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Knowledge Base/Material Specification Deviation Request (SCAR): How to Write a Good Deviation Request
Quality Assurance6 min read

Material Specification Deviation Request (SCAR): How to Write a Good Deviation Request

This article addresses a common challenge for process or equipment engineers: situations where supplier materials are slightly out of specification but are indispensable. It delves into the purpose and proper execution of a SCAR (Supplier Corrective Action Request), clarifying that it's not merely a sign-off but a documented justification for accepting a deviation. By understanding how to effectively evaluate non-conformances and their potential impact, readers will be better equipped to manage these situations confidently.

The day the yield report was released, silence fell over the room for three seconds

I remember one time, I had just walked out of a meeting when my phone started ringing non-stop. Xiao Li from Process Engineering anxiously said, "Bro, we're in trouble! The film thickness measurements for this batch of material show the average is within specification, but the CPK is only 1.08, and the supplier says this is their limit. The boss wants us to churn out a SCAR (Supplier Corrective Action Request) before end of day to let them ship, otherwise the production line will run out of material." My head started to spin. This kind of rushed SCAR is a typical headache. But honestly, as a process or equipment engineer, you'll always encounter situations where materials or parts are "just slightly off" specification, but absolutely essential to use. In such moments, a SCAR becomes your lifesaver.

What's the Problem?

To put it plainly, a SCAR is an application that says, "I admit this item does not meet standards, but I have sufficient reasons for you to accept it." It's not for blindly accepting substandard goods. Rather, when a supplier's material or part cannot fully meet your stringent specifications due to some special circumstances, you can use this document to assess whether this "non-conformance" will truly lead to product defects. You must convince everyone that even if this material has a slight "flaw," it will not have any substantial negative impact on product performance, reliability, or even customer experience.

How Is It Done in Practice?

To write a good SCAR, the key lies in "evidence" and "persuasiveness." When you encounter a situation like Xiao Li's, with a CPK of 1.08 and a DPMO of 6210 (6210 defects per million opportunities), you cannot simply write, "the supplier says this is their limit." Here are several things you need to do:

  1. Quantify Differences and Risks: Although your film thickness CPK is not good enough, what does the actual data distribution look like? Are there any points exceeding the upper and lower limits? If so, by how much? What impact will this deviation have on your product yield and reliability? Have you actually conducted experiments to prove that this "deviation" will not cause your product to fail? For example, you could state: "Although the CPK is 1.08, all measurement points fall within the specification's center value ± 1.5 sigma, and we have conducted a Reliability Test with this batch of material, which showed no abnormalities."

  1. Propose Specific Remedial Actions: Even though it's accepted this time, what about next time? Has the supplier proposed an improvement plan? For example: "The supplier has committed to adjusting process parameters and submitting an improvement plan for the next three batches, aiming to increase CPK to above 1.33." You can also propose additional in-house inspection or control methods.

  1. Assess Impact on Subsequent Processes: Will this deviation affect your next process? For example, if the film thickness is slightly high, will it affect the uniformity of subsequent etching? All these must be evaluated. In other words, you must prove that this "imperfection" will not trigger a chain reaction, harming subsequent processes.

The Most Common Pitfalls

The most common mistake I've seen is writing a SCAR by simply "copying whatever the supplier writes." Once, a new process engineer pasted the supplier's "root cause analysis" and "corrective actions" verbatim into the SCAR, and was severely reprimanded by the boss. The boss asked him: "Have you verified these actions yourself? Do you think they're reasonable?" His face instantly turned green. To be frank, suppliers will naturally present themselves as having done their best and solved all problems. However, you must use your own professional judgment to assess, verify, and even request further information. If the supplier claims their equipment is old, leading to insufficient precision, you must ask: What is your maintenance plan? Have you considered replacing the old equipment? You cannot simply accept passively. Honestly, you are signing this SCAR; if something goes wrong in the future, you bear the responsibility.

One Thing You Can Do Today

Take five minutes today to pull out your SCAR template and re-examine it.

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