Downtime calculations can be a bit confusing. Most people would assume that if a line has 4 machines, and they are each down for 5% of the time, that the total downtime (DT) is 4 x 5% or 20%. Thinking about another example will show a flaw: what if each machine is down 30% of the time? Would the total downtime then be 4 x 30% or 120%? Clearly, 120% cannot be true.
The mistake that is made is that if one machine is down, the clock stops on the other machines. As an example, let’s do the 30% DT case. Assume Machine 1 is down; the downtime for machines 1 and 2 is then:
0.3 + (1-0.3)*0.3 = 0.51
(1-0.3) is the time the line is up for machine 2 to be down on its own.
Figure 1. One cause for downtime on a stencil printer is poor response to pause. When this occurs, the first print after pausing may have to be reprinted. Choosing a paste with good response to pause can eliminate this downtime concern.
For all 4 machines the downtime is:
0.3 + (1-0.3)*0.3 + (1-0.3) (1-0.3)*0.3 + (1-0.3)(1-0.3)(1-0.3)*0.3 = 0.7599
A simpler way to calculate the DT is to calculate the uptime (UT) and subtract for 1:
Uptime = 0.7 x 0.7 x 0.7 x 0.7 = 0.2401
DT = 1 - UT = 1 – 0.2401 = 0.7599
This and many other aspects of electronic assembly are covered in SMTA’s SMT Process Engineer Certification. I will be teaching this certification workshop from July 20-22 in Jasper, IN. SMT Process Certification is a valuable asset to a process engineer’s career.