Reading time ( words)
Assist time is the human intervention required to keep a machine producing during normal operation. It does not include errors, maintenance, setup, and prototyping.
Improving machine process throughput requires more or faster machines. Lost time on a machine can’t be recovered without adding more time. Consider an SMT printed circuit board assembly (PCBA) with 500 component placements:
- If a line is stopped for one minute, for any reason, and its capacity is 5,000 components per hour (cph)
- 83 placements are lost or four PCAs per 24 hours will not get shipped to the customer, likewise
- 40,000 cph = 667 placements lost or 32 PCAs per day
- 100,000 cph = 1667 placements lost or 80 PCAs per day
There are many reasons for a machine to require assistance. PCB loading, stencil wiping, paste addition, feeder replenishment, and PCBA unloading—some require labor, some can be automated, but all will require some amount of time to complete.
The pick-and-place (PnP) machine is usually the bottleneck. Let’s simplify and only look at three assist operations required of every PnP machine (see Table 1).
Line A may be low volume; Line B, medium volume; and Line C, high volume. The maxi- mum achievable throughput would require instantaneous board transfer, registration and feeders of unlimited supply.
Considering the PCBA above with 500 placements, assuming all the parts are supplied on tape and reel with an average reel size of 3,500 parts/reel, the resulting throughput based on Table 1 assist times can be seen in Table 2.
For comparison, the maximum achievable throughput can be seen in Table 3.
As line throughput increases, assist time has greater impact and can justify increasing automation.
How to Reduce Assist Time
- Use manual dispense guns with paste supplied in cartridges rather than scooping from jars
- Automate under stencil wipe and paste deposition
- Reduce stencil fiducial verification when possible
- Reduce auto post print inspection points when possible. Programming a single scan across the length of a board may suffice
- Improve operator’s response time to a machine request for assistance
- Purchase larger reel sizes, splice small reels into larger reels, multiply feeders’ positions
- Segregate panels with X-outs so you only search for bad marks when necessary
- Have extra feeders loaded and ready, so that many systems continuously report which feeder will run out next
- Automate handling, loading, and unloading
- Reducing operator assist time up and downline of placement will allow faster response to feeder replenishment
- Prioritize operator response for the bottle neck in your line
- Locate machines to reduce operator travel distance
- Increase conveyor speeds
The numbers and calculations used here are greatly simplified. You can always create a simple spreadsheet with the assist variables you identify in your process for a better understanding of the impact on your throughput.