May, 2019


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Reducing CTE Mismatch Defects in Flip Chip Reflow


Continued from previous page


ensures intimate contact of the sub- strate with the carrier above the Tg of the substrate where the deforma- tion is at its worst. As temperature increases, suc-


tion force will be reduced. This is a normal phenomenon as air density increases with higher temperature, reducing the efficiency of suction. With a normal tin-silver-copper (SAC) reflow profile, the suction reduction will be about 30 percent. However, this is still sufficient to keep the hotter substrate held down on the carrier. This can be demonstrated with a ther- mocouple (TC) mounted on the carrier, just below the TC on the top side of the substrate. The top of Figure 2 shows the


heating of the substrate as humps, due to the absence of the heat spread- ing effect as the substrate lifts off from the carrier as it deforms. With suction, effective heat spreading occurs, which evens out the temperature.


Advanced Thermal Control (ATC) Below are the heat transfer for-


mulas related to mass reflow ovens. Convection heat delivered:


Target heat absorbed: This produces S-curve heat


transfer characteristics and the heat- ing rate will reduce when the target temperature approaches the convec- tion temperature. Common parame- ters for mass reflow ovens based on the parameters for the above trans- fer are: M = mass of the product and carrier; Cp = specific heat capacity of carrier; A = exposure area; t = expo- sure time; H = coefficient of convec- tion; and T = temperature. Control of the heating rate can


be achieved by adjusting the reflow parameters. The mass (M) of the product is an important factor. It is necessary to reduce excess area and weight of the carrier to minimize effects of forward heat sinking and backward conduction, which impact the thermal uniformity along the direction of flow. It is typical to see the rear of the


product with a higher temperature than the leading edge. The rear will act as a heat sink for the leading edge, while the leading edge will con- duct heat to the rear as it gets hotter. This effect can be reduced with smaller temperature setting differ- ences between the zones and avoid- ing high ramp rates, but may require longer zones or a longer oven. With the oven size, conveyor


speed and the product and carrier mass held as constant, the next parameter to investigate is the coeffi- cient of convection (H). The variables for convection heat transfer will be the area of coverage and impingement pressure. Plenum orifice size and dis- tance from target will determine the impingement force on the product. By optimizing the orifice size, distribu- tion and distance from product, the thermal transfer rate can be opti- mized to a preferred condition. This creates a controlled heating and cool- ing rate for the reflow process.


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Figure 3 (page 67) displays the


profiles with similar settings and some fine-tuning to accommodate the critical areas to control reflow and CTE differences. Unformed joints are not sensitive to CTE mismatch dur- ing heating. The area of critical con- trol starts from the reflow ramp at zone 5 to the end of the oven cooling zones. The calculations for heating and cooling rates are made over five second intervals. The control of the time above liquidus (TAL) is believed to influ-


Continued on page 67


Page 65


Figure 2: Window cover without suction (top) and with suction (bottom).


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