Electronic PCB assembly soldering: selective or wave soldering?

When producing ‘mixed technology’ printed circuit board (pcb) assemblies, once all of the surface mount technology (SMT) devices are in place, there are a variety of approaches available for attaching the plated through hole (PTH) components. Traditionally these have included either an operator soldering PTH devices by hand, or utilising automated equipment known as a wave or flow solder machines.

As our hunger for smaller, lighter, faster, more complex products packed full of the latest technology grows, we have seen a growing shift towards the use of double sided PCBs - which provide designers with twice the available space to build upon. Just compare any home computer or mobile phone you are using today with the one you had back in the 1980’s and it’s obvious to see how things have changed.

Adding components to both sides of a PCB instantly creates a challenge for CEM providers. There will often be a mix of SMT a PTH parts on one or both sides which can mean traditional wave solder is not suitable for automated production.

The purpose of this blog post is to explore the pros and cons between the traditional wave soldering process and the use of more advanced selective soldering machines for PCB assembly, such as those supplied by Ersa. 

The basics of the soldering process

Before we look at the differences I thought it would be beneficial to briefly explain the basic process of how both wave and selective solder machines physically connect the electronic components to the PCB.

Wave solder machine

There are 3 separate processes involved within a wave solder machine.

1. The first stage in the process is for the machine to apply a layer of flux to the underneath of the entire board in order to help clean the components and PCB in the areas requiring solder. Any impurities, such as the forming of oxide layers, can affect the soldering process which can then lead to poor quality solder joints.
   
2. Once the flux has been applied, the PCB is gradually heated up which both activates the flux prior to soldering and also helps prevent the board from suffering thermal shock.
   
3. Finally the PCB is passed over a molten ‘wave’ of solder. As the PCB moves over the wave, via a mechanical conveyor driven system, a connection is made between the electronic component leads, the PCB pads/holes and the solder itself which forms an electrical connection.

Miniature wave selective solder fountain

There are also 3 separate processes involved within a miniature wave selective solder machine however:

1. Flux is only applied to the components that need to be soldered not the entire board. The flux is accurately applied using a nozzle which can vary in size to increase or reduce the area to be soldered. This is particularly useful when attempting to solder densely populated PCB’s.
2. Once the flux has been applied, the PCB is gradually heated up in the same way as the wave solder machine, activating the flux prior to soldering and once again helping to prevent the board suffering thermal shock.
3. Finally, instead of passing over a ‘wave’ of molten solder, smaller, more accurate nozzles , are used to deposit the solder and form the connection.

So now we have covered the basic soldering process differences, what are the Pros and Cons of selective soldering compared with wave soldering?

Pros

1. Double sided PCB’s can be soldered without the need for glue (which holds components in place during reflow) or wave solder carriers.
2. By applying the flux locally, there is no need to mask off certain components and test points for example, avoid being soldered.
3. You have the ability to set different parameters for each component or location, for instance a pin connected to a ground plane may require more contact time to ensure solder flows to the topside filling the plated hole.
4. Selective solder machines are generally cheaper to run due to the reduced amount of flux and solder being used.
5. It’s possible to use a solder with no silver content and still get good wetting as the parameters for each joint are variable.
6. And finally you have the ability to process boards that cannot usually be wave soldered i.e. those that have tall PTH components on both sides. Arguably this is the most beneficial feature as the alternative is to hand solder.

Cons

1. A unique program must be created for each circuit board produced, similar to those used within automated SMT machines. The creation and ‘fine tuning’ of these programs can add additional time and cost to the process for Electronic Manufacturing Services (EMS) providers working within the low to medium volume, high complexity markets.
2. With the increased number of parameters available for each component, there is a risk (if not controlled properly) that process issues can rise. Whereas a wave solder machine has a small number of parameters to change (speed of conveyor, preheat temperature, amount of flux applied and wave height), selective soldering machines can have many more as each component in theory can be treated differently.

So whilst there may appear to be far more benefits in using a selective soldering machine to that of a wave solder machine, it’s important to put them into perspective. Wave soldering continues to be an efficient way for EMS providers to solder PTH electronic components to a PCB. Consideration must be made therefore as to how many of the boards that are being produced today, or required in the future are not suitable for this method of soldering. If the number is high and an automated, high quality alternative to hand soldering is required, miniature wave selective soldering could be a viable solution.

If you have found this content useful and would like to read more about electronics manufacturing click here to subscribe to our blog.

Image by: SparkFun Electronics