PCB Inspection Before Assembly: What to Check
Most of the cost of a bad build is committed before the first panel is etched. Checking the data package, the placement and the routing before the order is placed costs an afternoon and prevents the failure modes that a re-spin cannot recover from in time.
Start With the Data Package
The first check is that the files the shop receives are the files the designer intended. Layer count, board outline, drill table, stackup, impedance requirements and the surface finish should all be stated, and the names of the layers should leave no ambiguity.
An experienced engineer still makes mistakes, so the review should be a formal step against a checklist rather than a glance at the plot. The prototype file checklist exists for that purpose and turns the review into a repeatable task.
Simulate Before the Build
A simulation of the critical nets before the panel is ordered costs nothing but time. Signal integrity, impedance and thermal behaviour can all be checked in software, and the problems found there are corrected in the data rather than in a physical board.
The simulation also forces the designer to state the assumptions. Once the stackup, the drive strength and the termination are written down, the result can be reviewed by someone else rather than accepted on trust.
Placement and Its Consequences
Placement decides the routing, the thermal behaviour and the assembly sequence. Heat generating parts should not be clustered together, and the parts that need to be reached during test should be kept accessible rather than buried under a module.
The placement review should also consider the assembly order. A component that must be fitted after another must not be surrounded by taller parts, and a connector that has to be inserted by hand needs clearance for the tool and the operator.
Routing and the Reference Plane
The routing review looks at the return path before it looks at the length. A plane that is continuous under a fast signal, a ground via beside every layer change, and a route that does not cross a split are the three items that catch most of the problems.
Where the design has been adjusted on the bench, the changes should be reflected in the artwork rather than in a note. A modification made with a knife is a signal that the design data no longer matches the working board.
Documentation on the Board
The silkscreen should carry the reference designators, the polarity marks, the first pin of every connector and the board revision. That information is what makes the assembly and the subsequent debugging possible.
Where the design carries a configuration that a technician has to set, the legend should say so. A board that requires a specific resistor to be fitted for a variant but does not show which one is a board that will be built wrongly at least once.
Safety Around Test Equipment
A board under test is connected to instruments that may be referenced to earth. If the board is also connected to a supply that is referenced differently, the test leads provide a path that can damage both the board and the instrument.
The rule is to understand the ground of every instrument before a probe is connected, and to use an isolation transformer where the reference is uncertain. The precaution costs a few minutes and prevents a failure that no schematic can explain afterwards.
The Soldering Iron and the Board
A soldering iron with a leaking element can damage a device before the joint is made. The tip should be earthed, the insulation checked, and a low voltage iron used where a mains powered one is not required.
The same care applies to the temperature and the dwell time. A joint that takes several seconds to form is a sign that the tip is too small or the pad is connected to too much copper, and the answer is a larger tip rather than a longer dwell.
Electrical Checks Before Power
Before the supply is connected, the resistance between the rails and ground should be measured, and the input impedance checked against the expectation. A short that measures a few ohms is easy to find; the same short found with the supply connected destroys components.
The first power up should be current limited. A supply with a current limit turns a catastrophic failure into a warm component, and the measurement of the current drawn at a low voltage often identifies the fault before anything has time to heat.
Quantity and Cost Planning
The number of boards ordered should follow from the purpose of the build. A board for a functional check needs one or two; a board for an environmental test needs enough for every condition, plus spares for the units that fail for unrelated reasons.
Ordering too few is the more common error, because a board that is damaged during assembly cannot be replaced in time. The cost of an extra panel is small compared with a slipped schedule.
Reviewing the Production Line
The checks before the order are matched by the checks before the run. The factory production checklist covers the stencil, the paste, the profile and the first article, and the simulation results from the design stage become the reference for the first article inspection.
Together the two sets of checks form a loop. The data is verified before the board exists, the board is verified before the batch is released, and the layout simulation provides the numbers that both steps are compared against.
Process Control and Verification
On a design of this kind, inspection is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.
The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
Process Control and Verification
On a design of this kind, inspection is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.
FAQ
How many boards should be ordered for a first build? Enough for every test planned plus spares for units lost during assembly. A single board rarely survives the whole programme.
Is simulation necessary for a simple board? Not always. It becomes valuable as soon as the rise times, the current or the thermal load make the behaviour of the layout hard to predict.
What is the most common omission in a data package? The surface finish and the impedance requirements. Both change the cost, and neither can be inferred from the copper layers alone.
