Low Volume, Root Cause & Corrective Action

Applying volume production control practices to low volume or prototype manufacturing is impractical. I am writing here to familiarize customers with some of the variables in low quantity, high mix, complex PCB assembly.

Here is what I can tell you about a quantity of 5 or less being built by contract: Most organizations are not going to program pick and place equipment for quantity of ten or less, let alone buy all parts on tape & reel for low qty builds. Its not that they wont, its that the customer will not want to pay for it.

Some CM's may tell you they'll automate it (because the perception is that robots are more accurate than humans...right?) NO - here's the reality: Every new program set up will require that feeders and feeder pick positions need to be adjusted. The program essentially goes through a "debug" process while the feeder pick is fine tuned. What this means is that parts are not being picked (which means they are not being placed). Parts are possibly being dropped in transition from the feeder to the placement location. The first 3 to 5 boards could very likely have parts missing, parts placed in the wrong direction or orientation or any number of placement errors. Not a big deal getting through the learning curve if you're running 200 boards, but certainly a futile effort if all your building is 3 to 5 units!

We can justifiably reach the conclusion that 3-5 pieces is a highly manual operation whether you waste your time setting up pick and place equipment, or you prepare placement charts for print, place and oven reflow. Ultimately any workmanship errors fall on the contract manufacturer. Root cause, on the otherhand.... Not necessarily.


Human Error

The operator, being human, made a mistake, an oversight. Corrective Action? If you have an experienced hard working assembler or inspector, perhaps making less than 1 mistake every 10000 components placed or checked, is there a root cause or corrective action? If your placing over 600 components of 200 different types, manually, the opportunity for humanity to come through is great....and sometimes does.

Designed For Manufacturing (or Not)

I had one manual placement job in which a BGA device was placed on one side of the board and a device of weight and mass designed to be reflowed on second pass (topside of the PCB) was placed on the opposite side. These parts were designed to be reflowed on the same side, but you have to solder the second on the opposite side by hand because they drop during second pass reflow upside down. The surface mount pins of this device were almost inaccessible when placed next to each other. The operator could not make adequate contact with the soldered lead and the pad. He or she did their best to solder it. It appeared to be soldered but was not. Product ships then test engineering finds the lead to be open..

The root cause is the design flaw, not the operator. The corrective action is to layout both parts on the same side of the board so they can reflow in an oven as they were designed. A corrective action may also be to layout the parts to space them further apart to improve soldering iron access.

My point here in this article is that when you have a highly automated production job, the opportunity to identify root cause, implement corrective action and process improvement is significant. When you are building 3, 5 or even 10pcs of a PCB assembly, the process is highly manual, there often is no "process variable" to improve on. I also wanted to point out that often the root cause and corrective action can be outside of the contract manufacturers control.