Buying electronic components and PCBA from China

A PCBA prototype is assembled from 10 or 50 boards, and in that range the open Huaqiangbei market is the right route; a production run starts at 500 boards and there a component without lot traceability stops being an option. The reason is arithmetic: across 50 boards, a batch of remarked integrated circuits is caught by debugging the board and costs you an afternoon; across 5,000 boards the same batch passes incoming inspection, gets assembled, ships, and fails in the field at a rate that forces the product to be withdrawn.

The two worlds of component buying

The authorised distributor sells traceability and documentation at a price; the open Huaqiangbei market sells availability and speed with no guarantee of origin. They are not substitutes, and the expensive mistake is applying the criteria of the second to the problem of the first.

An open-market broker is easy to spot on 1688 by the catalogue: they sell part numbers from ten different manufacturers, whereas a franchised distributor carries one line or a few.

The worst case in the open market is not the integrated circuits, it is the passives: an 0402 resistor carries no marking, so there is nothing to inspect. A 5% part sold as 1% does not fail incoming inspection; it shifts a feedback loop and shows up months later as drift with no visible cause.

How a recovered or remarked component is detected

The checks run from cheap to expensive, and the order matters because the first four tests are not conclusive. They are resolved in Shenzhen on the same day the goods arrive:

1. Date consistency on the reel. That the marking on the parts matches the label and that the reel does not mix years. One labelled 2026 with parts marked 2019 is a relabelled reel and needs no further analysis.
2. Manufacturer packaging. A barrier bag with no humidity indicator card and no desiccant, a reel of a different dimension, or tape whose pocket lets the part rotate.
3. Package marking. Blacktopping consists of sanding the surface and re-engraving it with a laser: under a microscope you can see the sanding scratches and a typeface that does not match. A swab with solvent settles the doubt in a minute.
4. Electrical parameters against the datasheet. Leakage current when off, quiescent current, threshold voltage. A sample is measured against a reference part from an authorised distributor, not against the supplier’s specification.
5. X-ray fluorescence (XRF). Composition of the terminal plating; it detects recovery tinning, in which oxidised terminals from disassembly are re-tinned.

What decides the expensive cases needs an accredited laboratory: decapsulation and die inspection. Opening the package with acid and reading the die marking tells you which silicon is inside, and it is the only thing that distinguishes an original integrated circuit from a lower-grade one or from a rejected die. X-ray reveals whether there is a die, how many bond wires it has and whether the leadframe is the expected one. The test sequence is set out in SAE AS6171 and the open-market inspection criteria in IDEA-STD-1010, and they are applied to expensive part numbers, not to the whole reel, because they are billed per piece.

What has to be sent to quote a PCBA

Three files are enough to quote, and without them no budget is comparable: the Gerber files for the board, the bill of materials with part number and quantity, and the placement position file. The rest improves the offer, but it is not essential to give a price.

• Gerber in RS-274X or X2, plus the drill file in Excellon. With them goes the board definition: layers, thickness, copper, finish (HASL or ENIG) and which traces carry controlled impedance. The acceptability class of the bare board is specified on the order: IPC-6012 class 2 for consumer product.
• Bill of materials with designator, quantity, exact manufacturer part number, package and tolerance. A line that says “10 kΩ 0402” is not a specification: the assembler will buy the cheapest part that complies, and will be right to. Part numbers that are not fitted (DNP) and the accepted alternatives are marked.
• Position file (centroid, XYRS), with the same designators as the bill of materials. This is where the most frequent error sits: with a different rotation convention, the integrated circuits come out rotated 90 or 180 degrees, and a rotated board solders correctly, passes optical inspection and does not work.
• Schematic if the assembler is going to do fault diagnosis: without it they debug blind and every hour is billed. Firmware and the flashing procedure if they have to flash it, with the exact version: the wrong firmware is not a soldering defect and no visual inspection sees it.

Turnkey or consignment

Turnkey means the assembler buys the components and absorbs the supply risk; consignment means you supply them and keep that risk in exchange for control of origin and a lower cost at volume.

In practice a mixed version is used: the critical and expensive part numbers — the microcontroller, the radio circuit, the sensor — are consigned, and the assembler buys passives and connectors.

Two consignment details do not appear in the quote: the components have to reach the line in their barrier packaging and within floor life, and the assembler is not liable for parts they did not buy. If a consigned reel arrives damp and the parts crack in the oven, the loss is yours. On top of that, sending your own components to China to be assembled and re-importing the boards is a temporary export with its own procedure; in practice they are bought in China and delivered to the assembler.

The functional test is what separates a good lot from a bad one

Visual inspection and automated optical inspection detect soldering defects — bridges, rotated parts, missing material, insufficient fillets — and detect neither a counterfeit component nor wrong firmware. The functional test is the only one that detects both, because it powers the board and checks that it does what it is supposed to do.

The procedure and the acceptance criterion have to come from the client’s side, because they depend on the design. An assembler knows how to solder; they do not know how much your board is supposed to draw at idle. A usable criterion reads like this: output within ±2% of 5.0 V; quiescent current below 12 mA; the radio pairs and responds in under two seconds; the firmware version read from the diagnostic port is 1.4.2. Each one is a value with a tolerance, not an impression.

It is applied to 100% of production units, not to a sample, and the record matters as much as the result: the serial number is stored together with the measurement obtained, not a pass or a fail, because that is what makes it possible to trace a field failure back to a component lot.

Before that, two milestones are worth demanding: first-article inspection against the assembly drawing, and X-ray of the ball areas (BGA), where the solder sits beneath the package and optical inspection cannot reach.

MOQ: from 10 boards to a production run

From 10 to 50 boards is a normal prototype order, and its unit price is not usable for calculating production cost: what you are paying for is the setup. The stencil, the line programming, the first article and the test fixture are fixed costs spread across few units, and from 500 boards the quote starts to reflect the real assembly cost.

The floor for a prototype is not set by the assembler, it is set by the component: a full reel of 0402 passives is 5,000 pieces and an integrated circuit is supplied in reels of 2,500. Below that quantity you buy cut tape and the price per piece multiplies. The lead time is set by the part number that is missing, not by the assembly line, which is why the list is checked against real availability before committing to a date.

Table: what to specify, what to ask for and what to inspect

Certifications: those of the finished product, not of the component

A loose component or an assembled board imported as such rarely falls within the scope of a product certification; what triggers it is the finished product that incorporates it. In Mexico, a loose component usually falls outside NOM and what decides is the finished appliance: with a power supply or a battery it enters regulated categories, and if it emits radio it needs homologation before the CRT. In Argentina the requirement likewise depends on the finished product. In Brazil a radio module sold loose does require ANATEL homologation, in addition to the Inmetro of the finished product when it has a power supply. MeliPrep coordinates the laboratory and invoices the authority’s fees at cost.

For the part numbers on the bill of materials, the requirement asked for by contract is RoHS and REACH, part number by part number and not by family. The detail by category and market is in the certification matrix.

On an industrial purchase the criterion in the warehouse is not that of a consumer accessory: the part number and lot of every line are verified against the bill of materials, whatever does not match is held before it enters the assembly line, and it is consolidated for shipment. MeliPrep works that receiving with documentary supplier verification at USD 349 and an on-site audit at USD 549 within Guangdong.