Production and process engineers have learned new insights to board-wave interaction, leading to dramatic changes in wave solder procedures. This has emerged only in the last several years, when technology was introduced which could directly measure your board’s experience in your solder wave. Immediate and sharp improvements in board quality have been the result, propelling widespread acquisition of such technology.

This article discusses concepts of board-wave measurement and reports study results that employed commercially available technology to measure and respond to the key parameters of wave soldering. The purpose of the study was to determine the importance of dwell time and immersion depth measurement in wave solder quality.

Centrality of Board-Wave Interaction

Your wave soldering machine was built for only one purpose: To cause your boards to interact with your solder wave. You know this to be completely true; because when you look in your reflow oven you see no wave. In the reflow oven, chemistry supports your boards as they experience temperatures. However, this is not true for wave soldering.

In your wave machine, chemistry and temperatures are only supporting actors as they deliver your boards to your solder wave. That is why your process window for temperatures in a wave machine is wide and forgiving in comparison to that of the surface mount oven, and why precise control of your board-wave interaction produces such large benefits.

Your leads are in the solder wave for only a few seconds or less. Soldering is supposed to be achieved in a single pass and emerge defect-free. Since this event is so brief and today’s boards are so complex, your board must pass through your wave with precision. Thoughtful engineers have learned that seemingly slight board-wave process variations can cause large quality variations.

Board-Wave Interaction

Assuming that your board is parallel (a parameter also requiring accurate measurement) to your wave, board-wave interaction 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.

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.

Limitations of Thermal Profiling

Many continue to adhere to the notion that wave solder process control is primarily about temperatures, and therefore choose to rely strictly on temperature stickers, pyrometers or thermal profiling. While temperatures are important, they do not and cannot by their very nature address your board’s interaction with your solder wave.

Wave soldering without accurate board-wave data is a sure prescription for consistent defects, production crises and downtime. Experientially, production professionals understand this - they see the rework staff at their workstations and bear the brunt of management’s goals for throughput and quality.

This is true despite Herculean efforts in thermal management, wonderful progress in wave solder machine quality, and the continuous development of flux and solder chemistries. Yet ask a manufacturing engineer from where the majority of his assembly defects come and more often than not he’ll point to his wave machines.

So, legions of rework staff work every day, every shift, solely to correct defects off the production line. Rather than being viewed as compensatory activity for production failures and therefore something to be exorcised, current levels of rework are often deemed "acceptable" as part of the production process itself. The net result, as we shall see here, is significant exaggeration of production costs and serious under performance in wave soldering.

For example, adjusting your preheaters can never eliminate bridging caused by too long a dwell. Likewise for skipping caused by too shallow an immersion depth. The study results presented here show that the majority of existing wave solder defects can only be eliminated through accurate, direct measurement and control of your board’s interaction with the wave.