SPI Data and Print Process Feedback
An inspection system that measures the paste deposit is only useful if the measurement changes something upstream. The volume figures it produces describe the printer, the stencil and the paste, and the value of the data comes from how quickly that description reaches the person who can act on it.
What the Measurement Gives You
The system reports a volume, an area and a height for each deposit, and compares them with a target and a tolerance. The volume is the figure that predicts the joint, and the area and the height explain how the volume was produced.
A deposit that is short in volume and low in height is a stencil or a paste problem. One that is correct in height and short in area is a print that did not fill the aperture. Our paste volume notes describe the printing side of that diagnosis.
Targets and Tolerances
The target is the volume the joint needs, and the tolerance is the range that still produces an acceptable joint. A tolerance that is copied from a machine default rather than derived from the joint is either too wide to catch a problem or too tight to run.
Deriving the tolerance requires a correlation between the volume and the joint result, which comes from inspecting the joints that were made at the limits. Our SPI notes describe how that correlation is normally established.
Trends Rather Than Single Readings
A single board that is inside tolerance is not a signal. The trend over an hour is, because it shows the direction the printer is moving in before the deposit leaves the band.
That is the difference between using the system as a gate and using it as a process control. A gate accepts or rejects; a trend tells the operator to clean the stencil or to add paste before the next board is printed.
Feedback to the Printer
The feedback loop runs from the measurement to the printer settings. Print speed, squeegee pressure, separation speed and stencil cleaning frequency are the four that move the volume, and the order in which they are adjusted matters.
Changing several at once makes the trend unreadable, so the useful discipline is one change at a time followed by a measurement. Our stencil cleaning notes describe the maintenance item that is most often the cause of a slow drift.
Correlation With the Joint
The volume is a prediction of the joint rather than a joint. Where a deposit is inside tolerance and the joint is not, the cause is downstream: the reflow profile, the pad finish or the component termination.
That is why the correlation is established once and reviewed after any change to the paste or the profile. A correlation that was established with one paste is not valid for another.
Operator Response and Alarms
The system raises an alarm when a board is outside tolerance, and what the operator does next decides whether the alarm was worth raising. A defined response, written down, produces a consistent action; an alarm without a response produces a habit of over-riding it.
Defining the response is the step that is most often skipped when a system is installed. Our first pass yield notes describe how the result of the response is measured.
Data Retention and Use
The data is worth keeping against the board identifier, so that a failure found later can be compared with the deposit that produced it. Keeping it without the identifier produces a file that cannot be joined to anything.
Retaining it for the life of the product allows a batch to be compared with the batch that preceded it, which is what turns a measurement into a process history.
Process Control and Verification
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.
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.
A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
Checks Before Release
The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.
A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.
Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.
FAQ
Can SPI replace visual inspection of the paste? It replaces the routine part and it cannot judge a deposit that is displaced rather than short of volume.
Does every deposit need to be measured? On a critical assembly yes, and on a general one sampling is used and the trend carries the information.
What does gopcb provide for print measurement? We provide targets and tolerances derived from the joint rather than from defaults, correlation of volume with joint result, trend review with one change at a time, a written operator response to an alarm, data retained against the board identifier, and records per batch.
