A PCB Supplier’s Inspection Protocol Does More Than Minimize Escapes

It’s not easy to change suppliers for custom printed circuit board assemblies if you experience a high rate of quality escapes. Look at your contract manufacturing partners’ inspection processes to help prevent this problem.

A contract manufacturer should have an inspection process that minimizes escapes and also allows them to identify root causes and take corrective actions.

Image courtesy: Z-Axis

Image courtesy: Z-Axis

As a buyer you will probably be most concerned with their outgoing inspection, since an escape here means you have to deal with out-of-spec parts in your facility. But don’t overlook their incoming and in-process inspection as well. Here, a low rate of escapes is an indicator of process efficiency. It helps keep costs down and shipments on time.

A good inspection protocol will:

  • Clearly show what parameters must be inspected
  • Give assurance that these parameters actually are inspected
  • Provide tools to help inspectors consistently and accurately make the correct pass/fail determination.

First, consider what parameters should be inspected. An engineering print may have 200 dimensions. You really don’t want to pay your supplier to inspect every one of them. Surprisingly, very few of our contract manufacturing customers specify which dimensions are critical to quality (CTQ).

So our design engineers regularly get in touch with the customer’s engineering team to understand how they are using the board assembly and how it fits into the end product. From this we determine which dimensions are critical to function. If you can share that information with your supplier from the beginning, you will help them establish meaningful inspection criteria for your project.

Second, you should have assurance that those CTQ parameters actually were measured, and that they actually passed.  The problem with most inspection processes is that you can’t know if an escape was the result of an inspector’s mental math error, a measurement error, or a failure to measure at all. Your supplier will have no data to make corrective actions and effectively reduce escapes.  

It’s fairly easy to set up a protocol to address this. For example, at Z-AXIS we created a browser-based inspection protocol tied to our ERP system. The quality manger sets up each product in the system with its CTQ parameters, the criteria for visual inspection, and dimensions to be measured with their minimum and maximum values.

Image courtesy: Z-Axis

Image courtesy: Z-Axis

At each inspection step, any inspector can pull up the CTQ parameter list and enter the data they measure. The computerized system determines whether each measurement is in tolerance and displays a pass or fail message.

This computerized protocol is far easier and less error-prone for the inspector. Imagine a CTQ dimension of 0.080 +/- 0.050 mm.  If the inspectors are comparing their measurements to a print, they have to do the mental math to decide if a measurement of 0.135 is within tolerance. It’s easier if the inspection criteria lists the minimum and maximum values. It’s easier still if they can just enter a measured value and let a computer do the math.

The computerized protocol also provides an excellent tool for inspection process improvement. All inspection data is saved, along with the date and the inspector’s initials. In the event of an increase in escapes, we can study the records and look for root causes. Maybe an inspector needs more training, or a change in the manufacturing process has made inspection more difficult, or a specific workbench or tool needs to be improved for usability.

Even more powerful is the ability to mine the data for manufacturing process improvement opportunities. For example, the quality manager might note that a CTQ board dimension is consistently near the edge of its tolerance range. Depending on how that dimension is created, they may have a talk with the bare board vendor or work with our own manufacturing engineering team to adjust our de-paneling process. Either way they can bring the dimension back to the center of its tolerance range and helps assure continued high yield.

Inspection protocols that allow inspectors to pass or fail boards without recording the data will not give your supplier the information they need to solve problems and improve processes. Fortunately this is an easy fix if your contract manufacturing partner is willing to invest the effort.

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