New and improved underfill formulations are continually being developed for flip chip assembly.  These materials are designed to create a uniform and void-free underfill layer that protects the active surface of the die while improving the reliability performance of flip chip devices by distributing stress away from the solder interconnects.  Reflowable or fluxing underfills can reduce the number of manufacturing steps required for assembly of flip chips or Ball Grid Arrays (BGA) on printed wiring boards.  Figure 6-1 shows the reduction in processing steps between standard underfill and reflow underfill processing. 

From Figure 6-1, it is clear that reduced cycle time will be achieved by using a fluxing underfill.  However, a fluxing underfill may not necessarily be reworkable underfill. Reworkable underfills typically fall into two categories, chemically degradable and thermally degradable.  Chemically degradable underfills can be removed by application of isopropyl alcohol or methylethylketone (MEK).  Thermally degradable underfills are decomposed by the application of heat.  The thermally degradable material system can be applied as a thin layer of thermoset material.  This thermoset layer has excellent adhesion to other underfills, but it can be broken at the PCB interface due to poor adhesion upon heating to a designated temperature.  At the present time most thermally degradable underfills break down at temperatures between 200 and 250°C, which may preclude their use in lead-free soldering due to the higher soldering temperatures required. 

Suppliers that offer a fluxing underfill generally recommend a pre-bake step prior to dispensing and assembly to reduce bubble formation in the underfill, which has been linked to the release of moisture from the substrate during reflow. 

Some fluxing underfills require a post-reflow bake so care should be exercised when selecting a material to ensure a good match to the available equipment and process limitations.

Two variables that must be controlled to ensure reliable results when using a reflow underfill are the amount of material dispensed (Figure 6-2), and the force (Figure 6-3)  that is applied to the devices in the reflowable underfill. This is especially true of flip chip devices and chip scale packages as they have very little mass  and could float on top of the underfill if too much dispensed. 

Reworkable underfills have shown excellent results in drop testing, and several hand held device assemblers have begun the use of reworkable underfills to enable repair of high value, hand held products and to reduce manufacturing costs.  Manufacturers that build systems with sensors understand the need to control the stress on a device imparted by the assembly process.  Sensors are often susceptible to residual strain from assembly.

Residual stress caused by the cured underfill may cause a completely satisfactory sensor to be out of spec after final assembly.  Therefore, the manufacturer may elect to replace it with another sensor, and a reworkable underfill will substantially aid this process.