Time on Joint (TOJ) is not well documented in the literature for selective soldering. All solder operations require a minimum time for the tin-to-base metal intermetallic to properly form, and for the capillary action to pull the solder through the component hole. A reasonable figure is 1–6 seconds depending upon the type of surface finish, the thickness of the through-hole copper, and the clearance between the component lead and the hole. Longer than this range and copper dissolution can end the process. If the time is shorter, there may be insufficient time for the complete solder action and hole-fill to occur. The TOJ is the amount of time a component pad is in contact with the liquidous solder.

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The calculation is fairly simple:
TOJ = (Diameter of the solder nozzle) + (Diameter of Component Pad)/ Pot Traverse Speed

For example:
A 9 mm nozzle soldering a small 40 mil component pad can move fairly quickly.A 2-second TOJ would allow a traverse speed of 12 in/min.

A simple chart can be created in Excel for any combination of nozzle diameter, traverse speed, and component pad size.
Using the tiny 40 mil pad of the example above, one could create the following TOJ chart.

Traverse speed in inches/minute with the tiny 40 mil pad

If we use a larger 70 mil pad, notice how the traverse speed can be increased.

Traverse speed in inches/minute with the tiny 40 mil pad

These charts should be viewed as “goalposts” within which the other variables of selective soldering can be adjusted. As with any process, the object is to start with all of the variables close to the median value for their range. You may then vary each independently (and only one at a time) to achieve the optimum process results. Future blogs will discuss the other Selective Soldering variables and how they interact with the soldering process.

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