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Tech Tips: Die Attach Dispensing Methods

ie attach material selection and process implementation play crucial roles in any microelectronic assembly. The chosen attach methods ultimately affect die stress, functionality, thermal management, and reliability of the assembly. Die attach applications are designed to optimize mechanical attachment of the die to the substrate, to create a thermal path from the die to the substrate, and to create an electrical path for a ground plane connection. Some of the more commonly used die attach materials in the microelectronics industry today are epoxies, polyimides, thermoplastics, silicones, solders, and special low outgassing, low stress, anisotropic adhesives. D

Most fluid adhesives exhibit characteristics which adversely affect the dispensing process.

  • Thixotropy

  • Viscosity (5,000 cps to 100,000 cps;

sensitive to temperature and moisture)

Time-Pressure Dispensing Valve: The time-pressure dispensing valve (Figure 4-1) consists of a syringe containing adhesive which is directly attached to the dispensing tip. Adhesive is fed from the syringe using pressure in a time-controlled manner. Pressure is removed to stop material flow. Fluid flow is proportional to the amount and duration of the applied pressure. Since the air pressure is kept constant over time, as the syringe is emptied, dot sizes decrease because the plunger does not advance as far with each air shot. This variability can be adjusted by increasing the air shot size, but is often operator-dependent and can lower throughput. Time-pressure systems are the most economical dispensing solutions, but have a lot of variation in their results, and are limited in the minimum dot size they can produce.

  • Tailing

Adhesive selection is very application specific and dependent on factors listed in Table 4-1.

The following are the manufacturing process steps to attach a die to the substrate.

  • The die attach adhesive is dispensed on the die pad in a pattern to optimize attachment material coverage between the backside of the die and the substrate.

  • The die is placed on the substrate with a pick and place machine. As the die is placed, the adhesive spreads to cover the die attach pad.

  • The adhesive is cured, typically with heat.

  • In wire bonded applications, encapsulation and sealing takes place

to complete the assembly process.

The most common systems used in dispensing adhesives use a time- pressure dispensing valve, auger pump, positive displacement pump, or a jetting valve. Each technique has its unique advantages and disadvantages.

Figure 4-1: ime-Pressure Valve. Courtesy of EFD, Inc.

Table 4-1: Factors influencing adhesive selection.

continued on page 9

4

JANUARY 2010

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