If an RFQ states that they need a “PCB”, it’s likely that they really mean a functional board populated with parts. The essence of PCB vs PCBA. A PCB is the actual bare board (copper and dielectric) on which the traces, pads, vias, mask and finish are fabricated, but there are no components on it. A PCBA is the same board that parts have been mounted and soldered on, it is then inspected and tested to the agreed criteria.
The difference decides the order. If you only need the interconnect built from Gerber data and a fabrication drawing, you are asking for PCB fabrication. If you want an assembled and working unit, you’ll be requesting a populated one. The team may think that they have ordered parts to be installed, but a bare board can pass the electrical test and turn out to be the wrong deliverable.
What a PCB (Bare Board) Includes
A blank PCB is NOT a blank sheet – it is a finished fabricated board waiting for the “real work.” The fabricator is responsible for creating the laminate, etching or plating the copper pattern, drilling and plating holes as per the designed pattern, applying the solder mask and legend, adding surface finish as per agreement and, in most cases, performing an electrical test. The board is then taken out of the fabrication shop with the empty pads. No resistors, no ICs, no connectors!
The blank expression can be very easy to misinterpret. If it’s a good quality bare board, the copper is already the circuit. Pads are already designed to fit the footprints you selected. The surface is already finished to which the solder will wet. The missing element is the way that the components are assembled, not the interconnect.
Consider the bare board as a product in and of itself. It has acceptance criteria which include layer registration, copper geometry, hole size, mask openings, finish quality, and continuity or isolation (where the test method applies to it). The checks are not proof of sitting, soldering or functioning of a part. They merely demonstrate that the “interconnect” has the same “fabration package”.
What the fabrication package must say
A fabricator does not quote or fabricate from intent, but rather from manufacturing data. Usable package is usually comprised of Gerber or ODB++ output, a drill file and a fabrication drawing. The drawing should include layer count, desired finished thickness, copper weight, material direction, surface finish, solder-mask requirements, legend requirements, impedance notes (if applicable), and panel notes (if applicable).
The most common mistake is providing only a Layout PDF or a photo of a populated board expecting a quote that would match the final product assembly. A photograph cannot specify stack-up, copper weight or finish. Drill data is not available in a PDF schematic. If there are no other files, such as a picture of a board with chips on it, then the fabricator is being asked to guess what is underneath the board.
Once the file list is complete, use PCB design guidelines to check stack-up, trace, drill, and panel data before the quote request goes out. The goal is a package a fabricator can build without inventing missing notes.
What a PCBA (Assembled Board) Includes
A PCBA includes inspection or test of the assembly process that parties agreed to perform on the bare board and the components. Surface mount parts are placed, reflowed. Where they are still being used in the design, through-hole parts are inserted and soldered. Both are used on mixed-technology boards. The assembly is then tested, either electrically or functionally (as appropriate) and is inspected after soldering.
The assembled board is the end result that most product teams want: a board that they can power, program and drop into a fixture. It still contains the bare board. The difference is that the PCBA order includes parts, polarity, solder connections and the checks that ensure that they are soldered to that revision.
The assembly house will also typically handle the bare board build and component ordering in many programs. That’s a service model, not a change in the definition of “PCBA”. The populated product consists of the assembled board. On top of that definition is the question as to who buys the parts and who fabricates the laminate.
Turnkey, partial turnkey, and consigned assembly
Three buying models keep re-appearing. They refer to who provides materials, rather than who owns the design.
The difference between turnkey PCBA and consigned assembly is often not distinguished in most RFQs. A turnkey job is one in which the partner manages the board making, component procurement and assembly. Consigned: The customer provides the parts and occasionally bare boards, the partner deals with placement and soldering. Partial turnkey is in the middle of those extremes—where’s the customer kitting a few critical or long-lead items, while the partner is buying the rest, and managing the line?
None of those models convey design authority. The approved BOM, approved manufacturers and the rules for substitution remain with customer unless otherwise specified in a separate agreement. The trouble is you think that when they say “turnkey”, they mean that the supplier can replace any part he sees is missing without needing to check it out. A turnkey partner will be able to identify shortages and suggest substitutions. Those alternates have yet to be approved as a project decision.
State the model in the RFQ. “Please assemble this” does not indicate the type of boards and reels that will be sent by the shop or if the shop has to purchase both boards and reels. That being the case, a fabrication-only quote and a assembly-only quote end up as one confused number because someone missed that sentence.
The assembly data set
PCB and PCBA files are not identical to each other and there are overlaps. There is still a need for bare board information to be passed to assembly to ensure the board is populated with the correct components. It also requires a BOM listing approved manufacturer parts or named alternates, centroid/pick and place data, assembly drawings, polarity and orientation notes, special handling instructions and any test & programming requirements that should be in the build.
Most first-article arguments come into existence because of two file problems. The first one is a BOM that contains only the distributor SKU and no manufacturer part number, no approved alternate. That file is where the users marked “buy”. It does not specify which package, tolerance or voltage rating is acceptable. The second is a centroid file that is not the same as the footprints on the board. The machine will deposit what it has written in the coordinates. If those coordinates are part of an older layout, then, the first assembled panel turns into a polarity & offset study.
When the assembly package is ready and the deliverable is a populated board, PCB assembly is the service path that mounts parts by SMT or through-hole work. Keep fabrication data and assembly data on the same revision. A new Gerber set with an old centroid is still a broken kit.
When to Order a Bare PCB Only
Only fabrication is the proper sequence if other people will be installing parts on the board, or if the team is not ready to commit to components yet. All of these fit the case of “in house assembly lines”, “a contract manufacturer already owns the BOM” and “lab builds where only the interconnect is needed for continuity or impedance check”.
A bare board buy is appropriate if there is a prototype stage. Some teams would like to check the quality of stack-up, finish and holes on first articles before release of expensive (or allocation sensitive) parts. The following is a permissible control step. It does not excuse not to remember assembly. All the pad geometry, keep-outs, fiducials and test points still need to facilitate the next process.
The first question in ordering a PCB or PCBA is who owns it? If your bench or your CM will be producing SMT and through-hole, then send a fabrication package and maintain the BOM in your own purchasing system. Without anyone in your camp to mount the parts, a bare-board PO is just half the product.
Risks of treating fabrication as a separate silo
An inexpensive bare board can turn into a costly assembly. Pads that do not fit exactly like the footprint, finishes made by selecting the ones the fabricator had in stock, no test points, and parts that are pushed against a tall connector are all discovered after the laminate is paid for. The “savings” then shift to stencil changes, hand rework and yield loss.
The silo mentality manifests as follows: The layout is finalized to meet the fabrication date, the files are sent to the assembly house a week after the fabrication date, and the first DFA comment comes once the copper is being fabricated. The fabricator performed as called for by the drawing. The assembled board was taken over from the previous assembly, but had never been examined as a populated board.
Even if fabrication and assembly will be split up between suppliers, include assembly constraints in the fabrication package. The following issues are decisions made in fabrication and have consequences on assembly: finish, pad design and test-point access. The optimization of only the bare board unit price delays the cost. It does not get rid of it.
When to Order a PCBA
Get an assembled board when the end result needs to fit in a box. The PCBA path would include early prototypes that require firmware, low volume builds for field testing and repeat production for a frozen BOM. The buyer retains the ownership of the schematic, layout and approved part list. The supplier carries out the process agreed upon that package.
A PCBA order is also the more eco-friendly option, if the team doesn’t wish to repeat the RFQ for assembly upon receipt of the boards. A single revision, one kit and one set of first-article comments is easier to deal with than a bare-board shipment with a scramble for reels.
If the date is indeed a demo date, don’t make up a board date. Demo hardware refers to a PCBA. You will not get a system to boot with all the laminates empty.
What changes in the conversation with the supplier
A PCBA RFQ takes the bare-board discussion and expands it to include component availability, approved alternates, lifecycle status and test strategy. The number of layers and finish are still important. No longer all the talk. The shop should also be aware of which parts are sole sourced, which packages are fine pitch or BGA, and if the build contains programming or a functional test.
The danger is to only charge for the assembly labor. Labor is visible. Schedules go awry over component risk. Any part with NRND (12 weeks lead), or any unofficial part that adjusts package height will advance the date faster than stencil time will.
When the build includes buying parts, ask for component sourcing for PCBA as part of a turnkey or partial-turnkey plan rather than treating the BOM as a solved input. The sourcing conversation should name manufacturer part numbers, allowed alternates, and who approves a change when a line item goes short.
How the Same Project Can Use Both
It’s not always one and the same between a bare board and an assembled one. Both are typically used in a realistic flow. First articles can be plain boards for electrical and stack up review. Next release could be a small PCBA run for firmware and mechanical fit. Pilot and production is then re-run with the same assembled BOM but under controlled conditions.
The sequence will only work if the same fabrication data and the same parts list are used forwards with the revision notes. The “quick” layout tweak between bare board spin and the assembled spin is still a new assembly package. If the pad moves but the centroid does not, the second build will inherit first build’s coordinates.
As Guangdong PCB and PCBA partner, the fabrication and assembly data are reviewed prior to production for the two packages, so that they can be compared. This review is not intended to supersede the authority of the customer in the drawing of the design. It is a chance to catch a mismatch before the file(s) have been changed.
Keeping fabrication and assembly data in sync
The unfashionable part of PCB fabrication as opposed to PCBA assembly is revision control. If a pad moves then the centroid and the footprint need to move. If a part is being replaced, the BOM line, assembly drawing and any polarity marking on the legend should indicate the same package. The polarity arguments begin on the line by updating the schematic and keeping the pick-and-place file on the last spin.
Provide one revision identifier for each release which includes Gerber output, fabrication drawing, BOM, centroid, and assembly drawing. If it is only the BOM has changed, state it. If only the text type changes (e.g., from regular to bold), state that. With silent partial updates, the supplier must assume what the current file is, since the update is not complete.
The pitfall is a folder called “latest” which contains 3 BOMs and 2 centroid files. There will be someone that chooses the wrong one. Indicate rewrite on the PO and on the file names. Then exclude the unused files from the release folder.
Common Buying Mistakes Between PCB and PCBA
PCB vs PCBA is not a copper mystery, it’s a purchasing & file issue in most instances. RFQs submitted by start-ups and/or by seasoned OEM teams who are moving too quickly all have the same 3 errors.
Ordering a bare board when a working assembly is needed
The boards arrive. They pass electrical test. They also have no contents. The firmware programmer is unable to upload code. Mechanical team is not able to verify connector height. Now the buyer must find a second supplier, a second PO, and a second waiting period for parts.
This additional loop is the price of “PCB” being a synonym for “finished hardware”. If the acceptance photo in the engineer’s head is populated parts, the PO should say “PCBA”. The absence of the necessary personnel to assemble the shipment is not a successful project if it comes in bare board.
Sending a populated-board photo as the only “spec”
It is helpful to have a picture of a functioning unit to be used as a reference. This is not a fabrication drawing AND it is NOT a BOM. The number of layers, the weight of copper, the finish, hole plating, and manufacturer part numbers are not part of a JPEG. If there is a photo attached, then each missing parameter is an assumption. When the first articles don’t match the unit in the picture, then assumptions are rework.
If any original design files are lost state that in the RFQ. Reverse engineering is not the same as a repeat of a documented build. Don’t rely on a snapshot of the top-side to get a grain of truth from a fabricator about a six-layer stack-up.
Treating assembly as a commodity after the board is made
If the laminate is complete, adding assembly is an additional “commodity” step. It is not. The design, finish, fiducials, keep-outs and test-point access were already determined for the pad layout. Where those decisions are against the process, then the assembly house inherits them.
The outcome is a board that can be fabricated, but not to the desired geometry: hand soldering on pads designed for a different geometry; no space for a fixture pogo; or a finish that does not match the desired profile. During the layout’movement’, ask the assembly questions. Until the panels are in a box, a “simple populate” becomes yield meetings.
A Simple Decision Checklist
Before ordering a quote, answer these.
- Need a working board or a bare interconnect? When it is absolutely necessary to switch on, then you require the PCBA.
- Will the assembly be retained in house or in another company that you are already doing business with? If it is, it’s possible enough a bare-PCB order will suffice.
- Do you need to source components? If the shop will have to purchase parts then turnkey or partial turnkey, and provide an approved BOM.
- Is there a correlation between the revisions of fabrication and assembly data? If a pad, part or centroid has been changed, release the entire package.
- What can you say about this product that you wouldn’t need to look at a picture of? Label the files: Gerber/Odb++ file, Fabrication drawing, BOM, Centroid, Assembly drawing, Test notes (if applicable).
If they are unambiguous, the RFQ can be asking for the right thing on the initial send. If not, redefine the deliverable first before anyone bids on copper or placement time.