Flip chip is a method of chip attachment to a substrate with essentially no conventional package. It is becoming more commonly used for electronic assemblies than in the past. This Tech Tip will outline the methods used by the EMPF to assemble and validate flip chip.

1. Decide if flip chip is warranted for the design. Flip chip attachment is used for extremely high I/O devices where there is insufficient peripheral length to accommodate bond pads for all of the I/O needed for the function of the die. Often this can occur for DSP or Microprocessor chips with over 1,000 I/O connections needed per chip. However, flip chip is also used in some instances of low I/O chips that require very good signal integrity and a very low impedance to each I/O. This is known as low parasitics. A third place where flip chip is used is where size is at an extreme premium (such as airborne applications or medical applications such as “in the ear canal” hearing aids). Figure 5-1 illustrates several types of flip chip assembly.

2. Select the integrated circuit die (chip) to be flip chip attached. There are several key considerations in the selection of a chip (also known as a die) to be used in the flip chip attachment process. These are:

Bump type – Is the bump a solder bump or metal bump? Solder bumps are more convenient to deal with if the flip chip is to be attached to the assembly at the same time as the other surface mount components. Only one solder reflow bake process is needed to simultaneously attach the flip chips and the normal packaged chips and discrete parts. The metal bumped chips could, in contrast, require special solders and/or adhesives to accomplish the flip chip process, which would then require two separate attachment sequences to build the assembly. EMPF prefers the solder bumped die.

Availability – Sometimes all of the different dies to be attached are not available in solder or metal bumped versions. Dies that are made for the normal wire bonding of the I/O pads can be bumped individually using what is called a “stud bump.” This is a bump that is applied to the die pad with a ball-bonding wire bonder, by making the bond to the die pad and then breaking the wire off. A die can be converted in this manner from a face-up wire bondable die to a stud-bumped flip chip die. When the die is flipped, either solder or an adhesive (usually electrically conductive) can be used to make the bond at the stud bumps.

Die “Shrinks” – “Shrinking” the die by getting more die per wafer processed and lowering its cost should be considered. If frequent shrinks are anticipated, be advised that the substrate pattern will have to change to accommodate the smaller die pattern. If this is planned at a very frequent interval, however, the costs for new substrates can become an issue.

3. Select the substrate material to be used in the flip chip assembly. The most critical choice here is ceramic versus organic substrate material. Ceramic has a much closer CTE (Coefficient of Thermal Expansion) to most IC (Integrated Circuit) dies than do the common organic materials. An exception to this rule is that some organics, such as LCP (Liquid Crystal Polymers), can be engineered to have close CTE properties to some IC materials. If common organics, such as FR 4 epoxy-glass composite, must be used, an underfill epoxy will be needed to secure the die to the substrate to limit the CTE mismatch reliability consequences. Many ceramic materials have low enough CTEs that they can be used for flip chip substrates without underfill.

4. Select the underfill to be used. This choice can range from “no underfill” to various underfill materials. Underfill materials are usually filled epoxy systems that have intermediate CTE between the die and the substrate. Underfills dramatically increase the flip chip ability to withstand thermal cycling, but they add dispense and cure steps to the manufacturing process.

5. Determine the assembly sequence. Without underfill, flip chip (with solder bumps) requires no change in the normal surface mount process. If underfilling is required, provisions for the dispense and cure of the underfill material is necessary.

6. Rework and Repair. If rework and repair of assemblies is contemplated, the flip chip process with underfill presents a considerable issue. Reworkable epoxy underfill materials are not well proven, and so must be selected on a case-by-case basis. Flip chip with no underfill presents much less of an issue, but either can be accommodated.

Without underfill, solder flip chip presents only the challenge of fine pitch surface mount. If underfill is required, as for reliability on organic substrates, a more complex assembly process, including dispense and cure, is needed. Rework of underfilled flip chip then presents additional issues, such as removal of cured underfill before die replacement.