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Effects of Copper Pattern Density and Orientation on the Modulus of BGA Substrates
Keywords: Elastic Modulus, Mixing, Composite
Substrate material properties directly impact package mechanical performance. Estimation of substrate mechanical properties based on each component layer provides maximum flexibility and better accuracy. The component materials of a BGA substrate are the fiberglass reinforced epoxy layers, copper and soldermask. Even when the material properties (modulus and CTE) of each component are known, the behavior of the final substrate is not easily determined because factors such as copper density and pattern have significant effects. For example, the copper pattern may be oriented such that conductor traces run in one direction for a considerable length. The modulus in the direction parallel to the traces will be different from the modulus in the direction perpendicular to the traces. Package mechanical simulations typically do not mesh the details of the copper patterns, but rather average within each layer, thus neglecting the orientation effects. Therefore FEM (Finite Element Model) simulations may yield inaccurate stress and warpage results. This study utilized a test vehicle (TV) to study the copper pattern density and orientation effect on substrate modulus. The TV was a 2 metal layer copperclad laminate designed with 8 simple geometric patterns. One set of patterns had Cu traces running parallel in one direction. Each module had a single metal density varied as 0%, 25%, 50%, 75% or 100%. For example, the 75% pattern consisted of traces 150um wide with 50um spaces in between, and the 100% pattern was a solid copper plane. Measurements were made both parallel (x-direction) and perpendicular (y-direction) to the trace orientation. Also studied were a pattern of holes (continuous metal plane with unconnected holes) and a dot pattern (unconnected circular metal pads). Substrate modulus measurements were made using two instruments (DMA and Instron), each utilizing two test modes (tensile and 3 point bending). Data was collected from -65 to 260C. FEM simulations which meshed the exact copper patterns were used to calculate an overall substrate modulus. These correlated with the data better than simulations utilizing the more traditional method of layer averaging. The copper pattern had a significant effect; the modulus of substrates was always much lower in a direction perpendicular to the traces, relative to the modulus parallel to the traces. The amount varied depending on temperature and whether soldermask was present but could be as high as 35%. When measured parallel to the copper traces, the substrate modulus was observed to increase with copper pattern density as predicted by theoretical mixing rules. However, the substrate modulus in the direction perpendicular to the traces was not easily predicted theoretically. Therefore, more sophisticated averaging techniques and simulations are needed.
Burton Carpenter, Senior Member Technical Staff
Freescale Semiconductor, Inc
Austin, TX

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