The continuous increase in the consumption of semiconductor devices and the emergence of new applications in optical components — MEMS, LCD display, flip-chip, chip-onglass, and multichip modules — has created a vast demand for faster throughput and better die-bonding equipment for IC packaging. IC packaging requires a typical ramp rate of 100ºC per second to 400 to 500ºC ±2°C, and a cycle time of 7 to 15 seconds. Similarly, IC chip testing, which stresses chips between -40 to 125ºC while monitoring electrical parameters, also requires a faster cycle rate. To manufacture ICs of all types, a die bonder or die attach equipment is used to attach the die to the die pad or die cavity of the package's support structure. The two most common processes for attaching the die to the die pad or substrate are adhesive die attach and eutectic die attach. In adhesive die attach, adhesives such as epoxy, polyimide, and Ag-filled glass frit are used to attach the die. Eutectic die attach uses a eutectic alloy. Au-Si eutectic, one commonly used alloy, has a liquidous temperature of 370ºC, while another alloy, Au-Sn, has a liquidous temperature of 280ºC.

Epoxy adhesive has a process temperature of 250ºC. In both processes, the temperature profile of the die attach material must be precisely controlled to ensure complete curing of the adhesive or melting of the eutectic materials. In addition to fast heating, uniformity in temperature over the attaching material is crucial to minimize the defects at the bond line. To meet the demanding requirements in those applications, the heating device must be able to perform reliably with the following characteristics:

  • Provide a uniform temperature both during ramp up and steady state;
  • Heat up extremely fast;
  • Dissipate heat quickly to allow for fast cool-down;
  • Have minimum dimension change during temperature cycle;
  • Withstand compressive pressure during operation;
  • Be highly finished with a smooth and flat surface to enable heat transfer;
  • Be constructed with mechanical features such as grooves and holes for vacuum passage and/or curved surfaces;
  • Have rapid sensor response time/ short sensor response time for precise control of temperature profile; and
  • Operate under high-power density.

Investigating Ceramics

To tackle the stringent thermal performance required of these heating elements, the finite element model (FEM) was employed to understand and optimize critical material and performance variables. The model was used to simulate the effect of thermal conductivity on temperature uniformity, predict the effect that power densities have on the thermal stress of different materials, specify the power requirements for given heating rates. and lastly, evaluate cooling behavior under different implementation schemes. The model not only assists in establishing the material requirement, but also helps in fine-tuning the heating element power distribution to achieve a uniform temperature. From a thermomechanical point of view, thermal conductivity and the temperature coefficient of thermal expansion (CTE) are the two most important properties that dictate performance of a candidate material for heaters in die bond machines. To establish a semi-quantitative relationship between power density and stress, a model was created to predict the stress level under various power densities for two of the high-performance materials: alumina (Al2O3) and aluminum nitride (AlN), listed in Table 1. The maximum stress is found to be about three times higher for a high CTE and low thermal conductivity material such as alumina, versus a high thermal material like AlN. It was also found that stress increases much faster with temperature in the case of Al2O3 than that of AIN. It is clear that aluminum nitride is the preferred material choice to meet the fast ramp-up requirement. Thermal conductivity also plays a key role for achieving highly uniform temperature. It is possible to design a heater with extremely uniform surface temperature when a distributed power input pattern is optimized using highly thermal conductive heater matrix. Table 2 indicates that the ΔT (Tmax-Tmin) of 1.1ºC is achieved for AlN, while Si3N4 has a of 7.4ºC, which is about seven times larger. As shown in Figure 3, extremely high uniformity of surface temperature (steady state) can be designed by properly distributing the power within the heater properly. The cooler terminal side and non-symmetrical temperature pattern is a result of the presence of heat sink and constraint of power input at the location.

Critical Cooling

One challenging aspect of designing the die bonder heater lies in the fast cooling. Even if the heater has high thermal conductivity, its cooling rate is too long when only natural convection is involved. It is not surprising that a heater could take more than 250 seconds to cool from 400ºC to 50ºC, as shown in Figure 1. The cooling time is significantly reduced (~55 s) when forced air (20 m/s) is applied onto both heater surfaces for cooling purposes. The time can be further reduced to 8.7 s when assisted with water flow through the channel inside the steel block attached to the heater bottom, as shown in Figure 2. As predicted by the cooling model, a heater assembly design for achieving 10- to 15-s cycle time is quite feasible when water cooling can be designed into the system. Because of the unique combination of material properties such as large thermal conductivity >140 W/Km, small coefficient of thermal expansion of 4.5×10-6/ºC, high dielectric strength of 15KV/mm, high electric resistivity of 10×1014 ohm-cm, and large elastic modulus of 330 GPa, AlN ceramic is an ideal candidate for a heater matrix among the high-performance ceramic materials.

AlN Heater Design

Based on the results of the theoretical analysis for heater performance and design, Watlow has developed a manufacturing process and proprietary composition to realize an AlN heater that meets the aggressive requirements in semiconductor die bonding and IC testing applications. The basic structure of the high performance ceramic heater consists of the AlN matrix, the heating element with distributed wattage based on FEA to ensure the temperature uniformity (shown in Fig 3A), as well as a high power input capability and terminal. The basic structural units were assembled in a green state and then sintered in a nitrogen furnace to allow densification to take place. The resultant AlN heater is a nearly full-density ceramic compact with little or no porosity, which combined with uniformed grains (Fig. 3 B), ensures high mechanical strength and thermal conductivity. The mechanical strength (ASTM type A configuration) of an AlN processed heater has a mean of 371 MPa and Weibull modulus of 11. Following careful consideration of the environment and defining of the boundary conditions, the heating element pattern is optimized using the FEA technique.

Infrared images of the AlN heater reveal excellent temperature uniformity of ±2ºC at 400ºC steady state. In addition to uniform temperature distribution, the heater must provide a fast heat-up rate for the short die bonding cycle. Collected data indicates that an AlN heater takes about 10.5 s to reach 400ºC when powered at 250 W/in2 power input. When power input is increased to 1,000 W/in2, a linear temperature profile with a heating rate approaching 150ºC per second is achieved and takes less than three seconds to reach target temperature. Such a heating rate exceeds the typical 100ºC per second requirement for die bonding applications. Finally, a small overshoot of less than 5ºC at 400ºC can be achieved easily using a self-tuning PID controller even at 150ºC per second ramp rate. To validate the reliability of the heater, a series of heaters with dimensions of 55 × 10 × 1.5 mm with 25-mm no-heat terminal were produced and tested by cycling between 100ºC and 700ºC at power of 1,000 W/in2. A Weibull analysis indicates that the MTBF life expectancy of the AlN heater is approximately 460,000 cycles. As these experimental results show, a high-performance AlN ceramic heater offers significant advantages in terms of fast ramping and cooling as well as temperature uniformity, and is ideal for the most demanding die bonding and flipchip operations.

This article was written by Dr. Hongy Lin, principal scientist in charge of ceramic heater research and development, at Watlow Electric Manufacturing, St. Louis, MO. For more information, contact Dr. Lin at hlin@watlow. com, or visit http://info.hotims.com/10960-400