Surface-mount technology (SMT) is a method in which the electrical components are mounted directly onto the surface of a printed circuit board (PCB). An SMT component is usually smaller than its through-hole counterpart because it has either smaller leads or no leads at all.

Flexible printed circuits (FPC) are made with a photolithographic technology. Rigid flex printed circuit boards are boards using a combination of flexible and rigid board technologies in an application. Most rigid flex boards consist of multiple layers of flexible circuit substrates attached to one or more rigid boards externally and/or internally, depending upon the design of the application.

The ordinary SMT chip processing process is basically the same, because the flexible circuit board, flexible and rigid board, and hard board all need to pass the solder paste soldering process such as component placement and reflow soldering. However, there are some unique features for flexible boards and flexible and rigid boards.

1. Solder paste soldering process

As with the hard board PCB process, solder paste is applied to the flexible board and the rigid-flex board through the stencil and solder paste printer operations. Many SMT workers are deeply troubled by size and fragility. Unlike hard boards, the surface of soft boards is not flat, so they need to be fixed by means of some fixtures and positioning holes. In addition, flexible circuit materials are not stable in size. They can stretch or wrinkle by 0.001 degrees per inch under changes in temperature and humidity. More interestingly, these extension and wrinkle factors will cause the board to shift in the X and Y directions. For this reason, flexible mounting often requires smaller carriers than rigid board SMT.

2. SMT component placement

In the current trend of miniaturization of SMT components, small components will cause some problems during the reflow process. If the flexible line is small, extensions and wrinkles will not be a significant problem, resulting in smaller SMT vehicles or additional Mark points. The lack of overall flatness of the carrier will also cause displacement in the placement effect. SMT fixture is one of the leading factors to keep the surface of SMT mounting flat.

3. Reflow soldering process

Before reflow, the flexible circuit must be dried. This is an important difference in the process of mounting flexible and hard board components. In addition to the dimensional instability of flexible materials, they are also relatively hygroscopic, and they absorb moisture like a sponge (up to 3% weight increase). Once the flexible circuit board has absorbed moisture, it has to stop reflow soldering. The same problem exists for hard-board PCBs, but has a high tolerance. Flexible circuits require preheating baking at ~ 225 ° to 250 ° F. This preheating baking must be completed quickly within 1 hour. If it is not baked in time, it needs to be stored in a dry or nitrogen storage room.