By Mark Everly & Dan Block
Mark Everly is Principal Systems Engineer and Dan Block is a Laboratory Technician at Watlow
Aluminum Nitride has become a popular heating material that reduces epoxy or eutectic bonding time in die bonding applications. However, manufacturers need to shave time off the cooling portion of the bonding process too. One of the least expensive and potentially most effective cooling techniques is forced-air cooling.
In the relentless march to ever faster production, semiconductor manufacturers are always on the lookout for ways to rapidly cure the epoxies and eutectic solder materials used in die bonding and integrated circuit applications. Heat is often the preferred method of curing. After increased temperature activates the epoxy or melts the eutectic material, the package must be cooled so that the bond provides sufficient handling strength before removal from the fixture. This cyclic heating and cooling takes time. Anytime seconds or minutes can be shaved from both heating and cooling steps enables the semiconductor manufacturers to increase their production yield.
Recent advances in heater technology allow the use of aluminum nitride (AlN) for the structural matrix of heaters for heating packaged semiconductors in die bonding operations more quickly than other materials, shrinking heating time. Engineers at Watlow have developed an AlN matrix heater design with an integrated heat-generating resistor circuit that allows leads for electric power to be connected directly to the AlN matrix. A thermocouple has also been integrated with the AlN matrix including the attachment of a third set of leads to the matrix. This configuration created the necessary rapid development of heat, however, the AlN ceramic needed to be quickly cooled to allow the semiconductor package to be moved.
The engineers conducted preliminary evaluations of several alternatives, such as liquid water or oil cooling, thermoelectric elements, and heat sinks, to quickly cool the AlN-based heaters after the bonding materials reached their cure phase. Cost-benefit analysis of these alternatives indicated that forced-air cooling would be a good, inexpensive, and convenient choice for many applications that use the latest AlN heating technology.
s and standard
Forced-air cooling tests were conducted on heaters with integral fins and standard “flat-sided” heaters.
Compressed air offers several advantages over other types of cooling. It is widely available, there is no risk of leakage as with liquid or oil cooling methods, it accommodates a range of common heater operation temperatures, and it can be easily integrated into production equipment as it requires relatively simple, small, lightweight components and does not complicate electrical isolation requirements. Based on this information, Watlow engineers tested several configurations with compressed air to accelerate the cooling of the ceramic AlN heater.
The basic tests
For testing purposes, a simple support fixture was machined to include a duct immediately below the heater location. In the basic test configuration, the heater forms the fourth side of a rectangular duct and is supported by a groove machined into the duct walls. Compressed air is routed to this duct through a filter-regulator while a flow meter controls and measures flow. The air is injected into the duct near the center of the heater and exits the duct at both ends.
A thermocouple in the air stream measured temperature at the air inlet, and a thermocouple brazed into the heater measured the heater temperature. These temperatures were recorded in approximately 0.2 s intervals using an automated data acquisition system and were subsequently used to evaluate the performance of the system under various conditions.
Various airflows and air temperatures at the duct inlet were tested to determine the cooling rate’s dependence on these factors. Heaters with integral fins were tested in addition to the standard “flat-sided” heaters. These heaters measured 50.0 mm x 10.0 mm x 2.5 mm (1.9 in. x 0.39 in. x 0.10 in.). The fins added an additional 1.0 mm to the thickness of the heater but increased the surface area exposed to the airflow by a factor of three.
The test configuration consisted of a simple support fixture machined to include a duct immediately below the heater location. The heater forms the fourth side of the rectangular duct and is supported by a groove machined into the duct walls. Compressed air is routed to this duct through a filter-regulator and is injected into the duct near the center of the heater. The air exits the duct at both ends of the heater.
Results and observations
As expected, initial testing showed a strong relationship between the heater air flow rate and cooling rate. The presence of fins also increased the cooling rate.
This graph shows the time-temperature curves for the standard heater with 5 and 10 CFM air flow rates and for the finned heater with 5, 10 and 20 CFM air flow rates.
The calculated cooling rate and heat transfer coefficient data shown in Table 1 uses an approximation of the heater as a single lumped mass, implying an assumption of uniform temperature within the heater. This assumption is logical since the AlN matrix is very conductive with a thermal conductivity of approximately 140 W/m °C.
However, the temperature/time curves indicate that it takes more than 0.5 s after power is removed from each heater to reach the peak measured cooling rate. This behavior is true even for cases where the airflow is on during the heating process, indicating that the heater sensor measures a temperature lower than the peak heater temperature at the start of cooling due to its position within the heater matrix.
Additionally, it is likely that the local heat transfer coefficient is greater near the center of the heater where the incoming air impinges on the underside of the heater, and lower near the ends. The transfer of heat from the heater to the air is also retarded near the ends by an increase in the air temperature as it flows along the heater. The variation in local heat flux drives a temperature gradient in the heater during cooling that in turn causes an underestimate of the heat loss during the initial 0.5 to 1.0 s of cooling and an overestimate later in the cooling process. The data for average heat transfer coefficient presented in Table I is averaged over the time of the cooling process as well as over the area of the heater to compensate for the error in cooling rate measurements.
The cooling rate of the heater without flow indicates that approximately 4 to 5 W of heat were lost to the mechanical support structure and to the working surface of the heater when it was near 200°C, that is Qo is between 4 and 5 W. This heat loss is not significant relative to the heat flow of the forced air inside the duct and therefore moderate variations in heat loss other than to the airflow in the duct are negligible.
The average coefficients of heat transfer calculated for the finned heaters are lower than those for the standard heaters despite the fact that the reduction in cross-sectional area for flow actually increases the flow velocity in the duct. The difference is likely due to the temperature variation along the length of the fins; the fins are undoubtedly cooler at their tips. However, even with the reduced heat flux per unit surface area, the amount of heat transferred from the heater to the air is significantly higher for the finned configurations as the simplified equation, Qc = h A ?T, would indicate. The three-fold increase in surface area has a greater effect on the heat transferred than a 40% lower heat flux.
It should also be noted that the engineers used a less than optimal fin profile for the tests. Rather, it was selected for manufacturing convenience. Therefore, some improvement in cooling rate may be possible if increased cost is acceptable for a better fin profile. Longer fins with a tapered cross section may show improved results.
This graph shows the effect of reduced air temperature to 1°C (34°F), which is less than the effect of a small change in the air velocity.
As expected from the equations, reduced inlet air temperature improves the cooling rate proportionately to the increase in the difference between the heater temperature and the air temperature. A vortex tube was tested as a way to reduce the air temperature without the expense of adding a refrigeration system. The test set-up reduced air temperatures down to 1°C (34°F). Larger decreases in air temperature would have proportionately larger effects on cooling rate; however, achieving a very cold air stream may require significantly more capital equipment than using a larger flow of compressed air from an existing system.
In plants with existing cryogenic nitrogen or argon delivery systems, air (or more precisely “coolant”) temperature reduction can be a cost effective way to accelerate the cooling rate.
Additional testing to determine the acceptable limit for cooling rate should be done prior to final equipment design.
Heaters and associated equipment that must be cooled to a temperature near or below ambient respond better to cooled forced air than systems that operate at temperatures significantly above ambient temperature.
Thus, for applications requiring cyclic heating and cooling, compressed air is one method for cooling. The use of forced air as a coolant can help a system with an appropriately designed thermal cycle fulfill application needs without the disadvantages of high initial cost and complexity associated with other cooling technologies.
Mark Everly is the principal systems engineer for Watlow’s Single Iteration division. Everly holds a master’s of science in mechanical engineering from the University of Missouri – Rolla. Dan Block is a laboratory technician at Watlow and holds an associate of science degree in applied electrical technology from Vatterott College. For more information, contact Watlow at www.watlow.com.
Accurately estimating the cooling rate of a heater with a load or attached heated part requires special consideration. The average coefficient of heat transfer data, such as those shown in Table 1, can help you estimate heat flow from a surface of an AlN heater exposed to a specific airflow. However, the heated part may have significant temperature gradients, its thermal coupling to the heater may be less than perfect, or it may have time dependant sources of heat gain or loss. For these reasons, most designs must be verified with a more detailed approach that uses one of the following: finite element models with time stepping solvers, computational fluid dynamics, or physical testing if partial prototypes can be constructed at a reasonable cost.
Regardless of the verification approach, it is necessary to design a rough first approximation of the system for analysis or testing. In systems using forced-air cooling the following approach can identify a reasonable first approximation for a design that achieves the desired cooling time.
Identify the work piece to be heated and cooled, including its dimensions and material.
Select a heater of appropriate heating capacity and dimensions to achieve the heating rate and uniformity objectives. (Note; these two steps identify the components to be heated and cooled.) These components, including the work piece, the heater, and any other component not thermally isolated from the heater constitute the control volume for cooling (and heating).
Identify and quantify any significant sources of heat gain or loss from (or to) the control volume in addition to the forced airflow. Examples of heat gain or loss sources include conduction through structural supports, radiation to the environment, and convective heat transfer from exterior surfaces.
Estimate the needed cooling power by calculating how much heat forced air must remove and how quickly. Cooling power can be estimated by multiplying the heat capacity of each component by the cooling rate required, summing the results and adding any heat gains to the control volume during the cooling time period (Q = Qc + Qo = [( V Cp) dT/dt].
Select the volumetric flow rate of air to achieve a desirable air outlet temperature. Vf (air) = Q / ((air) x Cp(air) x T (inlet to outlet) )
Calculate the average temperature difference between the heater and the air: T = [(Heater high temperature + heater low temperature)/2] – [(air inlet temperature + air outlet temperature)/2].
Calculate the coefficient of heat transfer needed to support the cooling power required: h = Q / (A(heater) x T ) (where A(heater ) is determined in step 2
Select a flow passage size to achieve the flow velocity required to give the needed heat transfer coefficient. (Note that higher velocity usually means greater pressure drop, and that the impingement on the heater surface is important if the table values are used.)
If the needed heat transfer coefficient cannot be achieved with reasonable air velocity, fins or some other form of extended surface can increase the area (A(heater) in step 7) exposed to the flow. Keep in mind that the apparent coefficient of heat transfer will be reduced for an extended surface. Other adjustments such as increased volumetric air flow or increased heater length or width can also contribute to improved cooling and be considered if the initial values selected do not offer the desired performance.
Simple heat coefficient relationships
When designing future cooling systems, identify average coefficients of heat transfer corresponding to the data. Table I summarizes the information gained based on an analysis of the test data and on the following simple relationships.
Q = Qc + Qo = [( V Cp) dT/dt] Qc = h A T
qc = h T = Qc / A
Q is the total heat loss per second from the heater during cooling,
Qo is the heat loss per second from the heater excluding the air in the duct,
Qc is the heat loss per second to the air in the duct,
(V Cp) is an expression for the heat capacity of the heater (density x volume x specific heat capacity of the material), dT/dt is the cooling rate, or time rate of change of the heater temperature,
h is the average coefficient of heat transfer over the heater surface area,
qc is the average heat flux, or heat flow per unit area, to the air in the duct,
A is the area of the heater exposed to the air flow in the duct, and
T is the temperature difference between the heater temperature and the bulk air temperature in the duct.
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