Reflow Profile: Reflow Profiling with a Thermal Coupon on the Real Board

A reflow profile is a claim about what the board experiences, and the claim is only true where it was measured. An oven display shows the air temperature in each zone, which is not the same as the temperature of a joint on a board that is absorbing heat and moving through the tunnel.

Profiling with a thermal coupon is the way that claim is tested. The measurement itself is simple, and the interpretation is where most of the value lies, because the numbers that matter are not the ones the oven reports.

What Has to Be Measured

The profile describes the temperature of the assembly over time. The parameters that matter are the ramp rates, the soak temperature and duration, the peak temperature, and the time above liquidus. Each of them affects the joint in a different way, and each has an acceptable range.

The measurement is made at the joint rather than on the surface of the component, because the thermal mass and the emissivity of the package change what the thermocouple sees. A reading taken on top of a body will differ from one taken under a lead, sometimes by more than ten degrees.

Attaching the Thermocouples

Thermocouple attachment is the most common source of bad data. The bead has to be in contact with the feature being measured, and the wire has to be fixed so that it does not pull the bead away or conduct heat into it from the air.

High temperature tape, a small amount of adhesive or a spot of solder are all used, and each has a failure mode. Tape that lifts leaves the bead in the air; a large blob of adhesive insulates the joint and slows the response; solder gives the best contact and is the most permanent. The attachment method belongs in the profile record, because a change in it changes the result.

Choosing the Test Board

The thermal coupon should be a real board from the production panel, loaded with the heaviest and the lightest components, rather than a blank laminate or a dedicated test piece. The temperature differences across a real assembly are the ones the process has to accommodate.

The coupon should also represent the most demanding case. Where a panel carries several boards, the position of the measured board on the panel matters, because the edges run hotter than the centre. The profile that is recorded should be the worst case, not the average.

Peak Temperature and Its Window

The peak temperature has to be high enough to wet the surfaces and low enough to avoid damaging the components and the board. The window is set by the alloy and by the most sensitive part on the assembly, and on a mixed assembly it is often narrow.

The important number is the peak at the joint, and the difference between the peak at different points on the same board. A profile whose peak varies by fifteen degrees across the assembly is unlikely to give the same joint everywhere, and the variation is what limits the process rather than the nominal value.

Time Above Liquidus

The time above liquidus determines how long the alloy is fluid and how much intermetallic growth occurs at the joint interface. A short time may not allow the solder to spread; a long time grows the intermetallic layer, which is brittle and reduces the fatigue life of the joint.

The range is usually quoted by the paste supplier as a minimum and a maximum, and the profile should sit near the middle of it rather than at the limit. Where the assembly is thermally heavy, the time measured at the joint may be much shorter than the time the solder on a small pad experiences, which is why several thermocouples are used.

Soak and Flux Activation

The soak brings the whole assembly to a uniform temperature before the alloy melts and activates the flux. It is the part of the profile that equalises the difference between a heavy connector and a small resistor, and it is the part that is most often shortened to increase throughput.

Shortening the soak has a cost that appears later. The volatiles are not fully driven off, the flux may not be activated, and the large parts may still be cold when the small ones melt. The result is a profile that works on the first article and produces voiding or poor wetting on a different panel from the same build.

Profile Verification and Revalidation

A profile is valid for the combination of board, paste, oven and conveyor speed that produced it. A change in any of them invalidates it, and the change does not have to be large: a different paste batch, a different panel loading pattern or a different ambient temperature in the shop all shift the result.

Revalidation is therefore triggered by change rather than by the calendar. The triggers should be listed with the profile record, and the revalidation itself is a repeat of the original measurement on the same coupon type, compared against the same limits.

Common Profiling Mistakes

The most common mistake is profiling an empty oven or a board with no components. The second is leaving the thermocouples attached with tape that fails part way through the tunnel, which produces a curve that drops part way through and is often mistaken for a cooling zone.

The third is accepting a profile that sits at the edge of its window because the oven cannot do better. That is a capabability question rather than a profiling one, and it should be answered before the product goes to volume, with the limits described in the process tolerances as the reference.

Records and Repeatability

The profile record should include the coupon layout, the attachment method, the oven settings and the measured curve, so that the next run can reproduce it. Without those details, a curve is a picture rather than a specification.

The records also support the comparison between machines. Where the same product is built on two lines, the two profiles should be compared rather than assumed to be equivalent, and the comparison is the same exercise as the one described for any manufacturing process that has to be transferred.

Process Control and Verification

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

How often should the profile be checked? After any change to the oven, the paste, the board or the loading pattern, and periodically to confirm that nothing has drifted. A monthly check is a common interval for a stable line.

How many thermocouples are needed? Enough to cover the heaviest and lightest parts and the position on the panel. Six is a common number for a mixed assembly.

Can the oven display be used as the profile? No. It shows the air temperature in the zone, which is not the temperature of the assembly at any point in its travel.