First Pass Yield, Defect Pareto and Rework Cost

A yield figure on its own tells you that something is wrong and nothing else. The first pass yield, broken down by defect type and by location on the board, is what turns a number into an action.

Which Yield to Measure

The first pass yield counts the boards that pass every test without rework, retest or touch-up. It is the only figure that reflects the process rather than the repair capacity.

A final yield that includes rework can be high while the first pass yield is low, and the difference is the hidden cost of the line. Our quality notes describe how the acceptance is defined.

Defect Classification

Defects must be classified consistently, or the Pareto chart changes shape with the inspector rather than with the process. The classes should be few and mutually exclusive.

A defect that belongs to two classes will be counted in whichever one the inspector prefers, and the resulting trend is noise. Our defect notes describe the categories that are usually used.

Location Analysis

The same defect count means different things in different places. A bridging defect spread across the board is a paste problem; the same count concentrated on one component is a placement or a stencil problem for that aperture.

The analysis should therefore map the defects onto the board rather than only count them, and the map should be produced from the inspection data rather than by hand.

The Pareto and What Follows

The Pareto chart ranks the defect classes by frequency. The largest class is not always the one to address first, because the cost of a defect depends on where it is found.

A defect found at the end of the line costs more than one found after printing, and a defect found by the customer costs more than either. The ranking should use the cost at the point of detection.

Detection Point and Cost

Moving detection earlier reduces the cost of each defect and it also reduces the information available, because a defect caught after printing has not yet been through reflow.

The trade should be made deliberately, using the yield data to decide where a check pays for itself. Our paste inspection notes describe a check that is often justified this way.

Rework and Its Cost

The rework rate and the touch-up rate belong in the same report as the yield, because they are the cost that the yield figure hides. A high rework rate also raises the risk of latent damage.

Where rework is concentrated on one component, the design or the process should be changed rather than the repair made more efficient. Our repair strategy notes describe the trade.

Acting on the Data

Each improvement should be recorded with the defect class it was intended to reduce, and the yield should be compared before and after. A change that does not move the figure is a change that should be reconsidered.

Where the yield is stable and the target is unmet, the constraint is usually the design rather than the process, and the answer is a design change. Our manufacturability review notes describe where that decision belongs.

Process Control and Verification

On a design of this kind, inspection is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. 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.

Process Control and Verification

On a design of this kind, inspection is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Process Control and Verification

On a design of this kind, inspection is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Process Control and Verification

On a design of this kind, inspection is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Is a high yield always good? Not if it is achieved by rework and retest. The first pass yield is the figure that reflects the process.

How often should the data be reviewed? Often enough that a trend is visible before a batch is lost, and always after a process or a design change.

What does gopcb provide for yield management? We provide first pass yield reporting separate from final yield, consistent defect classification, defect mapping by position and component, Pareto ranking by cost at the point of detection, rework and touch-up reporting, and recorded improvements with the before and after figures.