Wave Solder Thermal Profiling and Preheat Control
A wave solder process has a profile in the same sense that a reflow process has one, and it is measured less often. The board passes through flux, preheat, the wave and the cooling, and each of the four decides what the joint looks like.
What the Profile Consists Of
The flux application sets the activity that is available. The preheat sets the temperature the board arrives at, the time in the wave sets the heat input, and the cooling rate sets the microstructure.
A measurement is taken with thermocouples on the top side and on the bottom side, because the two differ by the thickness of the board. Our flux notes describe the application that precedes it.
Top Side Preheat
The top side temperature is the figure that matters, because it is where the flux has to remain active and where the components are. It is usually specified as a range rather than a value.
Too low and the solvent is still present when the board meets the wave, which produces spatter. Too high and the flux is consumed before the solder arrives. Our profile notes describe how the measurement is arranged.
Contact Time and Wave Height
The contact time is the length of the wave divided by the conveyor speed, and it is the heat input that decides whether the joint fills. The wave height sets how much of the board is in contact.
A wave that is too high floods the board; one that is too low misses the pads at the edge of the wave. Both appear as a defect that varies with the position on the board. Our hole copper notes describe the barrel that the solder has to fill.
Cooling
The cooling rate after the wave sets the grain structure of the joint. A slow cool produces a coarser structure and a different response to thermal cycling.
The cooling is often the least controlled part of the process, because it happens as the board leaves the machine. Where it matters, it should be measured rather than assumed. Our alloy notes describe the consequence.
Deformation From the Wave
The wave applies a thermal gradient across the board, which bends it. A board that is supported only by the conveyor can take a permanent set, and the joints at the trailing edge carry the load.
The support and the conveyor speed should be set together with the profile, and the board should be checked for flatness after the process. Our warpage notes describe the measurement.
Profile Records
The profile should be recorded with the board identity, the alloy, the flux and the settings, so that a change can be attributed.
Where the profile is not recorded, a defect investigation has to start from the settings and the settings may not be the profile. Our fabrication notes notes list the records that should be kept.
Verification
The verification is the measured top side temperature at the worst case position, the contact time and a joint that fills the barrel on a sample.
The worst case position on a wave process is usually the edge of the board that is closest to the start of the wave. Our joint criteria notes describe the fill that is required.
Process Control and Verification
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. 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.
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. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.
Verification and Records
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. Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.
Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.
FAQ
Is the bottom side temperature enough? It describes the wave and not the flux or the components. The top side measurement is what the process is judged on.
Does a faster conveyor reduce the heat damage? It reduces the contact time and it may not fill the barrel. The speed should be set from the fill rather than from the damage.
What does gopcb provide for wave profiling? We provide top side and bottom side measurement at the worst case positions, preheat, contact time and wave height development, cooling that is measured where it matters, support and conveyor set with the profile, and records that tie the profile to the board, the alloy and the flux.
