Novel mass reflow method for organic substrates
The warp of an organic substrate is a major challenge for bond and assembly (B&A) operations. A portable vacuum fixture that eliminates the warp by holding the substrate perfectly flat during the chip attach process is introduced in this paper. Excellent C4 height uniformity with increased B&A yield is achieved by this novel method. The fixture also makes it possible to utilize all electrically good substrates irrespective of their incoming shape. Furthermore, the electrical performance of the substrate is no longer constrained by the need to modify its circuit design to reduce the warp. The vacuum fixture is designed to be compatible with an industry standard mass reflow oven. The design of the fixture is described along with the manufacturing challenges that were faced. A transient thermal model, leveraged to optimize the thermal mass of the fixture by a factor of two, is presented. The performance of the fixture is compared to other mechanical techniques used for clamping substrates flat. The fixture's unique ability to shape the substrate to match the warp of the chip at reflow temperatures is discussed. Finally, application of the technique to challenging chip-attach cases is explored.