HDI PCB Manufacturing: What You Must Specify, What You Can Compare, What You Can Only Verify

Asking for a price first is the most expensive step in HDI sourcing

An HDI requirement usually arrives as a board outline, a layer count, and a date. The natural first move is to send that to three or four suppliers and see what comes back. The quotes arrive, they differ by a lot, and the sourcing conversation becomes a conversation about why one number is lower than another.

That conversation cannot be concluded, because the numbers were never answers to the same question.

HDI is not a single specification. It is a family of build approaches that can satisfy the same outline and the same layer count in structurally different ways. When the request does not say which approach applies, each supplier quotes the approach it would prefer to build — usually the one that is cheapest inside its own plant. The spread you are looking at is partly competitiveness and partly a set of assumptions you never saw.

The failure here is rarely supplier selection. It is that three kinds of decisions, with three different owners and three different timings, got pushed into one request:

1. Decisions only you can make. They belong to the buyer because they define what "correct" means at incoming inspection. Leave one blank and you have silently authorised the supplier to choose for you.
2. Decisions you can compare. They only become comparable *after* the first group is closed. Comparing them earlier compares assumptions, not suppliers.
3. Decisions you can only verify. They are facts about a plant and a production run. During shortlisting you cannot obtain the conclusion, only the form of evidence that would later support it.

The rest of this page is about separating those three, the order they have to happen in, and what each one turns into inside the RFQ. Individual items are covered in depth elsewhere on this site; here each one gets a single decision test and a pointer, so the shape of the whole process stays visible.

Group one: the items you have to declare yourself

The test for this group is one question: if this item is left unstated, is there still a document that says whether the delivered board is right or wrong? If the answer is no, the item belongs to you, and no amount of supplier expertise can substitute for your declaration.

• Build structure and sequential level. The same finished layer count can be reached through different buildup arrangements, and they are not interchangeable in cost, lead time, or risk. Stating the structure is what makes the rest of the request readable. Depth: HDI layer count and lamination cycles in the RFQ.
• Microvia structure form. Whether vias are stacked or staggered, and where they are permitted to land, changes the build sequence. Declared, it is a requirement; undeclared, it is a supplier's convenience. Depth: stacked vs staggered microvia declaration.
• Via treatment in pads. Filled, capped, or left open is a decision with assembly consequences, so it cannot be deferred to whoever happens to receive the board next. Depth: via-in-pad filled and capped declaration.
• Impedance targets and who owns them. A target is only enforceable if the RFQ says who derived it, against which stackup, and who re-derives it if the supplier adjusts dielectrics. Depth: controlled impedance target ownership.
• Base material family. Material choice changes processing behaviour and availability, and a substitution accepted casually mid-quote changes what you receive. Depth: base material declaration in the RFQ.
• Acceptance class and inspection basis. The same board passes under one acceptance basis and fails under another. This is the single item most often omitted and most often disputed. Depth: build class specification and decision ownership.

None of these require you to be a process engineer. They require you to state a position, or to state explicitly that the supplier may choose within a named boundary. Both are acceptable. Silence is the only option that is not.

Why a gap in group one becomes a gap at acceptance

This is the part that is easy to underestimate. An undeclared item does not stay undeclared. It gets resolved — just not by you, and not in writing.

The sequence is consistent. The RFQ omits an item. The supplier builds to its own default, because it has to build to something. The board arrives. Someone at your end measures it, finds a feature that looks wrong, and reaches for the specification to decide whether it is wrong. The specification is silent on that feature. At that moment there is no technical dispute available, because a dispute requires two readings of one document, and here there is only one party's unwritten default.

What follows is a commercial negotiation with no basis, usually resolved by whoever has more leverage or less schedule pressure. If you are the one with a build date, that is you.

So the practical rule is that the declaration list and the acceptance list are the same list. Anything you want the right to reject later has to appear in group one now. This also explains why adding acceptance criteria after the quote rarely works: it changes what was priced, which legitimately reopens the price.

Two related mechanics are worth knowing, because they sit at the boundary of group one:

• Engineering queries are not a second chance to declare. The supplier will come back with questions before release, and some of your answers are irreversible grants of design freedom. Answering "as you suggest" closes the item without creating a document. See engineering query response authority and the lead-time clock.
• Application requirements are not yet buildable requirements. "It is for a 5G module" is a context, not a specification, and the translation step is where most group-one items are actually generated. See turning an application spec into buildable requirements.

Group two: what is actually comparable once group one is closed

With the declarations in place, comparison becomes meaningful — and narrower than most people expect. The comparable set is roughly:

• Price against an identical declared input. Not price against your outline; price against your declarations.
• Tooling and non-recurring charges, and what they cover on reorder. A low unit price with non-reusable tooling behaves differently over a programme than the opposite. See tooling and NRE ownership on reorder.
• Quoted lead time and what starts the clock. Two suppliers quoting the same duration from different start events have not quoted the same thing.
• Capacity commitment and allocation. Whether your quantity has reserved capacity or sits in a general queue. See supplier capacity allocation and commitment.
• Quote validity and the conditions that reopen the price. A quote without a stated validity basis is a number with an unstated expiry.

Everything else that looks comparable in a quote document is usually a restatement of group one. If one supplier's quote describes a different structure than another's, you are not looking at a price difference at all.

For the cost mechanics underneath this group, HDI cost drivers and quote optimisation covers which inputs actually move the number, and the HDI quote package checklist covers the file-level granularity that makes quotes land in the same format.

What two incomparable quotes look like

Incomparable quotes rarely announce themselves. The usual signs:

• One quote is noticeably shorter than the other. Brevity often means fewer assumptions were written down, not that fewer were made.
• The structure is described in different vocabulary. Different build vocabulary for the same outline is a signal that two different builds were priced.
• One quote carries conditions the others do not. A supplier listing conditions is frequently the one that read the request most carefully.
• Lead times differ sharply at similar prices. Often a different start event or a different subcontracting arrangement rather than a faster plant.
• A material or stackup "equivalent" appears without being requested. An equivalent accepted silently at quote stage becomes the specification at acceptance stage.

The response is not to pick the quote you like. It is to identify which group-one item each supplier filled in for you, put it back in the request, and reissue. Whether to reissue or to resolve item by item depends on how many items moved: one or two can be closed in writing against the existing quote, while a different declared structure means the quote was for a different board and should be replaced.

Where the choice is between HDI and a conventional multilayer build in the first place, that decision sits upstream of all of this — see HDI or conventional multilayer.

Group three: the things you can only verify, and what you can ask for instead

Some facts about an HDI order cannot be established during shortlisting, because they do not exist yet or do not exist in a form anyone can hand over. Asking for them produces a reassurance, not a fact.

What is in this group:

• Which steps happen in the supplier's own plant. Subcontracted steps are not a problem; undisclosed subcontracting is, because it changes who answers when something goes wrong. See in-house vs subcontracted process steps.
• Consistency between first article and later batches. Establishable only across a first article and at least one production lot. See first article inspection report and release terms.
• Whether prototype results carry into production. A prototype built on a different route does not predict the production board. See prototype service scope and production equivalence.
• Attributes that stop being measurable once the board is finished. Internal features are the clearest case: if the evidence is not generated during the build, it does not exist afterwards. See incoming acceptance and unverifiable attributes.
• Traceability depth on shipment. What arrives with the boards determines what you can reconstruct later. See shipment document package and lot traceability.

Since the conclusions are unavailable, what you shortlist on is the form of evidence: which records exist, who generates them, when they are generated, whether they travel with the shipment, and whether you can request them for a named lot. A supplier that can describe its evidence formats precisely is giving you something checkable. A supplier that answers with capability statements is giving you something that cannot be checked either way.

Capability evidence for a shortlist covers which evidence formats to request, and capability tiers for buyers covers how to read capability claims without treating them as verified facts. For how any of this is produced inside a plant, the HDI manufacturing process is the background reference.

The order, and the cost of taking it out of order

The three groups are sequential, not parallel:

1. Declare — close group one before any supplier sees a request. Use an internal pass first: for each item, either a position or a named boundary within which the supplier may choose.
2. Compare — issue the declared request to the shortlist and compare only group two. Any group-one item that comes back altered is a finding, not a quote.
3. Verify — specify evidence formats for group three and confirm they appear on the first order, before the quantity that matters.

Reversing one and two is the common error, and it costs twice: once in time, because the first quote round has to be discarded, and once in position, because you are now negotiating against a number that was anchored by an assumption you did not set.

Skipping three does not cost anything until the first problem, at which point it costs the whole investigation — the records needed to attribute the problem were never specified, so attribution falls back to argument.

What each group turns into in the RFQ

Each group has a different home in the document set, and mixing them is why RFQs grow long without getting clearer:

• Group one → the specification sheet. Line items with values or explicitly named supplier latitude. This is the section that incoming inspection will later read, so it has to be written as criteria, not as narrative. Granularity is covered in the fabrication specification sheet and what binds the quote, and the file-level package in the HDI quote package checklist.
• Group two → the commercial request. Price basis, tooling treatment, lead-time start event, validity, quantity and allocation. These are terms, not specifications, and they only make sense once group one is attached.
• Group three → the quality and documentation requirements. Evidence formats, report types, what ships with the boards, retention, and which named lot you can request records for. Test and inspection definition is covered in electrical test, netlist and coupon declaration.

Conflicts between drawing notes, specification sheet and commercial terms are their own failure mode, and worth checking before issuing — see the fabrication notes conflict checklist.

When to stop comparing and build something

Continued price comparison stops being useful at a specific point: when the remaining uncertainty is about whether the board is buildable as declared, rather than about which supplier should build it.

Indicators that you have reached that point:

• Group one cannot be closed because a requirement conflict has not been resolved internally.
• Suppliers' engineering queries converge on the same item, which usually means the declaration itself is not buildable as written.
• The build is new to your programme and no comparable board has been produced before.
• The structure sits near the edge of what the declared stackup supports, where a small change propagates through the whole build.

In those situations a prototype is the cheaper information, as long as its scope is defined so the result transfers. Prototype expectations are covered in prototype and low-volume quote expectations, and where a second source is the goal, equivalence has to be qualified deliberately — see second-source build equivalence qualification.

To start a declared request rather than an open one, the HDI PCB capability page lists what the specification sheet expects, our capabilities overview covers the surrounding scope including multilayer and assembly, and the quote request page accepts the declared package directly.

FAQ

Can I send an outline and a layer count and let suppliers propose the rest?

You can, and it is a reasonable way to gather build options early. What it is not is a quote round. Treat the responses as proposals, choose one structure, declare it, and then request prices. Otherwise each price refers to a different board.

If I do not know the right answer for a group-one item, what do I write?

Write a named boundary instead of a value: state that the supplier may choose within a stated constraint, and that the chosen value must be returned in writing before release. That keeps the decision reviewable and keeps an acceptance basis in place. It is the difference between delegating and omitting.

Which group-one item is most often skipped?

The acceptance basis. It is the one item that does not affect how the board is built but entirely determines whether the delivered board passes, which is why its absence only surfaces at incoming inspection. Build class specification and decision ownership covers how to state it.

Two suppliers quoted the same lead time. Is that comparable?

Only if both quoted from the same start event. Lead time can start at quote acceptance, at purchase order, or at engineering query closure, and the gap between those can be substantial. Ask which event starts the clock and put the answer in the order.

A supplier says it can hold a tighter capability than the others. Can I use that to choose?

Not as a conclusion, because it is a group-three fact. You can use it to choose what evidence to request: ask which records demonstrate it, for a named lot, in what format. The answer is checkable; the claim is not.

Does adding acceptance criteria after the quote reopen the price?

Usually yes, legitimately, because acceptance criteria change what has to be produced and inspected. This is the practical reason group one precedes group two rather than following it.

We already have a supplier. Does this still apply?

It applies on every reorder where anything changed. A repeat order against an unchanged declared package is straightforward; a repeat order against a changed structure, material, or quantity is a new group-one pass. Change control on repeat orders is a recurring source of surprises.

Should the prototype supplier be the production supplier?

Not necessarily, but if they differ, equivalence has to be qualified rather than assumed. The prototype result only transfers if the build route transfers with it.

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