Solder paste replenishment is typically based upon how the paste is performing on the stencil relative to its rheology, such as its viscosity, tackiness, and its ability to be released from the stencil. As the paste is exposed to the local plant environment during the day, it dries out loosing some of the viscosity agents and carrying vehicles like alcohol, thereby making it stiffer to roll and penetrate through the stencil.

A viscometer like a Brookfield viscometer, can be used to determine what the initial viscosity should be, as the paste should have been purchased to a specified viscosity and a comparison can be made to determine the differences between those two numbers, i.e. the old and the new.

The stiffer the solder paste becomes the more difficult it will be to stencil and its behavior on the stencil will change. I know this is difficult to express, but when it is first used it is smooth like a cream and as it dries out it becomes more pasty and it doesn’t roll easily along with the squeegee on the stencil. The squeegee will have trouble wiping it clean off the surface of the stencil. New paste can be added to make is spread easier and replenishment can be as easy as adding more solder paste.

Depending upon the type of solder paste used and the environment within the facility it can also absorb moisture which can or may change its reflow characteristics. The absorption of moisture into the solder paste may result in excessive solder balls after reflow, so the thermal profiles need to be closely monitored and compared to the finish solder joint quality to verify the solder paste is reflowing according to capability.

There is no formula that I know of as to when the solder paste should be changed, however experiments should be conducted to determine the working life of the solder paste on the stencil. Measure the time it takes for the solder paste to change characteristics on the stencil to predetermine when it should be changed.

The initial volume of solder paste applied to the stencil when beginning the shift needs to be enough to spread across the entire width of the squeegee and be around ½ inch high across its entire width.

Additionally, used solder paste should never be returned into the original jar as it will contaminate the original paste and will change the rheological properties of the remaining paste. Used solder paste should be properly discarded based upon the local environmental requirements.

Rule of thumb is to maintain a paste bead of 0.5 to 0.75 inches high. The first symptom of low paste bead is poor aperture fill, usually at the far ends of the board. Without an automatic dispenser, the operators must be training to maintain the paste volume. One clever method I've seen in the field was a "go/no go" gauge cut out of a plastic block which the operators used to measure the bead size periodically.

Along with this tool they would set a "batch limit" to pause production every 20 prints to alert the operator that another measurement was needed. This is a cheap solution, but it does negatively effect throughput, and does not guaranty 100% of the opportunities are prevented.

Another option is to monitor the paste height using optics positioned at either end of the squeegees which will trigger an alert when the paste bead reaches an unsafe level. On DEK printed this option is call the "Paste Roll Height Monitoring" feature.

The ideal method is to use the vision inspection on the printer to inspect each board printed to detect low paste. Monitoring the four corners of the board, a 2Di system will allow the printer to trigger an alert whenever the paste coverage on the inspected areas go below a safe amount. The down side to this method is cost to throughput. However, modern day 2Di inspection systems, such as Hawkeye found on DEK printers, set up easily in a minute or two, and can inspect these same sites in only a few seconds, greatly minimizing the cost of inspection on throughput or setup time.

http://www.dek.com/web.nsf/us/Hawkeye