When to use this
Run this whenever the solder paste changes, the board design changes significantly, or the oven has been serviced. Also run it if joints start looking dull, grainy, or uneven.
A reflow profile is the temperature the board passes through over time. The oven heats the air. The board heats more slowly, and every board heats differently. So you measure the board.
Do not use this if you are running tin-lead paste. That melts near 183 °C and needs a completely different set of targets. Use the tin-lead procedure instead.
Before you start
- The paste datasheet is open. Its numbers beat every number on this page.
- At least three K-type thermocouples, plus high-temperature tape or solder to attach them.
- A scrap board that matches production, including the largest and smallest parts.
- A profiler that logs temperature against time.
- The oven has been running at temperature for 30 minutes and has settled.
The shape you are aiming for
| Phase | Target | Why |
|---|---|---|
| Preheat ramp | 1–3 °C per second | Faster boils the flux and throws solder balls |
| Soak | 150–200 °C for 60–120 s | Lets thick and thin parts even out |
| Time above liquidus | 45–90 s above 217 °C | Long enough to wet, short enough to spare parts |
| Peak | 235–245 °C | Comfortably above melt, below the part rating |
| Cool-down | Up to 4 °C per second | Fast gives shiny fine grain; too fast cracks parts |
Procedure
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Read the targets off the paste datasheet
Write down its ramp rate, soak window, peak, and time above liquidus. Every paste is slightly different. The datasheet wins any disagreement with the table above.
Verify: you have five numbers written down, not four.
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Attach thermocouples to the scrap board
Put one on the largest thermal mass, such as a big ground pad or a connector. Put one on the smallest part. Put one in open board area. Solder them down if you can; tape lifts and lies.
Verify: tug each lead gently. Nothing moves.
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Run a baseline pass and log it
Send the board through at the oven's current settings. Do not change anything yet. You need to know where you are starting from.
Verify: all three channels logged a full curve with no dropouts.
If it fails: a flat or spiking channel means a detached thermocouple. Reattach it and repeat this step.
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Fix the ramp first
Check the steepest part of the preheat. If it exceeds 3 °C per second, lower the early zone temperatures or slow the belt.
Verify: the fastest channel stays at or under 3 °C per second.
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Close the gap between channels during the soak
Compare the hottest and coldest channels at the end of the soak. If they differ by more than about 10 °C, lengthen the soak.
Verify: the spread across channels is under roughly 10 °C when the board leaves the soak.
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Set the peak against the weakest part on the board
Raise the peak until the coldest channel clears 235 °C. Then check the hottest channel against the lowest maximum temperature of any part on the board. Overshooting cooks components, and the damage is permanent and often invisible until the product fails in the field.
Verify: coldest channel above 235 °C, hottest channel below the lowest part rating.
If it fails: if you cannot fit between those two numbers, stop. The board needs a layout or component change, not a hotter oven. Escalate to the design owner.
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Trim the time above liquidus
Measure how long each channel stays above 217 °C. Aim for 45 to 90 seconds. Shorten by speeding the belt; lengthen by slowing it.
Verify: every channel sits inside the 45–90 s window.
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Check the cool-down
Measure the steepest downward slope. Keep it at or under 4 °C per second. Faster looks tempting but stresses ceramic parts.
Verify: the drop never exceeds 4 °C per second.
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Run it twice more, then lock it
Two consecutive passes must land inside every window. Save the recipe under a name that includes the paste and the board revision.
Verify: two back-to-back passes pass on all channels, and the recipe is saved under its full name.
If it goes wrong
| You see | It usually means | Do this |
|---|---|---|
| Solder balls scattered around pads | Preheat ramp too steep, flux spattered | Slow the ramp toward 1–2 °C per second |
| Dull, grainy joints | Cooled too slowly, or too long above liquidus | Speed the cool-down; trim time above liquidus |
| Small parts standing on end | One pad melted before the other | Lengthen the soak to even the board out |
| Voids under a ground pad | Not enough time for flux gas to escape | Push time above liquidus toward 90 s |
| Brown flux residue, discoloured board | Peak too high or reflow too long | Lower the peak; re-check against part ratings |
| Cracked ceramic capacitors | Cool-down too aggressive | Reduce the cooling slope below 4 °C per second |
Afterwards
- Save the profile log alongside the recipe name and today's date.
- Tell the line operators the recipe name has changed.
- Remove the thermocouples and scrap the test board. Never ship it.
- Note the paste lot number. The next profile starts from this record.