Assuming that your board is parallel (a parameter also requiring accurate measurement) to your wave, board-wave interaction in wave soldering has three distinct, simultaneous facets that can be directly and accurately quantified:
1.) Dwell Time: - The amount of time a lead is in the solder wave. This needs to be controlled in one-tenth second increments.
2.) Immersion Depth: - How deep your board immerses in the solder wave. Since the very best waves have a wave height variation between 10 and 20 mil, this parameter is optimally gauged by its passage through a process window. The device used for this study employs 12 mil increments for this purpose.
3.) Contact Length: - The distance in which a lead passes through a wave.
Figure 1 illustrates the interrelationship of immersion depth and contact length for your boards, showing that in fact your immersion depth directly determines contact length. This in turn directly affects dwell time, since:
Dwell Time = Contact Length Conveyor Speed
What this means to the wave solder engineer is that your conveyor speed setting will not on its own control your dwell time in the wave. You must in fact have a means of accurately measuring and controlling your immersion depth as well.
Wave Shape
Many of us have experienced the frustration of running an assembly on two different wave soldering machines and seeing two very different board qualities emerge. Why do your wave machines produce different results when both are set at the same pump speed, conveyor speed, conveyor angle, solder pot height, preheat and solder temperature, are using the exact same chemistry, have the same maintenance schedules and show the same thermal profile?
As an industry, we have often retreated to accepting that “different wave machines have different personalities.” Others blame operators. Yet the answer is often simple and measurable: All wave machines produce waves that are different shapes.
Figure 2 shows the impact of wave shape on contact length. A wider wave will mean a longer contact length - and therefore dwell time - at the same immersion depth.
Limitations of Machine Set-up
What this all means is that in order to control your wave process you need to directly measure what your board actually experiences in the wave. Wave soldering machine settings can never assure repeatability. Your board does not see a conveyor speed; it does experience a dwell time. Likewise, your board does not know your pump speed; it experiences an immersion depth. Also, your wave machine settings do not tell you the wave machine’s variability. Therefore, parameters for wave soldering must be based primarily on guidelines for board-wave interaction, not wave soldering machine settings.
Assembly plants need no longer blame their wave solder machines, flux or personnel when their real challenge is the wave solder process itself. Your wave machine does not even purport to measure your board-wave interaction. Good equipment does not compensate for uncontrolled process. The best wave solder equipment in the world still requires a sound approach to process wave soldering optimization and control.
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