Rapid Ir Heating of Electronic Components in the Testing Cycle

Electronic devices are often tested at elevated temperature after they are manufactured. The time required for testing can be reduced by increasing the rate of heating from ambient to test temperature, particularly if the devices are not removed from their factory carrier trays. This paper describes a new rapid-heating method based on low-cost infrared heaters. A basic model is developed to evaluate the effect of design parameters on temperature uniformity across a component carrier and along the carrier. The model is compared to experimental results. NOMENCLATURE A aspect ratio in view factor calculation Adevice surface area of component (m) B aspect ratio in view factor calculation F radiation view factor Fo Fourier number H height of chamber wall (m) Ic collimated radiation intensity (W/m) Id diffuse radiation intensity (W/m) Qc surface heat transfer from collimated radiation (W) Qd surface heat transfer from diffuse radiation (W) QT total surface flux (W/m) T temperature (◦C) T average temperature of a device (◦C) V velocity of device carrier (m/s) ∗Author to whom all correspondence should be addressed. X aspect ratio in view factor calculation Y aspect ratio in view factor calculation a thermal diffusivity (m/sec) cp specific heat (J/kg·K) dA element of surface area of device (m) k thermal conductivity (W/m·K) m mass of a device (kg) n index in Fourier series t time (s) tc plane thickness of device carrier (m) x position variable in temperature distribution (x) α absorptivity ε emissivity λn eigenvalue ρ reflectivity Introduction In the electronic component manufacturing industry, most components are subjected to a full functional test before they are sold. Depending on the type of components, these functional tests may be performed at room temperature, at cold temperature, or at high temperature (−50◦C to 160◦C) depending on the type of component and intended market1 (Pfahnl et al., 1998a; Pfahnl et al., 1999). One of the inherent problems in testing at elevated temperature is the time required in bringing the components to test 1Under the hood automotive, aviation, and military components tend to be tested at the temperature extremes. 1 Copyright  2001 by ASME temperature and returning them to room temperature after testing. Currently, for device carriers holding a large array of components, thermal soak/de-soak chambers are used in the component handlers to raise or lower the device and carrier temperatures. These chambers tend to be large and very slow in conditioning the devices, so the actual time required to electrically test a component is very short compared to the total time required for the testing procedure2. Increasing the heating/cooling rate is possible by increasing air velocities or introducing turbulators (Pfahnl et al., 1998b), but a practical limit is quickly reached due to limited blower capacity, noise restrictions, and, for some types of carriers, the danger of blowing loose mounted components out of the carrier. This paper proposes a new method for rapid uniform heating of multiple components using infrared radiation. Infrared heating is not a new technology and has found broad application ranging from the drying of pulp in the paper manufacturing (Pettersson and Stenström, 2000) to preheating plastics in vacuum molding (Kalpakjian, 1995). Radiant IR heating has also been used extensively in the electronics industry, primarily in rapid thermal processing (RTP) systems, which use radiation for heating materials for CVD and ion implantation. These systems require very high heating rates (∼ 200◦C/sec), are designed to produce material temperatures up to 1100− 1200◦C (De Keyser and Donald, 1999), and can be critically affected by temperature distributions within components during heating. The system proposed in this paper for heating devices in the testing process has significantly different objectives than RTP technology. The primary goals are a low cost system, a small system footprint, and the capability to heat multiple encapsulated devices to a uniform final temperature no higher than 160◦C. The proposed method passes a carrier with multiple devices beneath the output of a single quartz-tungsten bulb having a prescribed radiation intensity. Two main challenges arise in heating components using this method, both of which involve heating components in different positions on the carrier to the same temperature. The first problem is to hold a uniform temperature for all devices along a line perpendicular to the direction of carrier motion. This requirement is not generally a problem if there are only 1–2 components across the carrier, but, for situations with 3 or more components, it is harder to keep the edge components at the same temperature as the central components. This problem can be addressed by suitable selection of the radiant heater, surface properties, and geometry of the thermal chamber, so that the radiation field is uniform across the device carrier. The second challenge in producing uniform device temperatures is in heating a device on the leading edge of the carrier to the same target temperature as a device on the trailing edge of the carrier. This problem is solely a function of the carrier type. For low conductivity carriers with high thermal resistance between the 2Conventional heating systems make take 20-60 minutes to preheat components, depending on the final temperature and component type, while the actual electrical test may only last a few seconds. device and carrier, such as plastic trays with loose mounted devices or polymer-sheet strip carriers, conduction along the carrier does not have to be considered, and leading-edge and trailingedge devices are easily heated to the same temperature without additional thermal control. For high conductivity carriers, such as lead-frame strip carriers and metal transport trays, conduction along the carrier can cause trailing-edge devices to be heated to a higher temperature than leading-edge devices. Under final steady state conditions, the temperatures of trailing and leading-edge devices will be the same, but the transient effect of carrier conduction must be analyzed to prevent overheating of components and to minimize the settling time after exiting the IR heating system. Modeling Three parts are involved in modeling the heating of devices using an IR source. One part is associated with the heating of the actual devices, another with the heating of the device carrier, and the third is associated with the coupling between the carrier and the devices. The main objectives of the modeling are to determine the feasibility of obtaining uniform device temperatures and to determine the parameters that are critical to the final design. Component Heating The radiant heat transfer from the IR source to an individual device can be calculated using traditional resistance network methods, but the method must be modified in order to account for the effect of collimating reflectors on the radiation field. Figure 1 shows a cross-sectional view of a proposed heating chamber and Fig. 2 presents an isometric view of the heater and carrier. To keep the size of the heating system to a minimum, the heater is assumed to be the same width as the carrier. Rather than analyzing the IR source and parabolic reflector, the emitted radiation from the IR bulb is separated into a diffuse and collimated component. The IR bulb and reflector assembly can then be treated as an imaginary surface with two specified radiant fluxes, one collimated and the other diffuse. The ratio of collimated to diffuse radiation will depend on the design and reflectivity of the parabolic mirror and the placement of the bulb. The radiant intensity from the bulb can be assumed uniform around its circumference. Then, if a parabolic reflector with the bulb set deep inside the reflector is used, the majority of the emitted radiation will intersect with the reflector and will be reflected as collimated radiation. Conversely, for a very shallow reflector surface, most of the radiation will be diffuse with only a small fraction of the total emission intersecting the reflector surface and becoming collimated. The position of the bulb and reflector are generally fixed for a given design, and the ratio of collimated to diffuse radiation cannot easily be changed. Modeling will be used to evaluate the effect of various collimated/diffuse ratios, but a good starting baseline is to assume half is diffuse and half is collimated. The radiation emitted from 2 Copyright  2001 by ASME