Surface-mount assembly places component terminations onto pads on the PCB surface. A typical solder-paste process then heats the assembly to form the joints. The outcome depends on the relationship among pad design, paste deposit, component placement and thermal processing; buying a pick-and-place operation alone does not define a complete assembly process.
Prepare data that agree with each other
The bill of materials identifies what to fit, the placement file identifies where to fit it, and the assembly drawing resolves orientation and special instructions. All three should describe the same revision and variant. Check reference designators and do-not-populate positions before releasing the package.
Verify unusual packages against the component manufacturer’s land pattern and handling guidance. Exposed pads, bottom-terminated packages and components with special moisture or temperature requirements deserve explicit review with the assembler.
Control the solder deposit
Stencil design or dispensing parameters establish the initial paste volume and location. Too little or too much material can create different defects, so inspect the deposit before assuming that a later reflow adjustment will solve the problem. Keep the paste identity and its handling instructions associated with the process.
For a new design, discuss stencil apertures and support under the board with the assembler. Features such as uneven support or difficult access can affect deposition even when the copper artwork is correct.
Review placement and thermal processing together
| Stage | Useful evidence |
|---|---|
| Placement | Correct part, orientation and location against the assembly data. |
| Reflow | A process profile appropriate to the actual assembly and materials. |
| Inspection | Defined checks for visible and hidden connections. |
| Electrical verification | A test method that addresses the product’s functional requirements. |
The oven setting is not the same thing as the temperature experienced at every joint. Use the assembler’s process-development method to establish a suitable profile for the populated board. Avoid copying a temperature sequence from an unrelated assembly.
Use defects to locate the weak process step
When a pilot build has a problem, retain the board identity, component lot and inspection evidence. Check whether the cause is design data, paste deposition, placement, material handling or thermal processing before changing several variables simultaneously.
Agree how rework is authorized and recorded, and include a post-rework check. A repaired board should remain traceable to the original issue and the action taken.
See the SMT assembly requirements and test and inspection guide when defining a production order. Package-specific data sheets and the selected solder-paste manufacturer’s process instructions govern the detailed process limits.
Move from reading to a build-ready project
Use the fabrication and assembly checklists to prepare drawings, production files and test requirements.
Open the DFM checklist