Solder paste is arguably the most highly engineered material in electronic assembly. It has many properties that must be favorable for its good performance. It must provide a well-defined solder paste deposit that resists cold and hot slump. The paste must provide adequate tack to hold the components to the PWB. As it travels through the reflow oven, the flux must clean off any oxides on the PWB pads and as the temperature increases, the oxygen barrier in the flux must protect the solder particles from oxidation.
There is one solder paste parameter that can make quite a difference in productivity: response-to-pause (RTP). There are times when the assembly line must be “paused.” An example would be when loading components on the component placement machines. During this pause time, some solder pastes will stiffen. When this happens, the first stencil print must be discarded. Cleaning the paste off the stencil can take up to ten minutes. If this process is performed several times a day, the lost production can be significant.
A good solder paste should be able to remain in a paused position (i.e., not being printed) on the stencil for more than an hour without significantly affecting its print performance. As mentioned above, pause situations occur when an SMT line needs to be stopped to replenish components on placement machines or for minor maintenance issues, for example. However, some pastes “stiffen” when printing is paused. This undesirable characteristic is called poor response-to-pause. Figure 1 shows the volume of solder paste deposits for three solder pastes as a function of pause time. In this experiment, the solder paste was placed on the stencil directly from the paste jar without mixing. Note that solder paste 3 has an initial printed volume of only 5300 mils3. In just three more prints it goes up to 9100 mils3. After pausing for one hour, solder paste 3 plummets to 7500 mils3. Note that solder paste 2 is much more consistent in the volume of the solder paste deposits, and solder paste 1 is the best.
Figure 1. Response-to-Pause Measurements of Three Solder Pastes.
If an assembler uses solder paste 3, they may have to reject the first PWB printed after a pause. Typically, this situation would require the assembler to wipe the board clean after the first print after the pause and reprint it. This operation would take several minutes.
Several minutes doesn’t sound like a big deal. However, I have worked with engineers to assess the productivity cost of this production time loss. In one study we found a productivity loss of 7%. For instance, if the assembly line was able to produce 10,000 PCBs within a certain time period with a solder paste that had good response-to-pause, it would only produce 9300 PCBs if the response-to-pause was like that of solder paste 3. This 7% productivity loss is due to the lost time performing the reprints after pausing.
SMT assembly has been around for about four decades. So, you might think that all solder pastes would have good RTP. Sadly, this is not the case. Therefore, good RTP is one of the first performance metrics to measure when evaluating a solder paste.