Many of us have experienced the frustration of running an assembly on two different wave 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 solder machines have different personalities." Others blame operators. Yet the answer is often simple and measurable: All wave machines produce waves that are different shapes.
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 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 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 optimization and control.
Dwell Time Baseline for Study
A major consumer electronics company tasked one of its North American facilities to perform a month-long study to assess the significance of dwell time optimization and repeatability. The assembly with the greatest volume, representing 19% of all the boards produced at that location, was selected for the study.
For this purpose, the Wave Solder Optimizer was used as direct board-wave contact sensors are necessary for meaningful data. Capable of performing four runs in a row, the device offered the convenience of taking multiple readings before downloading the data to a PC. Also, the Optimizer's LCD display allowed the reading of data immediately upon its exit from the wave machine. Another important capability: Direct measurement of immersion depth. The following steps were performed:
Step 1Parallelism was measured and established.
Step 2Measurement of the board's current dwell time, which was 1.0 seconds.
Step 3Measurement of the board's current immersion depth, which was 24 mil.
Step 4Assessment of board quality, showing a defect rate at 312 ppm, a level considered normal at the facility despite the amount of rework being performed, and excellent by industry standards.
Step 5Steps 1 through 3 were easily performed for three shifts in a row, twice per shift, since all data was obtained in a single run of the device through the wave machine. Step four was performed at the end of each shift.
If, prior to running the assembly, measurements showed a disparallelism, or a dwell time more than 0.1 second away from 1.0 seconds, or an immersion depth other than 24 mil, adjustments to the wave machine were made and additional measurements were taken to confirm that the desired board-wave experience was occurring. Areas unrelated to board-wave interaction were maintained throughout. These include, for example, flux types, preheat settings and solder temperature.
