From schematic to assembled PCB: making an electronic prototype

1. Schematic and component selection: availability, packages, life cycle.
2. Layout: placement and routing according to the manufacturer’s rules.
3. DFM review: check of the manufacturing and assembly rules.
4. Manufacturing files: Gerber, drill, outline, bill of materials, component positions.
5. Bare board manufacture (PCB).
6. Component sourcing.
7. Assembly: solder paste, placement, reflow, through-hole components.
8. Inspection, testing, then firmware programming.

As a guide, a prototype bare board is made in a few working days, whereas it is often the components, their availability and their sourcing lead times that set the total lead time: start critical purchases early. You can entrust all or part of the chain to a supplier: see electronics subcontracting.

The price of a prototype is made up of the bare board (area, number of layers, finish, lead time), the components and the assembly. Prototype manufacturers often combine several orders on one production panel, which makes small quantities affordable. At assembly, start-up costs (solder paste stencil, pick-and-place programming, line set-up) weigh heavily on a single board: the price of a few extra boards is often small compared with these costs.

Deliver a compressed package, clearly named, in a single version. ODB++ and IPC-2581 combine this data in a single format: ask your manufacturer what it accepts. Open your Gerber files in a viewer before sending: it is the quickest way to spot a missing layer. See also manufacturing file formats.

• Layers: one or two for simple boards; four or more as soon as density, signal integrity or power planes require it.
• Substrate: FR-4 is the most common; polyimide for flex, aluminium for LEDs, special laminates for high frequencies.
• Thickness and copper: 1.6 mm and 35 µm (1 oz) are the standard values; high current calls for more copper.
• Surface finish: HASL, economical, leaves a less flat surface; ENIG (electroless nickel immersion gold) suits fine pitches and BGAs.
• Capability: track width, clearance, drill diameter, blind or buried vias, controlled impedance. As a guide, tracks and clearances of around 0.15 mm (6 mil) and a 0.3 mm drill fall within most manufacturers’ standard; below that, it is a more expensive option.

The IPC classes describe the level of requirement: IPC-6012 for the manufacture of rigid boards, IPC-A-610 for the acceptability of assembled boards. Class 1 is for general products, class 2 for products whose service must remain durable, class 3 for high reliability (critical equipment, medical, aerospace). Class 2 is the reference for most industrial products: demanding class 3 for a prototype makes the job heavier if your final product is not required to meet it.

For controlled impedance, ask the manufacturer for its standard stack-up before routing: it sets the track widths, and changing it afterwards forces you to redo the routing.

The bill of materials (BOM)

A usable BOM gives, for each line, the designators (R1, C3…), the quantity, the value, the package, the manufacturer and its part number, and the accepted alternatives. Mark components not to be fitted. Check availability before freezing the package: an out-of-stock or obsolete component delays the whole project, and providing a compatible alternative for at-risk part numbers avoids having to redo the package. Choose who sources: you, the assembler (turnkey) or a mix of the two. The workshop often allows a surplus of small components for placement losses, and reels have minimum quantities.

The pick-and-place file

Exported from your CAD tool, it gives for each component its designator, its X and Y coordinates, its rotation and its side. Rotation conventions differ from one library and one machine to another: a polarised component (diode, electrolytic capacitor, integrated circuit) may be fitted the wrong way round. Attach an assembly drawing that marks polarity and pin 1, and ask for validation before launch.

• SMT (surface-mount technology): components soldered to the surface, without drilling, on both sides. Solder paste is applied through a stencil, components are placed by a machine, then passed through a reflow oven. This technology suits fine-pitch packages; the hidden joints of BGAs and, in part, QFNs are checked by X-ray.
• Through-hole: the pins pass through plated holes. It is mechanically more robust, hence its use for connectors, transformers, large capacitors and terminal blocks. Soldering is done by wave, selectively or by hand: more operations, and so a higher unit cost.

The mixed board is the most common: SMT first, then through-hole components. Favour SMT wherever possible, and keep through-hole for parts subject to stress or high currents. Assembly on both sides doubles the placement and reflow passes: for an economical prototype, assembling a single side is preferable. If you want to solder a first sample yourself, packages of size 0603 or larger and pitches of 0.5 mm or more remain manageable; BGAs are in practice soldered by reflow. In the European Union, the RoHS directive restricts the use of lead: assembly is generally done with lead-free solder, and any special requirement must be flagged. See the glossary: SMT and printed circuit board.

Five DFM checks before sending

1. Run the design rule check (DRC) with your manufacturer’s capabilities.
2. Check each footprint against the component datasheet: a wrong footprint means redoing the board.
3. Leave room around components for placement and rework, and a clear zone of a few millimetres at the board edge for the conveyor and routing out.
4. Add fiducials for machine vision and accessible test points.
5. Take care with the pads of small passive components: asymmetrical pads encourage the “tombstone” effect, where the component stands up during soldering.

Inspection and testing

The bare board can undergo an electrical continuity and isolation test. After soldering, automated optical inspection (AOI) detects missing, shifted and reversed components and solder bridges. In-circuit testing and functional testing require test points and a procedure: they pay off mainly in series production. For a prototype, the workshop often limits itself to inspection, and you test the first board yourself. Supply the firmware and the programming procedure if the workshop is to load it.

When you receive the first board

Check the appearance first (missing components, polarities, solder joints), then measure the resistance between each supply rail and ground before any power-up. Then power up in stages, preferably with a current-limited bench power supply, and validate the blocks one by one (power supplies, clock, communication) before testing the rest. Record the deviations: they feed into the next revision of the schematic.

• The number of boards, with price breaks (1, 5, 10) and any forthcoming series requirement.
• The complete package: Gerber, drill, BOM, positions, assembly drawing.
• The board characteristics: layers, thickness, copper, finish, solder mask colour, impedance if any, target IPC class.
• Sourcing: components supplied by you or bought by the assembler, accepted alternatives.
• Assembly: sides to assemble, through-hole components, soldering requirements.
• The testing and programming expected (inspection, functional test, firmware loading), protective coating, anti-static packaging.
• The lead time, the date on which you will have any components you are supplying, and the delivery location.

On Meaflow, posting a request is free and commission-free: several suppliers can send you their quote, which you compare before discussing through the messaging system. To structure your package, use the specification generator. For the rest of the project, read from prototype to series production, and discover the hardware start-ups sector as well as design offices for the design work.