SMT Quick-Tips 5: Selecting a Selective Soldering System
From: Author:Mark Hardy Publish time:2021-08-31 16:31 Clicks:0
Selective Versus Wave Technologies
It’s important to remember that Selective Soldering does not necessarily replace a wave machine, because wave soldering is still the most efficient method of processing boards with only through-hole components. But Selective is essential for a mixed technology board, and depending on the nozzle type used, can replicate the wave technique in a compact way.
Nozzle Types
Jet (or Wave) soldering is similar to wave in that it’s uni-directional and provides the same benefits as wave. Its smallest solder diameter is 4mm, it requires minimal maintenance, and it is fairly low cost. The nozzle type you use will depend on the make-up of the board and the location of SMDs on that board.
For instance, a fairly large row of connectors/leads not in close proximity to SMD components could use a wide nozzle to swipe (or wave) the entire row at once. A small area closed situated to an SMD would require a very small nozzle to avoid disturbing the surface mount device.
Jet nozzles attack the board at about the same angle (7) as a Wave machine, and can deliver a high volume of solder using a tapered tip which guides solder roll-off in one direction returning unused solder back to the pot from its trailing edge. In this scenario, the direction of travel (board or nozzle) is very important.
Jet nozzles are long-lasting and usually cost only a few hundred dollars. Some boards can benefit from a custom nozzle to solder an area as quickly as possible in a dip or drag process, or even a min-wave.
Pros: Many nozzle types, including custom configurations, can be used for most efficient speed and high quality, if the board population permits it.
Cons: Boards (or solder pot) can only move in one direction making programming a little more complex.
Wettable nozzles create a uniform bubble by virtue of the surface tension of the solder. Excess solder rolls off around the entire surface of the nozzle, rather than in a single direction, thus producing less turbulence from solder roll-off and helping to preserve solder quality. For this reason, a wettable solder can move in every direction, 360 degrees, or in either the x or y direction, and it is easy to control the bubble height. This combination provides maximum flexibility for a large variety of boards.
Wettable nozzles provide finer accuracy than jet type which makes it better suited to connections in close proximity to SMDs. It also produces less oxidation because there’s less contact with air. For this reason, wettable nozzles are also best suited for lead-free solder which tends to be more vulnerable to oxidation.
Pros: Provides pinpoint accuracy for highest connection integrity on densely populated boards.
Cons: Costs more than Jet wave and requires daily maintenance to prevent clogging.
Custom nozzle capabilities can be made to cover very large areas of a circuit board in a single pass while masking SMD components to speed production.
Contract manufacturers who assemble a wide variety of board configurations should make sure the machine they’re investigating can accommodate both types of nozzles and/or custom/hybrid configurations if they think they are ever going to need them.
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