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Selective Soldering

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Selective Soldering

Assembly is one of the important phases of PCB assembly. In this phase, components are added to the board by soldering and other techniques. The type of soldering would depend on the type of component used on the board. Wave soldering is employed for surface mount assembly, while selective soldering is a choice for soldering through-hole components to the circuit board. This soldering is commonly employed when through-hole components need to be soldered on both sides of the circuit board. Suntronic employs selective wave soldering machines to achieve consistent quality and process control when assembling boards having multiple grounds, power planes, or dense populations of SMT components on both sides.

What is Selective Soldering?

Selective soldering is a process that is used in electronic assembly to selectively solder components onto printed circuit boards. this type of soldering selects specific areas or components on the PCB. Whereas, traditional wave soldering or reflow soldering, simultaneously solders all components on the PCB.

The components are chosen using a program, which is generated using the PCB image. The program features lines where solder and flux needs to be applied. This selective soldering comprises three steps – flux application, preheating, and soldering using solder nozzle. We use automated selective wave soldering machines and software, which enables operators to set up the program easily.

Steps Involved in Selective Soldering

At Suntronic, we perform selective soldering following these steps:

  1. Preparation – The PCB is prepared by ensuring that all the components are placed in their designated places. The selective soldering machine is programmed using the centroid files. These centroid files are derived from the Gerber files.
  2. Applying Flux – Flux is applied to the board before placing it on the conveyor belt. This is done to remove the oxidized metal layer. Once this is achieved, the solder is applied. The application of flux also helps seal out air and prevent oxidation.
  3. Component Masking – Masking is done on the components of PCB that should not be soldered during the soldering process. Adhesive tapes or masking materials are used to protect these components.
  4. Selective Soldering Programming – Next, boards to be soldered are placed on the conveyer belt, which is then fed to the selective soldering machine. The machines are programmed as per the parameters like temperature, soldering time, soldering nozzle movement, etc.
  5. Solder Application – With the help of the machine’s robotic arm, molten solder is accurately applied to the designated areas or components of the PCB.
  6. Solder Process – The solder is heated by using a combination of radiant heat and convection. With this process, the solder solidifies and forms a reliable solder joint between the components and the PCB. the flux is activated by preheating the boards to 100 degrees.
  7. Cooling and inspection – After soldering, the PCB undergoes a cooling process to solidify the solder joints. Once the PCB is cooled down, the soldered PCB is inspected for quality assurance, checking for proper solder flow, fillet formation, and the absence of defects like bridging or insufficient solder.
  8. Cleaning – The PCBs undergo a cleaning process to remove any flux residues or contaminants left from the soldering process.

Important Questions on Selective Soldering Answered

  • What are the considerations made by your team during the selective soldering process?
    Our team makes several considerations during the process and the following are a few popular ones among them.
    • The size of PCB: We use small and large machines for selective soldering. The choice of the right machine will depend on the size of the PCB. If the PCB size is small, we use a secondary solder pot, which accelerates the soldering process. Also, if the size is small, we can configure two conveyors to process double number of assemblies.
    • Solder type: We use different types of solder alloys for soldering. We use both leaded and non-leaded solder for soldering. The lead-free alloys need high soldering temperatures than leaded ones. They are operated at pot temperatures between 270 degrees Celsius and 300 degrees Celsius. Lately, our experts have been using lead-free solders which can be operated at pot temperatures of 250 degrees Celsius and more.
    • Nozzles: These components enable the soldering of areas that are otherwise difficult to achieve. We use nozzles in different lengths and diameters to achieve consistent soldering across all areas. Although small nozzles in 1.5mm allow us to access restricted areas, they may require long process times.
    • Nitrogen Supply: We use nitrogen supply to prevent oxide formation of liquid solder. This nitrogen is supplied through a tank comprising liquid nitrogen. Apart from this, sometimes pressurized nitrogen bottles and locally generated nitrogen are also used for the purpose.  
    • Solder Pots: Selective soldering is one of the longest assembly processes. To cut down the times, we use multiple solder pot configurations.
    • Solder Flux: This chemical is used to clean the metal surfaces, which are to be soldered. We regularly work with no-clean fluxes/low-solids, water-soluble fluxes, and rosin fluxes. Each of these flux types has typical advantages. Our experts will choose the most appropriate solder type depending on the solids content, activity, and material type.  
  • Do you perform wave soldering?

Yes, we do perform wave soldering for through-hole and SMT assemblies.

  • What are the advantages of choosing selective soldering over wave soldering for through-hole assemblies?

We usually recommend selective soldering over wave soldering for through-hole assemblies for the following reasons:

    • It allows the operator to program the exact spots where molten solder is to be applied.
    • The operator can set the nozzle speed to achieve the adequate heating time required for the solder to fill through-holes.
    • The machines allow operators to set the temperature and solder amount.
    • This soldering produces reliable solder joints.
    • It doesn’t require glue as the wave soldering process.
    • Different parameters are set for each component to ensure effectiveness.
    • It requires no expensive wave solder pallets.
    • The overall process is cost-effective. 
  • Are there any special requirements for choosing selective soldering over wave soldering?
    There are certain conditions for which wave soldering or manual soldering may not be effective. Selective soldering works well in these conditions.
    • When tall components are involved, they may stop the wave from soldering the board properly.
    • When SMT and through-hole parts are kept closer to each other, there may not be enough space to install a protective fixture to protect SMT parts. If protection is not added, SMT parts may be damaged during the wave soldering process.
    • Manual soldering becomes difficult when boards with thick copper layers for ground and power planes are involved.
    • When through-hole pins are placed in dense configuration, it becomes difficult to solder those properly using the soldering iron. This is when selective soldering equipment with wide nozzles is used. These nozzles can solder rows of connector pins.

For several decades, Suntronic has been providing expert PCB assembly services. Our advanced engineering capabilities allow us meet complex fabrication and assembly requirements with ease. We ensure the board meets your exact design requirements and technical specifications. Feel free to contact us today to know more about our selective soldering capabilities and other PCB assembly capabilities. Our experts would be happy to answer your queries regarding the services we offer and make the right decision regarding PCB designs and choice of components.