Improving IC Packaging and System Performance
BY HONGY LIN, Ph.D., Watlow
Two common processes used to attach die to the pad or cavity of the package’s support structure are adhesive and eutectic die attach.
Adhesive die attach relies on adhesive materials such as epoxy, polyimide, and silver-filled glass frit. Eutectic die attach uses an eutectic alloy such as Au/Si eutectic, which has a liquidus temperature of 370°C, or Au/Sn, which has a liquidus temperature of 280°C. Epoxy adhesive has a process temperature of 250°C. 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. additionally, temperature uniformity over the attaching material is crucial to minimize the defects at the bond line.
The heating device must 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; exhibit rapid sensor response time/short sensor response time for precise control of temperature profile; and operate under high power density.
Tackling the stringent thermal performance required of these heating elements, the finite element model (FEM) was used to understand and optimize critical material and performance variables. The model simulates the effect of thermal conductivity on temperature uniformity, predicts the effect power densities have on the thermal stress of different materials, specifies the power requirements for given heating rates, and lastly, evaluates cooling behavior under different implementation schemes. The model not only assists in establishing the material requirement but also helps fine-tune the heating element power distribution to achieve a uniform temperature.
Figure 1.Cooling curve for an AlN heater under various cooling conditions.
From a thermo-mechanical perspective, thermal conductivity and the temperature coefficient of thermal expansion (CTE) are the 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), (Table 1). The maximum stress is about three times higher for a high CTE and low thermal conductivity material, such as Al2O3, vs. a high thermal material like AlN. Temperature-induced stress increases much faster for Al2O3 than AlN. Thus, AlN is the preferred material for meeting the fast ramp-up requirement.
Figure 2.Water-cooled heater assembly for fast process cycle application.
Thermal conductivity also plays a key role in managing a uniform temperature. It is possible to design a heater with extremely uniform surface temperature when a distributed power input pattern is optimized using a highly thermal conductive heater matrix. The ∆T (Tmax – Tmin) of 1.1ºC is achieved for AlN, while Si3N4 reaches 7.4ºC ∆T. Extremely high uniformity of surface temperature (steady-state) can be designed by properly distributing the power within the heater. The cooler terminal side and non-symmetrical temperature pattern is due to a heat sink and constraint of power input at the location.
One challenge of designing the die-bonder heater lies in rapid 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 400°C to 50°C, as shown in Figure 1. The cooling time is significantly reduced (~55 s) when forced air (20 m/sec.) is applied onto both the heater surfaces for cooling purposes. The time can be further reduced to 8.7 sec. 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-sec. cycle time is quite feasible when water cooling can be designed into the system.
Table 1.Thermal properties of alumina (Al2O3), aluminum nitride (AlN) and silicon nitride (Si3N4) at 25ºC.
Because of the unique combination of material properties, such as large thermal conductivity >140 W/Km, small CTE of 4.5 × 10-6/ºC, high dielectric strength of 15 KV/mm, high electric resistivity of 10 × 1014 Ω-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, a manufacturing process and proprietary composition have been developed to realize an AlN heater that meets the requirements in semiconductor die bonding and IC testing applications.
Figure 3. Schematic diagram of AlN heater structure (A) and microstructure (B).
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 (Figure 3a.), and a high-power input capability and terminal.
The basic structural units were assembled in a green state and sintered in a nitrogen furnace for densification. The resultant AlN heater is a nearly full-density ceramic compact with little or no porosity, which, when combined with uniformed grains (Figure 3b.), ensures high mechanical strength and thermal conductivity. The mechanical strength of an AlN-processed heater has a mean of 371 MPa and Weibull modulus of 11.
Table 2. Effect of thermal conductivity on temperature uniformity of a ceramic heater.
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 temperature uniformity of ±2°C at 400°C steady state.
The heater must also provide a fast heat-up rate for the short die-bonding cycle. Collected data indicate that an AlN heater takes about 10.5 sec. to reach 400°C when powered at 250 wsi power input. When power input is increased to 1000 wsi, a linear temperature profile with a heating rate approaching 150°C per second is achieved, and takes less than three seconds to reach target temperature. Such a heating rate exceeds the typical 100°C per second requirement for die-bonding applications. Finally, a small overshoot of <5°C at 400°C can be achieved using a self-tuning PID controller even at a 150°C-per-second ramp rate.
To validate heater reliability, a series of heaters with dimensions of 55 × 10 × 1.5 mm with 25-mm no-heat terminals were produced and tested by cycling between 100°C and 700°C at power of 1000 wsi. A Weibull analysis indicates that the MTBF life expectancy of the AlN heater is approximately 460,000 cycles.
Thus, 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 flip chip operations.
HONGY LIN, Ph.D., principal scientist, may be contacted at Watlow Heater Technology Center, 909 Horan Drive, Fenton, MO 63026; 636/349-5123; firstname.lastname@example.org.