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Reflow Soldering Cheat Sheet (DRAFT) by [deleted]

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Introd­uction: Reflow Soldering

Reflow soldering perman­ently glues components that are first tempor­arily stuck to their pads on circuit boards using solder paste that will be melted through hot air or other thermal radiation conduc­tion. Reflow soldering is implem­ented in a machine called a reflow soldering oven (Figure 3). As its definition implies, electrical components are tempor­arily attached to contact pads prior to soldering using solder paste.

This process primarily contains two steps. First, solder paste is accurately placed on each pad through a solder paste stencil. Then, components are placed on pads by a pick-a­nd-­place machine. Real reflow soldering won't start until those prepar­ations have been made.

1. Pre-He­ating

This step serves two purposes during reflow soldering. First, it allows boards to be assembled to consis­tently reach the required temper­ature to fully comply with thermal profiling. Second, it is respon­sible for expelling volatile solvents contained in solder paste. Otherwise, soldering quality will be compro­mised.

2. Thermal Soak

Similar to wave soldering, reflow soldering also depends on flux that has been contained in solder paste. Accord­ingly, temper­ature has to reach a level at which flux can be activated, or the flux fails to play its role in the soldering process.

3. Reflow Soldering

This phase occurs when the peak temper­ature is achieved, enabling the solder paste to be melted and reflowed. Temper­ature control plays a crucial role in the reflow soldering process. Too low a temper­ature stops the solder paste from suffic­iently reflowing; too high a temper­ature may cause damage on surface mount technology (SMT) components or boards. For example, a ball grid array (BGA) package contains multiple solder balls that will be melted during reflow soldering. If soldering temper­ature doesn't reach the optimal level, those balls may be melted unevenly, and BGA soldering may suffer due to rework.

4. Cooling

Temper­ature will go down soon after the top temper­ature is achieved. Cooling leads solder paste to solidify, perman­ently fixing parts on contact pads on boards.

Reflow Soldering

Applies to SMT and THT Assembly

Reflow soldering can be applied in both SMT and throug­h-hole technology (THT) assembly, but is used primarily in the former. When it comes to applic­ation of reflow soldering on THT assembly, pin-in­-paste (PIP) is usually relied upon. First, solder paste fills in holes on the boards. Then, component pins are plugged into the holes, with some solder paste coming out on the other side of the board. Finally, reflow soldering is implem­ented to complete soldering.

Wave Soldering vs. Reflow Soldering

Figure 2 illust­rates the difference between soldering process steps. The essential difference between wave soldering and reflow soldering lies in flux spraying — wave soldering contains this step, while reflow soldering does not. Flux enables dioxide elimin­ation and surface tension reduction in the material to be soldered. Flux works only when it's activated, which requires rigorous adherence to temper­ature and time control. Since flux is contained in solder paste in reflow soldering, flux content has to be approp­riately arranged and achieved.

Wave Soldering vs. Reflow Soldering

General Prefer­ences

Generally speaki­ng, reflow soldering works best for SMT assemb­ly, while wave works best for THT or DIP assemb­ly. Nevert­heless, a circuit board almost never contains pure SMDs (surface mount devices) or throug­h-hole compon­ents. In terms of mixed assembly, SMT is normally first carried out and then THT or DIP is performed, since the temper­ature required for reflow soldering is much higher than that required for wave soldering. If the sequence of two assemblies is inverted, solid solder paste will possibly be melted again, with well-s­oldered components suffering from defects or even falling from the board.