What hand assembly was hiding
A hand-built product can absorb a lot of documentation debt. The operator knows the board, reads the schematic, checks polarity by eye and reworks the odd joint. When the same design moves to an SMT line, that tacit knowledge is replaced by programs, feeders and a stencil — and every gap in the documentation becomes a defect.
The five issues below are the ones that show up first.
1. Documentation gaps
The line needs a BOM that matches the designators, an assembly drawing that shows orientation and polarity, footprints that match the parts actually bought, and a panel that fits the machine. A hand build tolerates "the 10 µF one near the regulator"; an automated line does not.
2. Silkscreen and reference marks
Silkscreen is the reference that lets a first article, an operator and an AOI recipe agree on what is on the board. Removing it to save a process step moves every check back to a drawing and slows the whole line.
3. Moisture and package cracking
Reflow heats the whole assembly at once. Absorbed moisture in plastic packages expands and cracks or delaminates them — the classic "popcorn" failure. Dry storage, floor-life tracking and baking before reflow are now part of the process, and they are dictated by the component's moisture-sensitivity level.
4. Feeders, reels and cut tape
Machine placement wants packaging it can index: reels, trays or sticks. Cut tape, loose parts and hand-sorted bags stop the line. Component packaging is a purchasing decision that has to be made at the same time as the BOM.
5. Quality gates moved earlier
On an automated line, defects are prevented at the printer and at the placement program, not found at the end. SPI after printing, a first article before the run continues, and an AOI recipe that is actually programmed to this board are the gates that replace the operator's eye.
Making the switch pay
The switch is worth it when volume, consistency or fine-pitch content exceeds what hand assembly can hold: 01005 and 0201 parts, 0.4 mm pitch packages, and any product where two boards must behave identically.
Four SMT lines place six million chips a day and three DIP lines handle 1.6 million through-hole pieces, with stencils cut in four hours, so a hand-built product can move onto the line without changing its design intent. Prototypes run in 1–3 days.
The assembly process is described on the PCBA manufacturing page; where the run is small, the prototype assembly page covers the fast path, and the quality gates note explains what the line checks.
