Flatness comes before the pulse
Hot-bar soldering works by pressing a heated face across several joints at once and holding a controlled pulse. If the face is not flat, pressure is not uniform, and the joint under the hollow sees less contact and less heat. No amount of pulse tuning repairs a mechanical gap: raising the setpoint to rescue the cold bond will overheat the bonds that were already in contact.
This is why we lap thermode working faces after grinding and check them against a reference plate before a thermode leaves the plant. For V, U and grooved profiles the same rule applies along the whole contact line, not just at the centre.
Three variables, in order
| Order | Variable | What to verify |
|---|---|---|
| 1 | Contact & flatness | Face flatness, alignment to the pad row, pressure distribution |
| 2 | Pressure | Repeatable force, even across the full face length |
| 3 | Pulse profile | Ramp, dwell and cool-down for the actual joint mass |
When a bond fails, engineers often start at variable three because it is the easiest to edit in the controller. In our experience the first two account for most of the variation between the first and the last joint in a row.
Checking the face, with one caveat
Three checks are cheap enough to do before every production run: a reference plate with a thin contact indicator, a pressure-sensitive film across the pad row, and a test bond on a scrap circuit that is then peeled and inspected. They show contact, force and result respectively.
Infrared thermography is useful for looking at temperature spread, but a polished metal face has low and uneven emissivity. Without an emissivity correction or a matte coating, an IR image of a bare thermode can show a gradient that is not there. We treat it as a comparative tool, not an absolute temperature measurement.
Where it applies
We grind V, U and grooved thermodes for enamelled wire from 0.05 to 0.4 mm, and for fine-pitch flexible circuits in the 0.3–0.5 mm range. On those joints the enamel is stripped and the bond formed in one step, so there is no pre-tinning station to compensate for a bad contact.
If your joint is a nickel tab or a copper busbar, the constraint changes again: stored heat and pulse energy dominate, and a wide blade or slotted face is usually the right starting point. The application table lists the six cases we tool for most often.
