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Knowledge Base/Writing Purchasing Quality Specifications (PQS): Clearly Stating Your Requirements to Suppliers
Supply Chain Quality6 min read

Writing Purchasing Quality Specifications (PQS): Clearly Stating Your Requirements to Suppliers

This article uses a jarring anecdote of a production line machine's 1.08 CPK and 6210 DPMO report, coupled with a supplier's "Your specifications said so" defense, to underscore a crucial point: quality failures often stem from inadequately defined requirements. It emphasizes the critical importance of a clear, quantitative Purchasing Quality Specification (PQS) in preventing issues caused by vague standards and lazy template reuse.

That Day the CPK Report Came Out, the Entire Room Fell Silent for Three Seconds

I remember several years ago, a new machine was just introduced to our production line. When the trial production data came out, everyone was initially full of confidence. However, in the meeting room, when the supplier displayed the CPK report, the screen showed a large 1.08, and the DPMO was 6210. You know what? The air instantly froze, and the entire room was silent for about three seconds. The responsible PM's face turned green. At that moment, I thought, "Oh no, now I'll have to rush to the supplier's site day and night to supervise." When we investigated, the supplier looked innocent and said, "But your specification document was written this way; we've met all the requirements!" Hearing that truly made me want to spit blood.

What's the Problem? Simply Put, It's "Not Knowing What We Want"

To be honest, often it's not that suppliers are unwilling to do a good job, but rather that we ourselves haven't clearly articulated "what we want." This "what we want" refers to the Purchasing Quality Specification (PQS). Many newcomers writing PQS simply take old templates, modify them, are too lazy to adjust the numbers, and even directly copy-paste some specifications. A PQS written this way might look impressive, spanning dozens of pages, but to the supplier, it's essentially a martial arts manual that says, "Do whatever you want."

In other words, the core spirit of PQS is: you must tell the supplier, clearly, explicitly, and quantitatively, what quality standards your product or service needs to meet. This should include not only the final product but also in-process controls, inspection methods, and recording standards.

How to Do It in Practice? Numbers Are the Hard Truth

When drafting a PQS, the most important thing is to replace all "vague" terms with "concrete numbers." For example, you cannot just write, "The product surface must not have obvious scratches." Saying that is as good as saying nothing. What constitutes "obvious"? What you find obvious, others might not, leading to endless arguments.

You should write:

  1. Scratch length: Must not exceed 0.5 mm.
  2. Scratch width: Must not exceed 0.05 mm.
  3. Number of scratches on a single surface: Must not exceed 2.
  4. Visibility: Must not be detectable by the naked eye when viewed from a distance of 30 cm under 100 lux illumination.

To give another example, if you have requirements for a critical dimension, in addition to specifying the tolerance range, it's best to further request a CPK value. For instance: "The thickness of a certain component is 1.25 ± 0.02 mm, and the mass production CPK value must be 1.33 or higher." This way, the supplier knows that even if all their products are within the tolerance range, if the distribution is too wide and the CPK does not meet the standard, the products will still be rejected.

Therefore, the key is that for every quality characteristic you deem important, you must provide a quantitative standard, preferably accompanied by a testing method.

The Most Common Pitfalls: Laziness and the "Good Enough" Mentality

The most common pitfalls I've seen are laziness and the "good enough" mentality. Many engineers writing PQS directly reuse specifications from the previous generation product, or even modify a few words from another product line's PQS and submit it. What's the result? The new product's process, materials, and application environment have all changed, rendering the old specifications completely unsuitable.

Another pitfall is only demanding the quality of the final product while neglecting in-process control. For example, you require solder joints to have no voids, but the supplier's process parameters are completely uncontrolled, and they only manually sort defects at the end. Although the finished products might meet specifications, the defect rate is alarmingly high, leading to high costs and unstable delivery times. We once had a supplier where the PQS did not include in-process parameters (e.g., reflow oven temperature profile, solder paste printing thickness), resulting in initial yield hovering around 85%. Only after we subsequently demanded these controls did we discover they weren't monitoring them at all.

Frankly, a good PQS is not just your requirement for a supplier; it is also proof of your understanding of your own product.

One Thing You Can Do Today

Open a PQS document you have on hand, find three of the most vague sentences, and change them into concrete numbers.

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