This part introduces an alternative to wave soldering called reflow soldering.
Reflow soldering basics
Q: What happened to wave soldering as a production arrangement?
A: It is still widely used due to its benefits. Among them is that it is necessary for through-hole components, and for wires and connectors soldered to the PC board edge. However, it is generally incompatible with the use of surface-mount components.
However, as the semiconductor and passive components evolved to smaller sizes, more contacts, or packages with tight lead pitch, the leaded package was simply too big and awkward, and restrained the shrinking of the overall PC board assembly. Surface mount packages and technology use contacts along the edge of the package, bumps under the package, or as tiny “gull wings” extending from the package, shown in Figure 1.
The result is that the entire board is much denser, and comments can be installed on both sides of the PC board, as seen in Figure 2. Since the board does not require drilling for component leads (although it still does need holes for vias), the manufacturing cost, complexity, and time, as well as drill-bit wearout, were significantly reduced.
Q: What is the technology used for soldering boards with surface mount technology (SMT) components?
A: It is called reflow soldering, and it is very different from wave soldering.
Q: What is the concept of reflow soldering?
A: It’s a very non-intuitive process that was developed beginning in the 1980s. Reflow soldering permanently “glues” components using solder, after they are first temporarily stuck to their contact pads on circuit boards using solder paste that will be melted through hot air or other means.
The reflow-soldering process
Q: What are the specific steps?
A: This process contains two steps: preparation and implementation.
For preparation, solder paste (with flux) is placed with precision using a stencil or high-speed dot printer on each component contact pad. Then, the components are placed on pads by a high-speed pick-and-place machine, and they stick to the paste and board.
For implementation, the board is run through the multizone reflow machine on a set of side rails or tracks, shown in Figure 3. The steps include:
- Pre-heating to ensure the boards to be soldered consistently reach the required temperature to fully comply with thermal profiling.
- Thermal soak – similar to the one in wave soldering – ensures that flux that has been contained in solder paste is activated, as it has a critical role in the soldering process.
- The actual reflow soldering 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 to SMT components or the PC boards. Heating is done using a convection, infrared, or vapor phase oven.
- In the final cooling phase, the temperature will go down soon after the top temperature is achieved. Cooling causes the solder paste to solidify, permanently fixing the components on the contact pads of the boards.
In most production systems, automated optical inspection is also used to ensure that each visible solder joint is done right. Of course, many ICs with high lead counts have contacts under their package body, so optical inspection is not possible. Therefore, the reflow process must be perfect.
Q: What is the “profile” that is so critical for reflow soldering?
A: It is a carefully defined graph of time versus temperature for the reflow process, with a typical one shown in Figure 4. It includes ramp-up, pre-heat/soak, ramp to peak temperature, reflow, and cooling phases. Vendors provide the specific profile for their components in the data sheets, and these must be followed to ensure proper and reliable reflow soldering.
For situations where the profiles of components differ from each other, some adjustments or compensation may be needed; in cases where there is an “outlier” component with a very different profile, the reflow may have to be done multiple passes (very difficult), or that component is not used.
The next part compares the two soldering techniques and highlights their key attributes.
References
Printed circuit board, Wikipedia
A Comparison of Reflow Soldering and Wave Soldering, All About Circuits
Wave Soldering vs. Reflow Soldering, Millennium Circuits Limited
What is the difference between wave soldering and reflow soldering?, RAYMING Technology
Comparison between Wave Soldering and Reflow Soldering, PCBCart
Reflow Soldering vs Wave Soldering: Difference and Process, NEXTPCB
Solder Reflow: An In-Depth Guide to the Process and Techniques, Wevolver
Wave soldering, Wikipedia
Reflow Solder Defects, Optima Technology Associates, Inc.
Troubleshooting Reflow Soldering Defects: A Practical Guide for Engineers, ALLPCB
Smarter Temperature Setup for Reflow Oven to Minimize Temperature Variation Among Components, IEEE Transactions on Components, Packaging and Manufacturing Technology
Head-in-Pillow BGA Defects, AIM Solder
Reflow Soldering Process Manual: A Comprehensive Guide From Design To Implementation, Zhejiang Neoden Technology Co., Ltd
Wave and Reflow Soldering, PIEK International Education Centre
AC03-CS Series Axial Cemented, Leaded Wirewound Safety Resistors, Vishay Intertechnology
PCB assembly with THR technology, Weidmüller Group
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