Automated Selective Soldering is a viable approach to enhance the affordability of certain electronics manufacturing operations. This step-by-step approach to selective soldering is meant to help decide which method is best.

Step 1: Decide if automated selective soldering can improve affordability in your product.

Is the electronic circuit card assembly (CCA) product mixed surface mount technology (SMT) and plated through-hole (PTH) technology? A"yes" to this question is imperative for selective soldering to be an economically viable option in the first place. Most cases of selective soldering are instituted to accommodate a small number of plated through-hole components that are usually manually soldered in an otherwise predominantly surface mounted assembly. These components are typically electrical connectors or high power supply components that are only available in through-hole mount versions.

Manual soldering is the obvious alternative and is typically used for prototyping a new assembly with a few through-hole mount components. Since there is an inherent inspection operation in any manual soldering application (by the operator doing the soldering), an automated selective soldering operation requires a planned inspection step to maintain acceptable joints.

Step 1 also asks the question is the product high or medium volume or is the product high mix and low volume? The answers to this question will indicate the appropriate hardware and software combination to secure the most cost-effective automated selective soldering operation for your product.

Step 2: Decide which type of selective soldering will be most effective for your product.

• Selective aperture pallet (masked) wave
• Mini solder wave with mask
• Programmed solder fountain
• Programmed laser

A large number of assembly processes used to perform selective soldering have been developed; each has economic advantages and disadvantages.

These processes include:

Selective aperture pallet (masked) over wave solder. This is the simplest form of selective soldering but must be balanced between the need to do selective soldering and the need to do production wave soldering. The selective aperture fixture effectively masks the areas previously soldered in the SMT reflow and non-selective wave soldering process while exposing only those areas to be selectively soldered. The fixtured PCB assembly is then passed over traditional wave soldering equipment to complete the process. While each fixture is specific for a PCB assembly, wave soldering is a routine operation that is very cost-effective for a small volume and high mix assembly.

Mini solder wave with mask. For higher volumes of products requiring selective soldering, the investment in smaller, dedicated wave solder equipment is economically justified by increasing the availability of the production wave solder equipment.

Programmed selective solder fountain. This process is slower than the two previous methods but much more accurate in terms of solder volume and joint quality. A small fountain of molten solder (Figure 4-1) is pumped to each through-hole joint of the PCB. The PCB may be fixed and the solder fountain moved underneath the PCB using a precision, 3-axis programmable gantry or the PCB may be positioned over a stationary solder fountain to perform the selective soldering process. Computer Aided Design (CAD)-based board layouts can be imported to accurately position the board and solder fountain. Since each solder joint can be individually created, no unique fixturing is required. This technique requires both dedicated hardware and software and can be justified for extreme low volume and high mix applications.

Laser selective soldering system. Instead of a solder fountain, the laser selective solder system uses the CAD data to position a laser (with solder wire feed) and directly solder any point on the board. The major advantage is the elimination of thermal stress to the surrounding board area. This non-contact soldering process produces consistent high quality solder joints at typically one per second with maximum flexibility. It may even be possible to eliminate stencils and solder masks from the circuit board to reduce manufacturing costs. This represents the most costly but the most productive method of selective soldering for very high mix, very high volume manufacturing.

Step 3: Select flux application method.

Regardless of selective soldering equipment, there are two types of selective flux applicators: a spray fluxer and a dropjet fluxer. The spray fluxer applies atomized flux spray to a specific area while the dropjet fluxer has more precise control over flux volume and card position. The final choice depends on the circumstances surrounding the soldering application.

Contact the EMPF with any questions regarding affordable selective soldering options for your product assembly at 610.362.1320 or via email at helpline@empf.org.