Certificate of conformity vs certificate of analysis

The short answer

A certificate of conformity and a certificate of analysis answer two different questions. The certificate of conformity says the supplier has checked the goods against the agreed specification and declares them conforming. The certificate of analysis reports the actual results of tests or measurements on a sample from the lot.

That difference matters when something goes wrong. A CoC tells you what the supplier believes. A CoA tells you what was measured, by which method, against which limits. If a plating thickness is challenged six months after delivery, the CoC gives you a statement and the CoA gives you a number.

Many shipments carry both. That is not duplication. The CoC covers the part as a whole, including dimensions, workmanship and marking. The CoA covers specific characteristics that were tested. Read them together, because neither document proves everything on its own.

What a certificate of conformity contains

A useful CoC identifies the issuer, the customer, the part number, the drawing revision, the lot or batch, the quantity, the date of supply and the specification the goods conform to. It ends with a signed statement of conformity, usually from a named person with a title and a date.

The scope of the statement is the part most buyers miss. A CoC can cover a single batch, a release, a purchase order or every part the supplier makes under a part number. A document that says the supplier conforms to all applicable requirements, without naming the lot or the revision, is close to useless as a quality record.

A good CoC also records exceptions. If a deviation was agreed, the certificate should reference the concession and the affected quantity. Silence on an agreed non-conformance turns a controlled decision into an audit finding later.

What a certificate of analysis contains

A CoA is a results table. Each line names a characteristic, the specification limit, the measured result and a pass or fail judgement. It should also name the test method, the sample or lot it represents, the test date and the laboratory that performed the work.

The characteristics depend on the part. A stamped copper contact may need chemistry, hardness, tensile strength and plating thickness. A machined aluminium housing may need alloy composition, temper and anodising thickness. A purchased cable lug may need conductivity and proof load. The CoA should only report what the specification requires, plus any characteristic the customer asked to see.

A CoA without limits is a data sheet, not a certificate. A result of 8.2 micrometres means nothing unless the drawing says the plating must be between 5 and 10 micrometres. Check that the limits on the certificate match the current drawing revision, not an older one.

Side by side

The table below sets out the practical differences. The rows are the questions a buyer should ask when a document lands in the goods-in area.

When to ask for which document

Ask for a CoC on every production shipment. It is the minimum record that the supplier stands behind the goods. Ask for a CoA where a measured property decides whether the part works, or where the process is hidden from you by the finish.

Material grade is the clearest case. If the drawing calls for C110 copper or 304 stainless steel, a CoA or a mill certificate is the evidence that the metal is what it claims. Plating is the second case. A tin layer that is too thin will corrode, and a layer that is too thick can crack at a bend. Neither condition is visible at goods-in.

Heat treatment and electrical properties also need data. Hardness after tempering, conductivity after drawing, and proof load after crimping all sit behind a process that a visual check cannot confirm. For medical and safety parts, the customer specification often names the document type, so follow that first.

Where EN 10204 fits

Metal buyers often meet EN 10204 inspection documents instead of a plain CoC or CoA. The standard defines types 2.1, 2.2, 3.1 and 3.2. Type 2.1 is a statement of compliance with the order, without test results. Type 2.2 adds results from non-specific inspection. Type 3.1 reports specific inspection results from the actual lot, signed by the manufacturer's authorised inspection representative.

Type 3.2 is the same idea, but the document is countersigned by an independent inspection representative or a nominated body. That extra signature is why 3.2 appears in pressure and safety work where the buyer wants a third party in the chain.

The common mistake is to accept a 2.1 document when the specification asks for 3.1. The 2.1 confirms the order, not the heat. Read the document type before you read the chemistry.

A distributor certificate needs the same scrutiny. The distributor may reissue the mill document on its own letterhead, which is acceptable if the original is traceable and the lot is unchanged. A reissued certificate with no reference to the original mill is harder to defend, because the link to the heat is broken.

What a weak certificate looks like

Weak certificates share a set of habits. They arrive as a generic letter with no lot number. They quote a standard but not a revision. They list results without limits. They use a scanned signature with no name or title. They refer to a part number that does not match the packing list.

A CoA that reports the same values for every lot is another warning. Real measurement varies. If three shipments of the same part show identical chemistry to two decimal places, the document may be a template rather than a record.

The safest response is not to reject the shipment automatically. Ask the supplier to reissue the document with the lot, the revision and the method. If the supplier cannot produce the underlying record, treat the certificate as a declaration only and decide whether that is enough for the part.

Check, link and file

At goods-in, match the certificate to the packing list, the purchase order and the drawing revision. Confirm the quantity and the lot. Then file the certificate against the shipment record and link it to the first article or PPAP file where one exists.

Retention should follow the product, not the finance calendar. A part with a ten-year service life needs records that last at least that long, and medical or safety parts may need longer. Store the file where the quality team can retrieve it by lot number, because that is how a future complaint will be traced.

A certificate is a record of a decision, not a substitute for one. The buyer still has to decide whether the stated scope covers the risk, whether the method is credible and whether the sample represents the lot. Reading the document is the easy part. Asking those three questions is the work.