This month’s Tech Tips focuses on some of the dispense parameters and patterns, die placement, reflow profile settings, and storage and handling steps used when processing flip chip (FC) or chip scale packaging (CSP) components with a reflow encapsulant (also known as no-flow reflow). This material combines both tacky flux and underfill, creating a one-step dispensing process. As seen in Figure 4-1 below, 4 of the 7 process steps are eliminated, requiring only a single dispensing step instead of two. In addition, only a single-pass reflow is required, using a standard SnPb profile to solder the eutectic bumps and cure the underfill epoxy.

1) Dispensing
Unlike traditional underfills, most reflow encapsulants do not require the substrate or the needle to be heated.

As typical with many dispensing applications, the size of the needle is based on the size of the component. However, for most FC processes, a 23-gauge needle is recommended.

The size of the component also determines the amount of
volume required when dispensing the reflow encapsulant material. Table 4-1 below shows examples of typical FC body sizes (in mils or mm) and the amount of volume required.

Table 4-1 also describes the proper dispense patterns to choose from, based on component size. Shapes such as a dot or asterisk (cross) are common to compression flow processes. These patterns allow for domed deposits, minimizing the chance for voiding, which is commonly caused by the placement of FCs or CSPs. A dot configuration is typically applied for dies smaller in size, and the asterisk shape applies to larger dies. Figure 4-2 shows an example of how these geometries appear from top and side views.

2) Die placement settings
The placement settings are important to the reflow encapsulant process, because it facilitates the comprehensive flow of the material. Table 4-2 describes what these recommended parameters should be when placing an FC or CSP package.
Figure 4-3 shows an illustration of how a die component would place on an asterisk (cross) shaped pattern.

3) Reflow profile settings
Modern reflow encapsulants are engineered to facilitate a single pass reflow process using a standard Sn63Pb37 flux soak profile. This profile (Figure 4-4, page 11) will solder the bumps to the pads and encapsulate them. This works in conjunction with other SMD components that are populated and soldered with standard Sn63Pb37 solder paste.

4) Storage and handling
Storage of reflow encapsulants is similar to most encapsulating materials – at least a -40°C environment to last until the expiration date. The pot life is also the same, in that it cannot be at room temperature for more than 24 hours.

The material should also be thawed according to the manufacturer’s recommendations. Table 4-3 shows a “time to thaw” example of Kester’s SE-CURE® 9126 reflow encapsulant. Most of these times are standard; however, check with your manufacturer for precise thaw specifications. Material should be used completely or discarded rather than frozen and stored for re-use. For further information regarding the use of reflow encapsulants, please contact the EMPF Helpline at (610) 362-1320.