Soldering is a crucial step in the of PCB assembly process that uses molten solder to bond the various components to the board. Solder is an alloy typically composed of a combination of elements; one of the most common for PCBs is lead-free (Sn-Cu) rosin core solder, but there are many varieties of solder available. The other essential ingredient in soldering is flux.

There are two primary soldering techniques used in PCB assembly: wave soldering and reflow soldering. Deciding which soldering technique is best for your specific application is just one of the reasons OEMs need to partner with a trusted PCB assembler like RiverSide Integrated Solutions (RIS). The team at RIS guide customers through the process and ensure the right PCBA technologies and techniques are selected for their specific needs.  Before diving further into the details of the different PCBA soldering techniques, it will first be helpful to review the steps involved in PCB assembly to better understand the role of soldering and where it fits in the process.

An Overview of the PCBA Process

Below are the primary steps in the PCB assembly process.  The steps and sequencing may vary slightly depending on whether SMT or THT technologies – or a combination of both – are employed; however, the following describes the key elements of any PCBA process.

1. Apply Solder Paste: the first step in PCBA is to apply the solder paste to the sections of the PCB where components will be surface mounted. The paste is a mixture of solder and flux that is used to permanently join the components to the board in future steps. Many PCB assemblies use a stencil to guide the solder paste and ensure it is delivered to the correct areas of the board and in the proper amount.
2. Place the Components: the next phase is to place the necessary components on top of the solder paste. This step is often called “pick and place”. Depending on the functionality of the PCB, the components being added to the PCB may be resistors, capacitors, inductors, sensors, or other electrical parts. This step can be performed manually or with automation, but either way it is imperative that the right components are placed in the specified location on the board.
3. Soldering: the purpose of this step is to permanently bond the components to the PCB. PCBA processes using Surface Mount Technology, SMT, typically utilize a reflow oven and conveyor system during the soldering step to melt the existing solder paste. With Through Hole Technology, THT, the component leads are inserted through the existing holes and a wave soldering method is often used where a wave of molten solder flows across the bottom of the board. In both cases, the molten solder joins the components and leads to the board – without compromising performance or structural integrity. The assembly is then cooled to solidify the solder once again and fix the components in place.
4. Inspect the Assembly: this phase of the PCBA assembly process is crucial. Proper inspection is necessary to ensure the prior steps were performed effectively. This step may entail manual, automated and x-ray technology.
5. Testing: the final step in the PCB assembly process is product testing to ensure the PCB functions as designed. There are various test methods such as flying probe, in-circuit testing, and others. Once testing is completed, the PCBA component assembly process is complete.

Primary PCBA Soldering Techniques: Reflow and Wave

As mentioned above, the main soldering techniques used during automated PCB assembly processes are reflow soldering and wave soldering. The soldering method that is best for a given application often depends on the type of PCB components: wave soldering is often utilized for a board with mostly through-hole components whereas reflow soldering is often the method employed for SMT components. Below is more information about each method.

Wave Soldering

With this method, the board is sprayed with flux and then routed through a wave of molten solder. Sometimes the PCB is sent through multiple waves of solder. The wave is typically created using a pump in the molten solder that creates a standing wave of solder for the board to pass through.  As the molten solder cools, it will harden and connect the components to the board. While wave soldering can be used for both THT and SMT assemblies, this technique is primarily used in the soldering of through hole components. This is because the solder will fill the through hole component vias more adequately, making a better connection. In recent years, many through-hole components have been replaced by surface mount components, thus, the demand for wave soldering has declined. However, there is still significant wave soldering where through hole technology is prevalent or where surface mount technology is not suitable.

Reflow Soldering

In reflow soldering, solder paste, which is a sticky mixture of powdered solder and flux, is applied to the PCB to temporarily bond components to their contact pads on the board. The assembly is heated and the solder melts to a molten state. The heating typically occurs by passing the board through an industrial convection oven. Upon cooling, the solder hardens to permanently bond the components to the board, forming the electrical connections for the assembly. For double-sided boards, reflow soldering is performed on one side then the PCB is flipped and the other side is soldered using the same process.

During reflow soldering, the solder paste must reach the eutectic temperature at which the specific solder alloy undergoes a phase change to a molten state. The temperature profile for a given PCB assembly is carefully set to achieve the eutectic temperature and allow for reflow of solder without overheating and damaging the electrical components.

While reflow soldering is primarily used when a board has many SMT components, reflow ovens may also be utilized for THT components by filling the holes with solder paste and inserting the component leads through the paste.

RIS – Your PCBA Partner

At RIS, PCB assembly is one of our core competencies. We provide reliable, seamlessly-assembled printed circuit board systems built with extreme precision. Our skilled engineers, technicians, and assemblers work to expertly put together a complete PCBA. When you choose RIS for your PCBA projects, you can expect the hands-on customer care of a small manufacturing operation and the capabilities characteristic of a much larger company with our complete electronics manufacturing services for OEMs in any industry. Our electronics assembly expertise includes:

• Surface Mount Technology (Fine-pitch, BGA (1 mm), µBGA (.4 mm), 01005, X-Ray, automated rework, and automated optical inspection (AOI))
• Magazine to Magazine handling
• Smart Shelving floor stock inventory control
• X-ray reel counting
• Lead-through auto-insertion (Axial, Radial, DIP, and Zierick Terminals)
• Wave Solder and Selective Solder Machines (No-clean & aqueous flux processes) RoHS and Tin/Lead capabilities
• Special Application Equipment (Robotic dispensing systems)
• Conformal coating (Silicone, Acrylic, Urethane)
• Potting (Epoxy, Urethane, and Silicone)
• Low-pressure molding technology
• Comprehensive product test

RIS is an advanced contract manufacturer providing robust solutions in circuit board assembly, product assembly, kitting, supply chain management, fulfillment, distribution, and reverse logistics. We employ more than 300 people and provide services to OEM’s across the world. We operate 3 state-of-the-art manufacturing facilities within the U.S, and as your one-stop-shop we have the capabilities, capacity, quality assurance standards, and resources to provide you with support for all of your manufacturing needs.

With all of the choices in contract manufacturers out there, we know it can be difficult to find someone that not only understands your business model but also has your best intentions in mind. RIS has proven to always be a win-win focused relationship. As your one-stop-shop, we have the capabilities, capacity, quality assurance standards, and resources to provide you with support for all of your manufacturing needs. We understand that supply chain management is difficult and very time consuming, so we urge our customers to utilize us in the fullest capacity. Our total-package solutions include:

· Extensive supply-chain network

· Purchasing and inventory management

· Dedicated Program Manager

· Warehousing and drop-shipping capabilities

· Flexible order fulfillment

· Product assembly: sub-assembly and full product

· Scalability to meet your needs

Contact us today at (507) 523-3220 to see how we can help with your manufacturing project, or click here for a quote.