA assemblies were developed. These technologies paved the way for the higher density, higher performance and affordable semiconductors that have revolutionized the industry.

Underfills also protect interconnects and the active surface of the chip from moisture and other environmental factors while providing mechanical support for flip chip assemblies. By bridging the assembly mechanically, the underfill prevents cracks in interconnects that often result from circuit board flexure during drop testing.

What are some of the properties required of the underfill to ensure reliability?

Underfills are formulated to be dimensionally stable, so that they can withstand thermal and mechanical shock. This will also maintain the alignment of the chip and substrate, which in turn, will minimize stress on the solder joints. A Master Bond system which provides outstanding dimensional stability is EP3RRLV. Featuring rapid curing at temperatures as low as 230-250°F, this no-mix formulation offers high mechanical strength properties, superior thermal conductivity and exquisite flow properties, with an unlimited working life at room temperature.

During the selection process for the epoxy, one thing to be kept in mind is the modulus of elasticity of the underfill. It should be high enough to ensure good mechanical coupling between the chip and the substrate. The CTE values of the substrates being coupled should also be considered.

Common epoxy resins have CTE values that are roughly three times that of solder, so formulators add fillers, such as aluminum oxide or silica, to reduce the CTE. Fillers also increase the modulus and enhance the dimensional stability of underfills. Based on the selection, as well as the size of the filler and the content added thereof, the viscosity of the product might increase slightly.

Both silica and aluminum oxide help improve dimensional stability. Aluminum oxide is renowned for its thermal conductivity which conducts heat better. A relatively small amount of silica filler should greatly enhance dimensional stability of an underfill without significantly increasing its viscosity, whereas a larger amount of aluminum oxide is required for effective heat transfer, at the expense of an increase in the viscosity.

Two Master Bond systems that can be used as underfills are EP30 and EP30AO. EP30AO is filled with aluminum oxide and offers good thermal conductivity as well as a lower CTE index than EP30. Both products offer good flow properties with EP30 having a lower viscosity. EP30LV-1 has an even lower viscosity than either EP30 or EP30AO.

What kinds of failure might flip chips face?

The predominant types of failures in flip chips are delamination or cracking. During environmental testing, flip chip assemblies are subject to high temperatures, various degrees of thermal cycling and humidity testing. Should moisture penetrate the underfill, the excessive heat applied during solder reflow process may produce enough vapor pressure to stress the joints. If the resulting stress exceeds the adhesive strength of the underfill, delamination between the underfill and the die, or between the underfill and the substrate, may occur. To compensate for this possibility, underfills are formulated to resist moisture and have enough adhesive strength to overcome high vapor pressures that may build up within the underfill during solder processing.

To maintain their thermomechanical properties throughout wide temperature ranges flip chips are exposed to, underfills are typically formulated to have high glass transition temperatures. The underfill material selection also involves using a material which can withstand some of the thermal cycles during testing.

What does the future hold for semiconductor components and their adhesion?

Continued advances in underfill formulation and processing, promise to pave the way for faster, simpler assembly and packaging processes, and increased product quality. New products and processes are being constantly developed to speed up curing cycles.

Master Bond continuously formulates various epoxies which can be used as underfills. Based on specific requirements, Master Bond is also capable of custom formulating a suitable product.

About Dr. Walter Brenner

Dr. Walter Brenner has been the Technical Director of Master Bond Inc.--a leading manufacturer of high performance adhesives, sealants, coatings, potting and encapsulation compounds and impregnation resins--located in Hackensack, New Jersey, for over 35 years. Educated at City College and Brooklyn Polytechnical Institute, Dr. Brenner received his PhD in Polymer Chemistry from Brooklyn Polytechnical Institute. He served as a renowned Professor of Chemical Engineering at New York University and has served as a consultant for various US Government agencies. He has authored three technical books and is the holder of numerous patents. Dr. Brenner is also credited with being the first person to develop electron beam radiation curing.