Soldering is a process for attaching electronic components to a printed circuit board (PCB) to form an electronic assembly. There are two automated methods for accomplishing this – wave soldering or selective soldering (Figure 3-1). Both expose the PCB to a molten wave of solder, but in wave soldering, the entire board passes over the wave, simultaneously soldering all connections, while in selective soldering, individual connections see the wave serially, in a programmed order.

Wave Soldering
A standard wave solder machine consists of three zones: the fluxing zone, the preheating zone, and the soldering zone. The fluxing zone contains either a spray or foam fluxer. Using a series of nozzles or an ultrasonic head, a spray fluxer applies a fine mist of flux to the underside of the PCB. A foam fluxer consists of a porous cylinder inside a tank of flux. When air is passed through the cylinder, a head of flux foam is created which rises up to touch and coat the PCB as it passes overhead. For either type of flux application, the flux quantities are precisely controlled – too little flux causes poor solder joints, while too much flux causes cosmetic or other problems.

The PCB then enters a preheating zone which uses convection or infrared heaters to increase the board temperature. Preheating activates the flux, removes any flux carrier solvents, and prevents thermally shocking the board with a sudden exposure to the high temperature of the molten solder wave.

The soldering zone uses a tank to hold a large quantity of molten solder and a pump to create a wave with a precisely controlled height. When the PCB is moved over this tank, the solder wave contacts the bottom of the board and sticks to the solder pads and component leads via surface tension, creating a reliable mechanical and electrical connection. Since the entire board is soldered in one pass, the process provides rapid throughput.

Wave solder fixtures may be required for PCBs where bottom side components cannot be exposed to the solder, or the board is either too small or has a non rectangular shape and cannot be conveyed easily. The fixture is made of a high temperature material that is not wetted by solder. Openings in the fixture expose components to be soldered while other areas are protected from seeing the solder or heat.

Selective Soldering
Often, there are interconnect hardware, displays, or other components that cannot withstand the exposure to the high temperature involved in the wave soldering process. While hand soldering can be considered, the process is slow and inconsistent; the quality of the results is entirely dependent upon operator skill, and can vary widely from day to day and hour to hour.

More and more individual, offline, or challenging soldering operations are being completed with the use of selective soldering machines. This is especially true for high volume assembly operations or for components that simply cannot be effectively hand soldered. Selective soldering is precise, programmable, fast, and consistent. In order to increase productivity volume, and achieve greater product consistency, automating soldering is necessary.

Selective soldering machines will precisely solder only the components necessary without disturbing nearby SMT chips or other heat sensitive components. These are programmable machines with a small solder wave. Using computer controlled three axis motion, either the board is precisely moved over the solder wave (Figure 3-2), or the solder wave is moved directly under the board.

Initial programming is accomplished by teaching the positioning system all the locations on the PCB to be soldered. This can be done by importing a PCB CAD file containing all the start/stop positions for the devices to be soldered. In addition, an imaging system can be used to accurately view the start and end of a row of terminals to be soldered and place the positions into memory, building a process path and script, component by component, manually. After teaching these positions for all sites, the X, Y, and Z positions, speeds, solder wave height and other parameters can be adjusted to optimize the process.

The cycle begins by using a jet or spray to apply flux to the programmed sites. With accurate positioning, the flux can be applied with minimal overspray, avoiding any neighboring components. Next, the solder wave is automatically positioned under the component to be soldered and the pot rises (or the board is lowered) to “wet” the first pins. The solder wave travels the length of the component, soldering the through-hole leads to the PCB. At the completion of the travel, the solder pot lowers (or the board rises) and the next site is positioned. All programmed sites are soldered in the same cycle.

Due to the variety of board configurations, ground planes, and component layouts, the best way to selectively solder can only be determined through experience. Multi-row components, such as connectors, require the solder pot to be moved at an angle at the end of the row to prevent solder bridging. If the board has very tall components on the solder-side, the distance between the board and solder pot must be considered when planning a soldering path. The solder pot must be able to clear all the components when soldering or moving to a new location. The leads on the board should just contact the solder wave without colliding into the nozzle. Thinner boards can warp when heated, so adjustments must also be made to the soldering distance.

To avoid swapping nozzles, the largest nozzle should be used that can perform the desired soldering operations on the entire board. Larger nozzles have solder waves that apply more heat to the board and are easier to maintain, they can accept larger board variations without missing pins or flooding.

The solder pot wetted surfaces are constructed of materials which are capable of withstanding aggressive no-lead solders. The heaters can bring the solder safely to temperature within an hour. Solder is re-circulated using a speed-controlled motor coupled to an impeller assembly. A nitrogen blanket ensures an extremely consistent and repeatable solder wave shape while minimizing dross, icicles, and solder bridges. The solder temperature is tightly controlled and the capacity of the solder pot ensures sufficient solder mass for even the largest assemblies.

The change to lead free solder as part of the required RoHS compliance has many board assemblers concerned about lead cross contamination. The ability to swap solder pots in and out of the machine allows most users to simply have two solder pots, one for lead based solder, and one for lead free. Each pot is equipped with its own solder pump and delivery system.

Wave soldering and selective soldering can both be used for electronic assemblies. Depending on the mix of technologies on each board, the process selected may be an easy decision. Wave soldering provides the fastest process since all components are soldered in one pass. Selective soldering can be customized to any board configuration and any heat sensitive components, but is the slower technique. Some form of automated soldering should always be considered to increase product quality, decrease soldering defects, and reduce labor costs. If you would like a demonstration of wave soldering or selective soldering, please call the EMPF Helpline at 610.362.1320.