Soldering Anomaly Investigation on an LED Driver Board
An LED driver board can pass a visual inspection and then show a local temperature rise, an unstable output or a difference between boards from the same batch when the customer tests it. Replacing the power device is the common reaction and often the wrong one, because heat is a result rather than a cause. The paste, the pad, the component and the reflow history all take part in producing it, and the investigation is more productive when it works from the position and the batch.
One Board or a Fixed Position
If a single board is affected, the part number, the joints, the placement position and the surrounding circuit on that board are examined. If the whole batch shows the problem in the same area, the stencil, the paste data, the placement coordinates and the reflow conditions are examined, because a fault shared by a batch has a cause shared by a batch.
An LED driver board order should retain the board data, the bill of materials, the coordinates, any material change and the first article record. Without that correspondence it is difficult to establish later whether an anomaly is linked to a particular reel of material or to a particular programme adjustment.
When the fault is reported, the description is more useful if it states the conditions under which it appears, the test connections, the duration, the number of boards and where on the board the anomaly is. A message saying that the board gets hot leaves the factory with nothing to work from.
<img src="https://www.gopcba.com/wp-content/uploads/2026/08/HDI-PCB-design-2.jpg" alt="LED driver board under thermal imaging” />
A Large Pad Does Not Want More Paste
A power device with a heat spreading pad needs an appropriate volume of solder, but an opening that is too large lifts the body during reflow, distributes the solder unevenly, or allows the part to move. Too little solder leaves the connection under the body weak. The opening is therefore evaluated from the package and the pad rather than by scaling one rule to every power device.
SPI observes the area, volume, height and offset of the deposit after printing. If the anomaly always appears in the same region of the board, the support under the board and the condition of the stencil are examined, because adjusting the oven cannot correct a print that was wrong before the board arrived at it.
The Profile Is Right and the Board Is Still Uneven
A correctly set oven does not mean that every joint experiences the same temperature. An LED driver board carries copper area, a transformer, power devices and small passives, and their thermal masses differ, so the surface heats unevenly. A reflow profile measured with thermocouples on representative positions shows what each region actually sees.
If a large joint is under-wetted while small components show signs of overheating, the conveyor speed, the zone temperatures and the distribution of heat across the board are re-evaluated. Raising the temperature of the whole oven is the wrong answer, because it trades one problem for another elsewhere on the board.
Thermal Imaging Alongside Electrical Testing
Thermal imaging locates the region where the temperature rises, and it cannot by itself prove whether the cause is the joint, the component or the circuit. The image is therefore read together with a functional test carried out under the supply, the load and the acceptance conditions the customer specified, with the position of the anomaly recorded at the same time.
If the temperature returns to normal after a joint is reworked, the original joint and its print history are investigated further. If the problem moves when the device is replaced, or remains where it was, the parameters of the material and the surrounding circuit come into question. Neither outcome is conclusive on its own.
A power-up check is not substituted for the load and acceptance test the customer defines, and where a special safety or reliability requirement applies, the standard and the method are supplied by the customer so that the result means what it is supposed to mean.
A Batch Problem Is Not Solved Board by Board
When the same position fails repeatedly, the affected batch is stopped and the stencil, the paste records, the material, the placement programme and the reflow state are checked together. Touching up the boards one at a time deals with the boards that exist and does nothing about the next one.
After a repair, the board repeats the appearance check, the functional test and any test the customer specified. Boards that have passed, boards awaiting review and boards that have been repaired are kept in separate flows so that a board which has not been retested cannot be packed by mistake.
Our SMT assembly lines handle the power device placement and the paste control, PCBA testing runs the functional test under load, and the records are held under quality management.
Keeping the Record That Makes the Analysis Possible
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An investigation is only as good as the record behind it. The board revision, the programme revision where the board is programmed, the material lots consumed, the paste data from the print stage, the first article result and the reflow profile all belong to the batch and are kept with it. When a customer reports that some boards from a batch behave differently from others, that record is what allows the difference to be traced to a reel, to a printing drift or to a change in the oven rather than guessed at.
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The reflow profile is worth saving for the specific board rather than only for the product family, because the copper distribution of a driver board determines how the heat is shared. A profile carried over from a different board of the same family may be close enough to work and far enough to produce the marginal joints that only appear under full load.
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Working With the Customer on the Test Conditions
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Where a board is judged under load, the load itself has to be defined. Which output current, which duty cycle, which ambient temperature and how long the board is run before the measurement is taken all change the result, and two operators testing the same board with two different sets of conditions will reach two different conclusions while both are being careful.
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The position of the measurement matters as much as the conditions. A temperature taken on the surface of a device, on its tab and on the board beside it are three different numbers, and only a stated position can be compared with a number taken later. For a driver board, the current drawn at the same time is often more informative than the temperature alone, because it shows whether the device is conducting as it should.
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Where the acceptance condition requires a measurement the factory cannot make, the customer is told rather than the test being replaced by a simpler one that produces a result of the wrong kind. A power-up check presented as an acceptance test is worse than no test, because it creates a record that appears to mean something.
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Validating a Change on a Small Batch
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Once a corrective action has been identified, a small number of boards is used to verify it before the parameters are changed for the whole order. The paste condition, the joint appearance, the temperature distribution and the functional data are compared before and after, which shows whether the change produced a real improvement or whether the board happened to behave better on that occasion.
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Only when the comparison supports the change is it applied to the batch, and the change is recorded with the reason so that the next order for the same product starts from the corrected process rather than from the original one.
FAQ
Is a hot device evidence of a bad joint? No. The part number, the placement and the surrounding circuit have to be excluded first, and the thermal image is one input rather than the answer.
Why not raise the oven temperature? Because the board heats unevenly, and more heat may damage the components that are already at their limit while failing to correct the joint that is short of it.
What stops the fault returning? Identifying where in the process it originated, correcting that step, and verifying the correction on a small number of boards before the batch resumes.
