Practical Considerations For Attaching Surface-mount Components
What automated soldering methods can be considered for Mini-Circuits surface-mount components? There are two basic methods: reflow and wave soldering. Generally, reflow soldering can be done when (l) there are only surface mount components, or (2) these are present together with through-hole components and the latter will be soldered in a separate (wave soldering) step. The surface-mount components can be located on both the upper and lower sides of the board, when reflow is used. Wave soldering is suitable for through-hole components mounted on top of the board as well as for surface-mount components on the bottom of the board. Both ceramic and polymer-based boards can be accommodated.
Reflow requires applying a controlled amount of solder and flux to the areas where connections are to be made. A common technique is to print a pattern of solder paste on the board, and then put the components onto their places. They tend to stay in position because of the stickiness of the paste. Optionally, components can be held with chip-bonding epoxy. When heat is applied, it must produce a time-temperature profile suitable for accomplishing several process steps.
First, solvent evaporation is done at temperatures up to about 100 degree C. Second, flux reduces metal oxides as temperature rises to the solder melting point, typically 183 degree C. Third, as temperature continues to rise, the solder particles in the paste melt and wetting and wicking in the joint area begin. In the next step when temperature reaches a peak around 215 degree C, surface tension shapes the fillet of fully molten solder. It takes only a few seconds for proper solder wetting at that temperature. The length of time that the work is actually above 200 degree C is usually limited to one or two minutes to avoid damage. Many plastic encapsulated components can withstand several minutes of solder-melt temperature when re-flowed. Even at the higher temperature experienced in wave soldering such components tend to withstand several seconds immersion in molten solder.
There are several techniques for applying the heat needed for re-flow. Two of the most common are infrared and vapor phase. Infrared heat sources operate at very high temperature and are placed at the inner walls of the chamber, not in contact with the work. The actual temperature of the work is strongly affected by its mass, geometry and composition, as well as bel speed. Organic materials such as epoxy-based boards tend to absorb the IR radiation and conduct the heat to metal parts which, by themselves, would tend to reflect the IR away. On the other hand, in the vapor-phase soldering process the vapor surrounding the work is maintained at the optimum solder-wetting temperature. This is accomplished by boiling an inert liquid in a tank. The boiling point is approximately 215 degree C. When the work is held in the vapor just above the liquid, the heat at that temperature is transferred to all surfaces quite uniformly.
This method applied molten solder to the bottom side of a circuit board after a preheating sequence, with cool-down following the soldering. There are variations to the method, mainly characterized by the shape of the solder wave; the choice among them is influenced by the type of assembly and the components being mounted. Solder temperature, typically 260 degree C, is higher then the peak temperatures used in the re-flow method. The temperature profile experienced by the board and components increases during preheat to about 150 degree C at a rate similar to re-flow, but then rises rapidly when the solder wave impinges.
Components housed in the molded plastic cases may be better suited to re-flow soldering because of the possibility of micro cracks developing at the leads if they are wave soldered. Resistance to this defect is enhanced when moisture is baked out of the plastic material before soldering by either method; 24 hours at 125 degree C is usually effective.