The short answer
Class 2 and Class 3 do not describe two levels of soldering skill. They describe two levels of consequence if the assembly fails, and therefore two levels of evidence required before it ships.
The process that produces a Class 3 joint is largely the same process that produces a Class 2 joint. What changes is which conditions an inspector is allowed to accept, how much of the output gets inspected, how much documentation follows the order, and how much scrap the supplier has to absorb. That is where the price difference lives, and it is why a supplier who quotes Class 3 at Class 2 prices is telling you something about how they intend to inspect.
The practical consequence for a buyer is uncomfortable: putting "IPC-A-610 Class 3" on a purchase order does not buy a reliable product. It buys a stricter acceptance judgment on a product whose reliability was already decided by the design, the materials and the process control. Those are three separate purchases and they are frequently confused.
What the class actually changes
IPC-A-610 is titled Acceptability of Electronic Assemblies. The word to notice is acceptability. The standard tells an inspector what a finished assembly must look like to be accepted for a given product class. It contains photographs and illustrations of acceptable conditions, process indicators and defects, and it organises those judgments by class.
It does not tell anyone how to make the joint. That is a different document. The distinction matters commercially because a supplier can run a perfectly controlled soldering process, follow every process requirement, and still fail an inspection if the design was not built to the class being demanded.
Three things change when you move from Class 2 to Class 3.
The judgment on the same physical condition changes. A condition that is acceptable at Class 2 can be a process indicator at Class 3, and a condition that is a process indicator at Class 2 can be a defect at Class 3. The board does not change; the verdict does. This is why the class cannot be negotiated after the boards are built.
The inspection coverage changes. Class 3 work is normally inspected at a higher rate — full visual inspection rather than sampling, and in many programmes additional X-ray or cross-section on top of that. Inspection is labour, and labour does not scale down with volume the way material does.
The disposition of non-conforming units changes. A unit that would be reworked and shipped under Class 2 may have to be scrapped under Class 3, because the rework itself is a deviation the class does not want. Scrap is a yield cost, and it lands on the supplier's price.
One more thing changes, and it is the one buyers notice least: the class propagates upstream. If the assembly is Class 3, the bare board is expected to be built to a Class 3 construction, which is a different standard with its own tighter requirements. A programme that specifies Class 3 only at the assembly stage has specified half a requirement.
The three classes, and what each is for
The classes are defined by the consequence of failure, not by price or by market segment. Getting the definition right is the difference between over-specifying and under-specifying, and both are expensive in different ways.
Two structural rules apply across the whole document, and they are worth knowing before you argue about a specific joint.
A condition that is a defect at a lower class is also a defect at a higher class. The classes only get stricter. There is no condition that Class 3 tolerates and Class 2 rejects.
The class is chosen by the product's use, not by the customer's preference. A buyer who specifies Class 3 for an indoor industrial controller is not buying reliability, they are buying inspection and scrap, and the design may not support either.
| Class | Defined for | Failure consequence | Typical products |
|---|---|---|---|
| Class 1 | General electronic products | Function at delivery is the main requirement; service life is short and cosmetic imperfection is tolerable | Consumer accessories, simple lighting, disposable devices |
| Class 2 | Dedicated service electronic products | Continued performance and longer life are required, but uninterrupted operation is not critical | Industrial controls, instrumentation, computing, telecom, non-safety automotive |
| Class 3 | High performance / high reliability electronic products | Continuous performance on demand is required; the equipment may operate in harsh conditions and failure is not recoverable in the field | Medical, aerospace, defence, safety-critical automotive |
Where the money actually goes
When a supplier quotes Class 3 at a meaningful premium over Class 2 for the same board, the difference is almost always in four places. Ask which ones, and the answer tells you whether the quote is real.
Inspection labour
This is usually the largest single line. Class 2 programmes commonly run automated optical inspection plus sampling, with human visual inspection on a defined sample. Class 3 programmes commonly run full visual inspection on every unit, which converts a sampled cost into a per-unit cost. On a high-mix, low-volume order, that per-unit cost can be larger than the material cost of the assembly.
Scrap and rework
A tighter acceptance judgment rejects more units, and a rejection is not free even when it is reworked. Rework consumes labour, consumes the risk of a second failure, and in Class 3 is itself constrained by the rework standard. Units that cannot be reworked within that standard are scrapped, and the scrap rate on a first article is always worse than the scrap rate on a mature build. That learning curve is priced into the first order whether or not anyone says so.
People and records
Class 3 programmes are expected to run with certified inspectors and documented evidence: inspection records tied to serial numbers, material traceability, coupon and test data, and a route card that shows who did what. That is administrative labour, and it is the cost that is easiest for a supplier to under-quote because it does not appear on a bill of materials.
Process window
Higher reliability usually comes with materials that are harder to process — higher Tg laminate, sometimes thicker copper, sometimes a surface finish chosen for the class rather than for the price. Those materials narrow the process window, which raises the cost of holding it. The material side of this is covered in the materials page.
A Class 3 quotation that is within a few percent of Class 2 is not necessarily wrong, but it does mean the extra inspection and records are not being paid for. Ask what the inspection plan is per unit, and whether the record set is included.
The two standards that get confused
Most of the arguments about class come from mixing up two documents that are meant to be used together.
IPC-A-610 is the acceptance standard for the assembled board. It defines what an inspector accepts, flags or rejects on a populated assembly.
J-STD-001 is the process standard for soldering. It defines materials, equipment, temperatures, cleanliness and the process controls that produce the joint. It also has classes, and they are meant to match the class you specified for acceptance.
A purchase order that names only IPC-A-610 specifies half of the requirement. It says what the result must look like but not how it must be made, which leaves the supplier free to reach that result by any route — including one that produces a joint that passes inspection and fails in the field.
There is a third document that matters and is almost always missing from purchase orders. IPC-6012 is the qualification and performance standard for rigid bare boards, and it also has classes. If your assembly is Class 3, the bare board should be specified to IPC-6012 Class 3 as well, and the bare-board acceptance is governed by IPC-A-600 rather than by IPC-A-610. A programme that names IPC-A-610 Class 3 and says nothing about the bare board has specified the populated half and left the other half to the fabricator's default. Whether a bare board can reach Class 3 at all is decided earlier, in the stackup and the geometry it allows, which is covered in the reference stackup builds.
That gap is worth checking on any existing purchase order. It is common, it is cheap to fix, and it is the kind of thing an auditor finds before a customer does.
Why Class 3 on a PO does not buy reliability
This is the part that costs buyers the most money, because it is the part that feels like it should not be true.
IPC-A-610 judges what can be seen. Reliability is decided by things that cannot be seen, or that are not visible at the moment of inspection.
A solder joint can meet every visual criterion of the highest class and still fail a thermal cycle, because the failure mechanism is an intermetallic layer that grew too thick during a reflow profile nobody qualified, or a void at the pad interface that is hidden under the joint. Neither condition is visible. Both are process outcomes, and both are controlled by the process specification, the profile and the material — not by the acceptance standard.
Three things therefore have to be bought separately if reliability is the goal.
A design that supports the class. Pad geometry, component spacing, thermal relief and via placement all set the ceiling on what the process can achieve. A design with pads too small for the class will produce either rework or a quietly relaxed inspection, and the second outcome is worse because it is invisible.
A qualified process, not just a compliant one. The reflow profile, the paste, the stencil and the cleanliness controls should be qualified against the actual board, not inherited from a similar one. This is where a supplier's process capability shows up, and it is the thing a certificate cannot tell you.
Verification that matches the failure mode. If the product fails by thermal fatigue, the evidence should be thermal cycling. If it fails by drop or vibration, the evidence should be mechanical. An inspection report answers a different question, and a programme that accepts an inspection report in place of a qualification test has bought paperwork.
The useful question is not "are you Class 3 capable" but "what would you do differently on this board if I asked for Class 3, and what evidence would you give me". A supplier who can answer the second question specifically is worth more than one who answers the first one enthusiastically.
What is actually different at the joint
We are deliberately not reproducing a table of acceptance values here. IPC-A-610 organises its criteria by table and figure, the values are tied to specific conditions, and the revision matters: the current revision is IPC-A-610J, published in 2024, and criteria have moved between revisions. Copying a percentage out of a secondary article and putting it in a purchase order is how programmes end up arguing about a number that neither party can find in the actual document.
What can be said reliably is the direction of the differences. Across solder joints, the higher class tightens four dimensions.
Wetting and fillet formation. The higher class requires a more complete and more uniform wetting of the pad and the lead, with a concave fillet and a contact angle that shows the solder actually wetted rather than sat on the surface.
Barrel fill and vertical fill in plated holes. Through-hole joints are judged on how much of the plated barrel the solder filled and how completely it wetted the barrel wall. The higher class narrows what counts as filled, and it is the criterion most often quoted incorrectly in secondary sources.
Placement and overhang. Component offset and end overhang tolerances tighten, which matters most for fine-pitch parts where the solder joint is the only mechanical connection.
Cleanliness and surface conditions. Residue, contamination, solder balls and splashes that are tolerable at a lower class become reportable or rejectable at a higher one.
Each of those is a judgment about a visible condition. None of them says anything about the void under the joint, the intermetallic thickness or the fatigue life. That is the boundary of what the standard can do, and knowing where the boundary is, is most of what a competent quality conversation consists of.
How to write the requirement
A requirement that cannot be inspected is not a requirement. The following form is short enough to put on a purchase order and specific enough to argue about.
State the acceptance standard with its revision and class: IPC-A-610J Class 3. Naming the revision matters because criteria differ between revisions and the supplier's certification may be against an older one.
State the process standard with its class: J-STD-001 Class 3. Without it, the acceptance requirement has no process counterpart.
State the bare-board standard: IPC-6012 Class 3, accepted to IPC-A-600. This is the half that gets left out.
State the inspection plan: what is inspected, at what rate, by what method, and whether first article includes cross-section or X-ray.
State the record set: which documents ship with the order and for how long they are retained.
State the rework policy: which standard governs rework and repair, and whether rework is permitted at all on this programme. For many Class 3 programmes the answer is that rework is allowed only by written approval, and that should be on the order rather than discovered later.
State who bears scrap: a first article at Class 3 will produce scrap, and which party pays for it should be agreed before the build rather than after.
A supplier who asks questions about two or three of those lines is engaged. A supplier who accepts all six without comment has probably not read them. The other standards that belong on the same order, and the record set that should follow it, are collected in the quality and standards section.
What to look for in an inspection report
Most inspection reports are one page and answer none of the questions they are kept for. Six fields make the difference between a document that supports a decision and a document that supports a filing cabinet.
The rate, stated as a rate. Not "inspected" but "100 percent visual, plus AOI on both sides, plus X-ray on BGA locations". Sampling is legitimate; an unstated sample is not.
The unit. Inspected per board, per panel or per lot changes what the number means. A 2 percent defect rate per panel is a different problem from 2 percent per board.
The revision. IPC-A-610 without a revision is a reference, not a criterion. The current revision is J, published in 2024, and criteria have changed between revisions.
The person. Class 3 programmes are expected to run with certified inspectors, and the certification should be named rather than implied. It is a checkable fact.
The disposition. What happened to the units that failed: reworked to which standard, scrapped, or accepted under deviation with whose signature. A report with no disposition has hidden the interesting part.
The traceability link. Which serial numbers or lots the findings belong to. Without it, the report describes a process rather than a shipment.
Two traps are worth naming because both look like good news.
A report that shows zero findings on a first article at Class 3 is unusual enough to ask about. It can mean the process is genuinely mature, or it can mean the inspection threshold was interpreted loosely. The question to ask is what the inspector would have had to see in order to write a finding down.
An AOI report with a very low false-call rate can mean the machine is well tuned, or it can mean the threshold was widened until it stopped complaining. Library coverage and threshold settings are the two numbers behind that result, and neither usually appears on the summary.
The first article is where this matters most. On a new build, ask for the first article report before the balance of the order ships, and check that it includes a microsection if the class or the application calls for one. A first article that is only a functional pass tells you the board works today, which is not the question.
What we can and cannot do
We build to a stated class, and we can tell you what changes in the build when the class changes: which inspection steps are added, which materials are substituted, what the record set contains, and what the realistic first-article yield looks like.
What we cannot do is certify that a board will be reliable because it passed inspection. No supplier can, and a supplier who offers it is selling the wrong thing. Reliability evidence comes from qualification testing against the failure mode, and that is a separate programme with its own cost and schedule.
We also cannot make a design meet Class 3 that was not laid out for it. If the pad geometry cannot support the class, the honest answer is to change the design or to accept a lower class with a documented rationale — not to inspect harder and hope.
The evidence a buyer should ask for, and the questions worth asking of any supplier including us, are collected in the quality management page and the bare-board testing page. If the assembly involves component sourcing as well, the PCBA testing page covers what changes on the populated side.
To review a quality requirement against a specific board, send the drawing and the class you intend to specify and we will return the build and the record set that go with it, including the parts we think your design cannot currently support.
Questions, answered
Before your next build
Is IPC-A-610 Class 3 the same as high reliability?
No. IPC-A-610 is an acceptance standard for what an inspector can see. A joint can meet every visual criterion and still fail thermally because of a process condition that is not visible. Reliability has to be bought through design, process qualification and testing.
Do I need both IPC-A-610 and J-STD-001 on a purchase order?
Yes, if you want the requirement to be complete. IPC-A-610 defines what is accepted on the finished assembly; J-STD-001 defines the process that produces it. Naming only one leaves the other to the supplier's default.
What revision of IPC-A-610 is current?
IPC-A-610J, published in 2024. State the revision on the purchase order, because criteria differ between revisions and a supplier's certification may reference an older one.
Does Class 3 apply to the bare PCB as well?
Not through IPC-A-610. Bare boards are covered by IPC-6012 for qualification and performance, with IPC-A-600 for visual acceptance. A Class 3 assembly normally calls for a Class 3 bare board, and that has to be specified separately.
How much more does Class 3 cost than Class 2?
It depends on the board and the inspection plan, and the difference is mostly inspection labour, scrap and records rather than material. Ask for the inspection rate per unit and whether the record set is included; those two answers explain most of the gap.