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In the last column, we introduced the various types of through-hole (or thru-hole) soldering techniques, which include: Manual, Dip, Drag, Wave, and Selective.
While manual soldering is still in practice today, it is usually not found in a production environment because it involves highly skilled and labor intensive work producing very low numbers of boards. Dip and drag were introduced many years ago as lower cost alternatives to wave soldering, but have become outdated forms with the advent of more affordable and highly accurate wave systems.
Today, wave soldering is the most common and most efficient form of through-hole soldering available. It involves a solder pot large enough to handle the width of the largest boards you expect to process. By pumping hot solder through a nozzle in a way that the bottom of the board surface encounters the wave caused by the nozzle, the resultant hot solder waterfall creates a single point of contact across all the connection joints on the board, eliminating any potential bridging. The system usually integrates a fluxing station, a pre-heat station, and a wave station in a conveyorized system, using fingers or pallet-type board mounts.
Wave soldering is a time-tested technique, but there are many variations of systems to look for depending on your needs. In all of them, three operations are involved:
1. Applying the flux
2. Applying pre-heat to activate the flux
3. Applying solder
The first step in selecting a wave system is to identify the largest board size (length x width) the machine will be expected to process. This will determine the machine size and, just as importantly, its cost, from a benchtop unit handling 8” boards (starting in the $10,000 range), up to free-standing machines with capacities up to 24” using a very large solder pot (costing as much as $80,000 or more). The larger sizes will also accommodate much more solder, which also contributes to the cost of ownership.
A larger system provides greater flexibility. It can not only handle larger boards, but it can also handle multiple smaller boards conveyed side-by-side to increase throughput. A prospective buyer may consider production throughput by calculating number of boards through a total cycle time from start to finish varying from 2.5–4 minutes from flux to solder.
Features of wave solder
There are two types of wave:
• Laminar wave produces a very smooth laminar flow of solder much like a waterfall. This is the most widely used type.
• Turbulent wave sends solder up into the board with a highly turbulent wave used for thru-hole components such as pins with long legs that are fairly closely positioned, and for denser components. The turbulent wave helps to wick off (pull solder from the component’s stem into the component joint) excess solder to reduce icicles or bridging.
Some machines are available with dual pots that can be used independently or together on the same board. SMD components are often processed in a turbulent wave machine.
Editor's Note: This article originally appeared in the December 2015 issue of SMT Magazine.