Process Considerations
Flip Chip Assembly Contents
Solder Flux:
The variety of techniques for fluxing include dipping the solder bumps into the flux as well as stencil printing, pin transfer and flood or spray coating of the substrate. Precision application (dip, stencil, pin transfer) is preferred to minimize residue and cleaning challenges.
Flip chip bumps placed on peripheral bond pad
Die Placement:
Due to the self-alignment characteristics of solder, a bump pitch of 200-225 microns only demands placement accuracy of +/- 75-125 microns and +/- 1 degree rotation. This compares to a placement accuracy requirement of +/- 40 microns for 0.4 mm pitch QFPs or +/- 50 microns for 0.5 mm pitch CSPs.
Reflow:
Conduction or vapor phase reflow provides optimum process control and ultimate solder joint consistency and reliability. However, more commonly available infrared and convection reflow equipment leads to more widespread application of these techniques. As with surface mount reflow, even temperature distribution and controlled temperature ramping, both up and down, are critical to yield and quality.
Flip chip eutectic solder bump reflow to substrate.
Underfill:
Underfill enhances thermal and mechanical vibration fatigue reliability as well as mechanical shock resistance of the flip chip solder joints. Most underfill application relies on capillary flow and thus demands well controlled material properties including viscosity and adhesion as well as the cured state properties of thermal transfer, coefficient of thermal expansion and mechanical strength.
Encapsulation:
Either liquid encapsulation or transfer molding may provide the physical protection for the die. For high volume applications as well as those which can be fit into standardized mold cavities, transfer molding provides the lower cost alternative. Transfer molding also ensures consistent surfaces for marking. Larger modules and die mounted to large, mixed technology substrates generally demand liquid encapsulation. Liquid encapsulants come in two types, silicone or epoxy based. Silicone systems offer excellent moisture resistance and high compliance, but remain difficult to mark and difficult to handle. Epoxy systems provide improved adhesion and marking relative to silicone as well as more consistent appearance and smaller "keep outs" or die spacing specifications.
Marking:
Laser, ink jet, stencil or stamp marking techniques may be employed although laser and ink jet simplify serialization of parts. Key factors include permanence, visibility and contrast. For silicone based liquid encapsulants laser marking works best. With epoxy liquid encapsulants and mold compounds ink based marking with either ink jets or pin printing provide maximum clarity and contrast.