Processing mechanics for flip-chip assemblies

In this paper, a non-linear finite element framework has been implemented t o simulate the sequential build-up of a flip-chip package. A generalized de formation model with element removal and addition is used to activate and d eactivate the underfill material layer to simulate flip-chip package fabric ation. Using process models, one can determine the warpage stresses at any intermediate stage in the process. Tn addition, topological change is also considered in order to model the sequential steps during the flip-chip asse mbly. Geometric and material nonlinearity which includes the creep behavior of underfill and solder balls, and temperature-dependent material properti es are considered. Different stress-free temperatures for different element s in the same model are used to simulate practical manufacturing process-in duced thermal residual stress field in the chip assembly. This approach (th e processing model established in this paper) is in contrast to the non-pro cessing model employed by many researchers, which is shown to yield overly conservative and sometimes erroneous results, leading to non-optimal design solutions. From the finite element analysis, it is found that the strains and deflections obtained from the non-processing model are generally smalle r than those obtained from the processing model due to the negligence of th e bonding process-induced residual strains and warpage. Furthermore, the fa tigue life for the outmost solder ball predicted by the processing model is much shorter than that predicted by the non processing model based on the Coffin-Manson equation. On the other hand, in order to prove the soundness of the framework established in this paper, the test results obtained by us ing the laser moire interferometry technique are compared with the results achieved from the proposed numerical analysis vehicle. It is shown that the deformation values of the hip-chip package predicted from the finite eleme nt analysis are in a good agreement with those obtained from the test. (C) 1999 Elsevier Science Ltd. All rights reserved.