PCB assembly: SMD, THT and what a prototype changes

PCB assembly is putting the parts on a bare board and soldering them. Engineers say "get it populated", purchasing writes "PCBA" or "assembly services". Same work. This article is about how PCB assembly goes in practice, step by step, and what changes when the job is 2 to 5 boards instead of 5,000. If you want the price and the lead time first, they are on the page about prototype PCB assembly in the Netherlands.

How PCB assembly works, step by step

It starts with solder paste. A stencil, a thin steel sheet with an opening on every SMD pad, lies on the bare board and the paste is squeegeed across it. Each pad then carries a layer of paste about a tenth of a millimetre thick. The amount matters: too little gives an open joint, too much a bridge.

Then the board goes to the pick-and-place. The machine takes each SMD part from a reel, tube or tray and puts it on its position. It reads that position from the centroid: per designator an X, a Y, a rotation and a side.

In the reflow oven the paste melts. The board follows a temperature profile: preheat, a hold so the flux can do its job, then a short time above the melting point of the alloy (around 217 degrees for SAC). The liquid solder pulls each part to the centre of its pads. That self-alignment is why a machine-placed 0402 sits straighter than a hand-soldered one.

If the board has SMD on both sides, this happens twice. After that comes everything that cannot go through the oven or does not fit the machine: THT connectors, large electrolytics, transformers, screw terminals. Those we solder by hand. Last comes inspection: is everything there, in the right place, pin 1 the right way round, no bridges.

What a prototype changes

In a run of 5,000, setting up the line is a fraction of the work. For 5 boards it is nearly all of it: the stencil, the feeders, the programme and the profile take the same effort for 5 as for 5,000. An assembly house built for volume therefore pushes a small order to the back of the queue, or says no. That is not unwillingness, it is arithmetic.

DeltaProto does the opposite: different boards every day, in small numbers. The minimum is 2 boards, with no surcharge for small quantities. From a few hundred boards an EMS is the better choice, and we will tell you so.

Two things go differently here than in volume. First, an engineer looks at your BOM and your files before the order is placed. A footprint that does not match the part number, or a centroid with the bottom side mirrored: better to hear it beforehand than to find it on the sample. Second, we build one sample first, the First Proto. You test it, and more than 90% of our customers change something afterwards.

On cost: five boards with about 120 placements come to 840 euro of assembly today at 10 working days, and 930 euro at 5, ex VAT. Components and the bare PCB are added at cost, so whatever Mouser or Digi-Key charges that day, with no markup. There is no setup or NRE line afterwards: the price in the calculator is the price you pay.

Fine-pitch parts: solder it yourself or have it assembled?

The question we hear most from labs: can't I do this myself? For an 0603 and a DIP, yes. Below that it becomes a gamble, and prototype SMD assembly is exactly where a stencil and an oven make the difference.

A QFN with 0.5 mm pitch is routine with a stencil and reflow. The right amount of paste sits on every pad and on the exposed pad, the machine puts the IC in place, and in the oven the solder pulls it straight. With a hot-air gun you have none of those three. A BGA goes through the machine and the oven the same way. Where a BGA does get exciting is the bare board underneath it: read how we struggled with an interposer at 0.5 mm pitch.

We also assemble single pieces for labs and universities. Two boards with one fine-pitch ADC on them is a normal order.

Your own parts and your own PCB

Three parts already in the lab, or one out of stock everywhere? Send them along. The rules are on the consignment page and come down to this: each part type in its own bag, the designator and quantity on every bag, and the project code on the packing slip and on the box. Reel, cut tape, tube or loose does not matter. The consignment list in your project says how many we need. Mixed is normal: three parts from you, the rest bought by us.

What we buy comes from Digi-Key, Mouser, LCSC, Farnell, TME, RS, Arrow, Verical and Würth Elektronik, only what is in stock there, and you pay what the supplier charges us. If a part is unobtainable but still sits on an old board, send the board and we desolder it free of charge. What we do about alternatives is on the parts and suppliers page.

You may also supply the bare PCB yourself. If you do not, we buy it per board from fabs in Europe or Asia, in whatever stackup your design needs. Whether it is 2 layers or 6 with ENIG makes no difference to the assembly.

What to prepare

Three exports from your CAD package, as they come out:

• the PCB data as a Gerber and drill set
• the BOM, as csv or xlsx
• the centroid, the pick-and-place file

KiCad calls the centroid a pos file, Altium a pick and place export. Both go straight in; mapping columns, converting units and repairing a broken CSV is our job. ODB++ is welcome but not needed. Put a part number on every BOM line, so there is no doubt about which variant we buy. And take one critical look at your silkscreen. The centroid says how a part is rotated, not whether the rotation in your library matches the real part. Pin 1 on every IC and a bar on every diode is the cheapest check there is.

Boards ship from the Netherlands, so inside the EU there are no customs and no import duties. Our two labs are in Amsterdam and Enschede, and you are welcome to visit during the build. Prefer an NDA before you send files? Ours is on the downloads page.

Rework and the second sample

A sample is finished only once you have tested it. Usually there is something afterwards: a pull-up that needs a different value, a connector that is mirrored. That is part of the process, read why sample 2 is not a failure. You put the changes on the List of Change and we build the next sample from it, within the same order. Three samples per order are included, more if you need them, each with its own revision.

If it is a small thing, swapping a part or adding a wire, there is no need for a new sample. Send the existing one back with the free return voucher and we modify it.

Want to know what assembling your board costs? Upload Gerber, BOM and centroid in the calculator. You read the price in seconds, about 95% accurate, and the quote follows within hours after an engineer has looked at your files. No account, no sales call.