HDI PCB Supplier Selection: Which Process Steps Are Not Done In-House
"We manufacture HDI" does not answer the question you asked
You send the RFQ and three quotes come back. All three letterheads say HDI. All three attach a capability table. All three confirm laser drilling, via fill plating and fine lines. The prices differ by a visible margin and the lead times by more than a week.
At this point most buyers compare price and lead time, then hunt for differences in the capability tables. What the capability table cannot show you is this: across those three suppliers, not every step on your board necessarily happens under one roof.
"We are an HDI manufacturer" describes order scope — they are willing and able to deliver this class of board. It says nothing about shop-floor boundaries — which steps their own equipment and their own people perform, and which steps mean loading work-in-progress onto a truck, sending it to another company, and trucking it back.
Quotes have no field that distinguishes these two things. Capability tables are formatted as can / cannot, not as in-house / outside. So a shop that places five steps outside and a shop that runs everything itself look identical on your comparison sheet.
This is not an argument for refusing subcontracting. Subcontracting is not a defect. Several steps in the HDI chain are inherently capital-intensive and machine-specific, and a mid-sized shop placing one or two of them outside is ordinary industrial specialization — sometimes more stable than building the capability in-house. The real problem is not whether steps are outsourced, it is not knowing that they are, because then your assumptions about lead time, liability and change control all rest on a continuous production line that does not exist.
Where step ownership typically breaks in the HDI chain
What separates HDI from conventional multilayer is concentrated in exactly the most equipment-hungry steps. Each one below has a mature specialist subcontracting market, and each one does get placed outside.
Laser drilling. Microvias are not a mechanical-drill job; they need a dedicated laser drill. This is the classic buy-the-machine-or-buy-the-service dividing line in the HDI chain. A shop can be fully capable of every other step and send only this one out.
Via fill and filled-via plating. Filling microvias, filling and capping via-in-pad, involves dedicated fill and plating lines with a different chemistry regime from standard plating. It is often bundled with laser drilling at the same subcontractor, and sometimes split.
Fine-line imaging. Past a certain line-width and spacing class, the exposure equipment, environmental cleanliness and develop control all step up a tier. A shop's standard line may handle coarse geometry while the fine class borrows someone else's imaging.
Thin cores and sequential lamination. HDI build-ups frequently require multiple lamination cycles. Handling thin cores, controlling expansion and contraction, and managing vacuum press programs are separate skills. Lamination is subcontracted less often than laser drilling, but it does appear on higher build-up counts.
AOI, cross-sectioning and electrical test. Inspection is the easiest to overlook, because it does not change the board — it only produces conclusions. But subcontracting inspection changes who signs that the board is acceptable. Sending cross-sections to an independent lab is common and sensible; the issue is that the entity named on the report is not the supplier named on your PO.
Surface finish. Some finishes carry higher line investment and environmental permitting thresholds, and smaller shops routinely send them out.
Put those together and a scenario emerges that buyers rarely picture: from material issue to shipment, an HDI board may cross the gates of two or three companies, while your PO carries one name.
An outsourced step turns lead time from a line into a relay
The lead time you receive is a single number. If every step is in-house, that number sits on top of a queue position on one production line — the board appears on the supplier's own production board, at a station someone can name.
With a subcontracted step, that number is several intervals spliced together, and the middle one is not on the supplier's board.
Three categories of time get added. Transit: the board ships out and ships back, consuming calendar days unrelated to any process. The subcontractor's queue: where your board sits in that queue depends on the subcontractor's own order mix and on the priority it grants your supplier — a priority you cannot observe and your supplier sometimes cannot guarantee. Re-entry: the board comes back and may not connect straight to the next station, so it re-enters the in-house queue.
The consequence of stacking these is not "slower on average" — it is higher variance. When an in-house step slips, the supplier knows the cause, knows the remaining duration, and can use overtime or resequencing to compress it. When a subcontracted step slips, all the supplier can do is call and chase, then pass the same uncertainty on to you.
Which is why the most revealing expediting question is not "how many more days" but "who physically has the board right now." If the supplier has to make a phone call before answering that, you have learned there is a segment outside its control.
There is a less obvious consequence too: the ability to expedite or jump a queue evaporates in the subcontracted segment. A supplier can clear its own line for your hot order. It cannot make another company clear theirs — unless it already holds a priority arrangement there, which it will generally not volunteer.
Liability breaks at the handoff
This is the most substantive risk in subcontracting and the least discussed.
Boards arrive. Something fails. Failure attribution requires evidence: records from a specific step, parameters from a specific lot, curves from a specific machine. When everything is in-house, those records live in one company's system, the supplier can pull them, and you have a contract that lets you ask.
When the failure point sits in a subcontracted step, the evidence chain has to cross a company boundary. Your supplier must request records from its subcontractor, and that subcontractor has no contractual relationship with you — no disclosure obligation toward you, and no motivation to cooperate beyond the commercial weight your supplier carries there.
The familiar deadlock follows: the supplier sincerely reports that it is "pursuing it upstream," what comes back upstream is not sufficient to assign cause, and your final answer is a relayed explanation unsupported by primary records. You can neither refute it nor accept it.
What should actually be settled at RFQ stage is not "who pays if it fails" — indemnity language is usually clear enough — but:
- Record accessibility. Whether the supplier has the right to obtain process records from the subcontracted step and disclose them to you. This has to already exist in its agreement with the subcontractor; requesting it after the fact generally fails.
- Retention responsibility. How long cross-sections, coupons and process parameters from the subcontracted step are retained, and in whose custody.
- The signing entity. Whether the entity signing acceptance on the shipment report is the same as the supplier on your PO. If inspection is subcontracted, two names appear here — and what you need to confirm is that your supplier carries full endorsement responsibility for the subcontractor's conclusion, not merely that it forwarded a report.
There is a layer beneath this: completeness of liability is not the same as enforceability of liability. Writing "the supplier is responsible for all processes" into a contract is easy and legally sound. But when the evidence sits with a third party that will not cooperate, that clause gives you grounds for a claim, not a root cause. Most of the time you need the root cause, because you still have to keep building.
Change control and freezes only cover the in-house segment
Once volume production stabilizes, buyers typically require a process freeze: no changes to stackup, materials or process route without prior notification and approval.
That requirement carries an implicit assumption — that every step is under the supplier's control, so its change control process covers all of them.
With a subcontracted step, there is a gap: changing subcontractors does not count as a "process change" under many shops' internal definitions. The reasoning is internally coherent — the specification did not change, the drawing did not change, the acceptance criteria did not change, only the company performing one step. Their change control form may not even have a line item for subcontractor substitution.
From your side, though, what changed can be quite material. Different subcontractors run different equipment, different programs, different process windows. The same data package can yield acceptable boards at two of them without necessarily behaving identically on the dimension your board happens to be sensitive to. And the moment you discover this is usually after a lot arrives with a shifted parameter distribution, while every document is unchanged and everyone states that nothing was modified.
Closing the gap takes one sentence at RFQ stage: subcontractor substitution constitutes an engineering change requiring prior notification, with an agreed notification point (before the change takes effect) and first-lot handling (whether first article confirmation is required). This is the highest-leverage term in the whole topic — it does not constrain the supplier's normal business, it only requires that you be told.
Conversely, if a supplier will not accept that term, the signal is clearer than any capability table: it needs to preserve the freedom to change its process constitution without your knowledge.
Questions that surface step ownership
"Do you subcontract?" carries no information. Almost everyone subcontracts something, and asked that way the answer is usually "the main processes are all done in-house" — where "main" absorbs everything.
Informative questions are step-by-step and demand specific answers. The groups below are ordered by leverage.
Group 1: ownership per step. List the steps your board will actually run — laser drilling, via fill plating, fine-line imaging, lamination, surface finish, AOI, cross-section, electrical test — and have the supplier mark in-house or outside on every row. Do not accept a paragraph. Require the table. A paragraph can always route around one particular step; a row-by-row table cannot.
The table has a side benefit: it shifts the conversation from "are you capable" to "how does this board flow," and the second is engineering language that their engineers will engage with.
Group 2: governance of the outside steps. For every row marked outside, pursue three things: whether the subcontractor is fixed (single named party, or selected by availability), whether you can obtain process records, and whether substituting the subcontractor triggers change notification. As above, the third matters most.
Group 3: how lead time decomposes. Do not ask only for total lead time. Ask how many days sit in the subcontracted segment, whether transit is included, and the historical range of queue variation there. The value is not in the number — it is in whether the supplier can answer at all. A supplier that decomposes it manages that segment; one that can only give a total is gambling along with you.
Group 4: inspection and signature. Who performs outgoing inspection and cross-sectioning, who signs the report, and whether your supplier fully endorses that conclusion. If inspection is outside and the supplier only forwards results, you do not have a counterparty carrying complete responsibility for acceptance.
Group 5: expediting. Ask directly: if this needs to be pulled in, can the subcontracted segment compress, and by what mechanism. An answer of "we have a good relationship with them" means no.
None of these require disclosure of commercial secrets, and none require a site visit. They all ask for interface information: who holds the step, who can pull the records, who notifies a change, how the time divides. A well-managed shop answers these easily, because those are already the dimensions along which it manages its subcontractors.
When "all in-house" is not the answer you want
The counterweight is worth stating, or this reads as an argument for picking the fully integrated shop.
All in-house does not mean every step is done well. A shop that sends laser drilling to a specialist with newer equipment and far higher volume may hold better actual process capability on that step than a shop that bought a machine, does not keep it loaded, and has limited program experience. Equipment under your own roof does not guarantee a steadier process window.
The word "in-house" is elastic. Another plant in the same group, an affiliate in the same industrial park, an own crew running on leased lines — all of these may be counted as in-house. That is not deception, it is customary usage. So the per-step table deserves one follow-up: does in-house mean inside this facility.
A shop that governs its subcontracting well beats one that conceals it, and beats one straining to keep a step in-house. The test is not the count of outsourced steps, it is three things: fixed subcontractor, accessible records, notified changes. A subcontracted step with all three has a risk profile close to an in-house step. An in-house step with none of them still leaves you without records.
So the conclusion here is not a preference, it is an action: move step ownership from unknown to known, then negotiate lead time, liability and change control against the real boundaries. How much is subcontracted does not matter much. Whether you know, and whether you agreed terms for it, matters a great deal.
Minimum actions before you send the RFQ
- List the steps your board actually runs and require the supplier to mark in-house / outside on each row; do not accept a narrative paragraph.
- For every outside row, pursue: fixed subcontractor or not, record accessibility, and whether substitution triggers change notification.
- Decompose lead time into in-house / transit / subcontracted queue, and ask for the historical variation range of that queue.
- Confirm who performs and who signs outgoing inspection and cross-sectioning, and the scope of your supplier's endorsement of that conclusion.
- Add one line to the PO or contract: subcontractor substitution is an engineering change requiring prior notification, with the notification point and first-lot handling specified.
- Pin down what "in-house" means: is it within this facility.
This list does not change your supplier preference. It changes how many columns your comparison sheet has. Those extra columns hold information you previously discovered during a lead-time slip or a liability dispute.
FAQ
Does subcontracting mean the supplier is not capable?
No. Several steps in the HDI chain are capital-intensive and machine-specific, and handing one or two to a specialist with more concentrated equipment and volume is ordinary industrial specialization — sometimes with better process stability. The problem is not knowing, because then your lead time, liability and change assumptions rest on a continuous line that does not exist. Judge the governance quality of the outsourced step, not the count of outsourced steps.
Can I just ask "do you subcontract?"
That answer carries no information. You will typically hear "the main processes are in-house," where "main" absorbs every specific case. The effective approach is a table of steps with in-house / outside marked row by row. A narrative answer can route around a particular step; a row-by-row table cannot.
For an outsourced step, should I worry more about lead time or quality?
About evidence. Lead time problems surface eventually and the board tells you about quality problems, but failure attribution needs an evidence chain that crosses a company boundary — and the subcontractor has no contract with you and no disclosure obligation toward you. So what to lock at RFQ stage is not indemnity language but record accessibility: whether your supplier has the right to obtain and disclose process records from that step. It must already exist in their agreement; requesting it afterward generally fails.
Why does "changing subcontractors" need its own notification clause?
Because many shops do not classify it as a process change — specification, drawing and acceptance criteria are all unchanged, only the company performing one step. But different subcontractors run different equipment, programs and process windows; the same data package can produce acceptable boards at both without behaving identically on the dimension your board is sensitive to. Without the clause, the failure mode is a lot arriving with a shifted parameter distribution while every document is unchanged and everyone says nothing was modified.
When a supplier says "in-house," can I take it at face value?
The term is elastic in this industry: another plant in the same group, an affiliate in the same park, or an own crew on leased lines may all be counted as in-house. That is customary rather than deceptive. It is worth asking "do you mean inside this facility" — for lead time and record accessibility, in-house across facilities is not as different from subcontracting as it sounds.
What is special about subcontracted inspection?
Inspection does not change the board, it only produces conclusions, which is why it gets overlooked. What it changes is who signs for acceptance. Sending cross-sections to an independent lab is common and reasonable; the issue is that the entity named on the report differs from the supplier on your PO. Confirm that your supplier carries full endorsement responsibility for that conclusion rather than merely forwarding a report — otherwise you have no counterparty fully accountable for acceptance.
Related reading
- HDI product and process scope: HDI PCB
- Capability evidence to request without a site visit: HDI Supplier Capability Evidence Shortlist
- How to verify capability tiers: HDI Manufacturing Capability Tiers Buyer Guide
- Capability is not capacity allocation: HDI Supplier Capacity Allocation and Commitment
- Submit the step table with your inquiry: Request a Quote
Need PCBs for Your Project?
Get an instant quote — prototypes from $2, delivered worldwide.
Get Instant Quote