HDI PCB for 5G Modules, Handsets and Automotive: What Changes in Your RFQ

"We need an HDI board" is not a specification. It narrows the process family and almost nothing else. Two buyers can send that same sentence with the same layer count and get quotes that differ in scope, lead time assumptions and what the supplier promises to verify.

The difference is not the fab being inconsistent. It is that your end application decides which RFQ inputs stop being optional. A 5G front-end module makes material and impedance data mandatory before anyone can price it. A handset main board makes build-up structure and finished thickness mandatory. An automotive control board makes traceability and change control mandatory, sometimes before the technical review even starts.

If you know which inputs your own category makes mandatory, you can put them in the first RFQ instead of discovering them across three rounds of supplier questions.

!Four-spindle CNC drilling machine for printed circuit boards with its acrylic hood raised, showing four spindle heads on a gantry above a gridded backing panel and a control column with monitor

The build sequence is a purchase, not an inheritance

Before splitting by application, one point applies to all three. HDI is described by a build-up notation: 1+N+1, 2+N+2, up to 4+N+4 on the PCB168 HDI PCB page. Each step up is another sequential lamination and another laser drilling pass.

That matters commercially because the notation is not a property your design inherits from its layer count. It is a routing decision with a cost attached. A 10-layer board can often be built as 1+N+1 if the escape routing from the fine-pitch parts can be resolved in one microvia level, and it may need 2+N+2 if it cannot. Same layer count, different number of process cycles.

So when a supplier asks "which build-up do you need?", the honest answer for many buyers is "whichever one your DFM says is sufficient". Say that explicitly. Asking for 4+N+4 because it sounds safer buys process passes you may not need. Asking for 1+N+1 on a design that cannot route in one level gets you a quote that will be revised later.

Practical wording for the RFQ: state the constraint, not the build. "Densest escape is a 0.35mm pitch BGA, 412 balls; please propose the minimum build-up that routes it and quote that." The fab now owns a decision it is better equipped to make, and you get a comparable basis across suppliers.

5G modules: material and impedance data come first

For a 5G or mmWave module, the first supplier question is almost never about layer count. It is about material, because the material choice constrains the stackup, the stackup constrains impedance, and impedance is the thing your design actually has to hit.

The PCB168 HDI page lists FR-4, BT and ABF as base material families. What a 5G RFQ has to add is which one your design is qualified against and at what operating frequency, because a laminate that behaves acceptably at 2.6GHz and one intended for mmWave are not interchangeable choices the fab can make for you.

What becomes mandatory in this category:

• Named laminate or an explicit equivalence rule. Either give the specific laminate you designed to, or state in writing that equivalents may be proposed for engineering review. Do not leave it blank — a blank material field means the fab quotes its default, and your impedance model was not built on the default.
• Impedance targets with tolerance and reference layers. Single-ended and differential values, the tolerance band, and which layer each controlled trace references. Without reference layers the fab cannot verify your target against its own stackup calculation.
• Operating frequency range. This is what tells the supplier whether loss and surface roughness matter enough to raise questions before quoting.
• Coupon requirement, stated yes or no. If you need impedance test coupons on the panel, say so in the RFQ. It affects panel layout, and adding it after the fact is a change.

Suppliers ask about material first in this category because it is the input with the longest consequence chain. Everything else can be adjusted; a material substitution invalidates your simulation.

For background on the frequency-driven side of these decisions, see high-frequency PCB material selection, impedance control in PCB design and 5G mmWave PCB design challenges.

Handsets and wearables: build-up and finished thickness come first

For a handset main board, a smartwatch board or any space-constrained consumer product, the constraint set inverts. Material is often ordinary. What is tight is geometry.

The first question here is usually about finished thickness and build-up, because thin boards with multiple sequential laminations are where handset HDI gets difficult. The PCB168 HDI page states 4-20 layers, 2mil/2mil minimum line width and spacing, a fine-line capability listed as 3/3mil, laser drilling at 0.075mm, microvia diameter from 0.1mm, and thin-board capability among its features. A handset RFQ has to say where in that range it sits.

What becomes mandatory:

• Finished thickness with tolerance. Not a target you hope for. The number your mechanical enclosure was designed around, plus the tolerance you can actually accept.
• Whether any-layer interconnection is required. Any-layer is on the capability list, and it is a materially different build from a stacked-microvia structure limited to outer levels. It changes the number of lamination cycles, so it changes the price.
• The tightest component pitch on the board, with its part count. This is the input that determines whether one microvia level is enough. Give the worst case, not a typical value.
• Panelisation and array requirements if assembly follows. If this board is going into an SMT line, the array format is not a detail to settle later.

The thing buyers most often omit in this category is the tolerance on thickness. A thickness with no tolerance is treated as nominal, and thin sequential-lamination builds are exactly where nominal and actual diverge most.

Related reading: HDI PCB manufacturing process and the multilayer PCB stackup guide.

Automotive: traceability and change control come first

Automotive HDI is where the technical inputs stop being the first thing anyone talks about. The first question is frequently procedural: what standard and class are you buying to, and what happens when something changes.

This is the category where buyers most often underestimate the RFQ. The board itself may be less aggressive than a handset board. The obligations around it are heavier.

What becomes mandatory:

• Acceptance standard and class, named explicitly. IPC-6012 for the bare board and IPC-A-600 for visual acceptance, at the class level you require. If assembly is in scope, IPC-A-610 and J-STD-001 at the stated class. Name the standard and class; do not restate table values from memory in the RFQ, and do not leave class unstated.
• Change control terms. Whether the supplier may substitute laminate, surface finish or sub-supplier without written approval, and who signs off. For automotive programmes the usual answer is no unqualified substitution, and that has to be in the PO, not in a conversation.
• Traceability granularity. Lot, panel or serialised board level. Each level is a different amount of work on the fab floor and a different record you can request later. Decide before quoting.
• Documentation retention and reporting. Which reports accompany each shipment and how long records are kept.
• Operating temperature range and any thermal cycling expectation. This is the technical input most likely to drive material discussion in this category.

The reason to front-load these is asymmetry. A missing impedance target costs you a supplier question. A missing change-control clause costs you an argument after a substitution has already shipped.

If assembly and traceability are both in scope, see PCB assembly and PCB quality inspection methods.

The four inputs all three categories need

Whatever your end product, these four appear in every complete HDI RFQ. They are the reason a fab can quote at all.

1. Fabrication drawing with tolerances. Dimensions, hole tolerances, layer sequence, surface finish, markings. If a value matters, it belongs here rather than in an email.
2. Gerber or ODB++ plus a drill file with tool assignments. State which holes are laser-drilled microvias and which are mechanically drilled, or state that the fab should propose it.
3. Stackup intent, or explicit permission for the fab to propose one. Either is workable. Silence is not — silence produces a default stackup nobody verified against your design.
4. Quantity structure and phase. Prototype, pilot and production volumes, and which one this quote is for. HDI cost per unit moves substantially with panel utilisation, so a quantity-less RFQ gets a cautious number.

Anything beyond the published capability figures — tighter line width, smaller microvias, unusual aspect ratios, exotic materials — should be marked in the RFQ as requiring engineering confirmation rather than assumed. That single habit removes most of the back-and-forth in the first round.

For the general capability picture, see capabilities, multilayer PCB and rigid-flex PCB if your design is partly flexible.

A one-page input sheet

Put this at the top of the RFQ. It fits on a page and removes the first two rounds of questions.

"Critical" means the supplier will most likely stop and ask before quoting if it is missing. "Required" means it belongs in a complete RFQ. "Optional" means it is useful context but not usually blocking.

Ready to send one? Use the quote request or contact form, and attach the sheet above with your data package.

FAQ

Does a higher build-up number mean a better board?

No. It means more sequential lamination and laser drilling passes. The right build-up is the minimum one that routes your design. State your densest escape constraint and let DFM propose the build.

Can I ask for HDI without specifying the build-up?

Yes, and it is often the better approach. Give the layer count, the tightest pitch and the finished thickness, then ask the fab to propose the minimum sufficient build-up and quote it.

Why does the supplier ask about material before layer count for 5G work?

Because material constrains the stackup, the stackup determines impedance, and impedance is your actual requirement. A material change invalidates the simulation your design was signed off against.

What is the most commonly missing input on handset HDI RFQs?

Tolerance on finished thickness. Without it the number is treated as nominal, and thin sequential-lamination builds are exactly where nominal and actual diverge.

Is automotive HDI technically harder than handset HDI?

Not necessarily. The board is often less aggressive. What is heavier is the procedural load: named standard and class, change control, traceability granularity and documentation retention.

My design needs something beyond the published capability figures. Should I leave it out of the RFQ?

No. Mark it explicitly as requiring engineering confirmation. Omitting it means the fab discovers it during DFM and the quote changes.

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