Troubleshooting: Key Checks Before Release
Troubleshooting a reflow defect is easy when the cause is obvious and slow when it is not. A structured sequence keeps the investigation from moving to the least likely cause first, which is usually where time is lost.
Start With the Defect Class
The class decides the likely area. A bridging defect points at the paste volume and the placement; an open points at the paste, the pad condition and the profile; a tombstone points at the imbalance between the two pads.
The class should be assigned consistently, because a defect recorded as “poor joint” cannot be analysed. Our defect notes describe the classes that are usually used.
Establish the Distribution
A defect that appears on every board in the same place has a different cause from one that appears on a few boards in different places. The first is a design or a program issue, the second is a process variation.
The distribution should be established before anything is changed, because the change destroys the evidence. Our yield notes describe how the map is produced.
Check the Inputs First
The paste batch, the stencil condition and the component batch are the items that change without a process change being made. They should be checked before the profile is adjusted.
Where a defect appeared on a particular day, the batch records are the fastest route to the cause. Our paste notes describe the records that exist.
Then the Process Settings
The print, the placement and the profile are the next group. Each should be compared with the recorded values from when the process was known good, rather than with the values that are assumed to be correct.
A setting that has drifted is a cause and it is also a symptom, so the reason for the drift should be established. Our <a href="https://www.gopcba.com/reflow-oven-profile-verification/” title=”profile”>profile notes describe the verification that catches the drift.
Then the Design
Where the process has not changed and the defect persists, the design is the remaining variable. A pad that is too small, an aperture with a poor area ratio or a thermal imbalance between two pads produces a defect rate that no setting removes.
That conclusion is worth reaching deliberately rather than by elimination, because it changes the action from a process adjustment to a design change. Our area ratio notes describe one of the checks.
Change One Thing
A change should be made one at a time and its effect measured on a defined sample. Changing the profile, the paste and the stencil together produces a result that cannot be attributed.
The measurement should be the defect rate for the specific class, not the overall yield, because the other classes add noise. Our sampling notes describe how the sample is defined.
Confirm and Record
Once the cause is established, the confirmation is a run at the restored setting with the defect absent, not a single board that looks right.
The finding should be recorded with the cause and the evidence, so that the same investigation is not repeated at the next occurrence. Our failure analysis notes describe how the record is structured.
Process Control and Verification
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.
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.
Verification and Records
A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.
FAQ
Why not start with the profile? It is the setting that is easiest to change and it is rarely the cause when the defect has a clear pattern. Starting there destroys the evidence and consumes time.
What if the defect rate is very low? A low rate needs a larger sample to measure. The investigation should establish the sample size before the change is made.
What does gopcb provide for defect troubleshooting? We provide a structured sequence from the defect class through the distribution, the inputs, the process settings and the design, one change at a time with a defined sample, confirmation on a run rather than a board, and a recorded cause that prevents the repeat.
