Hot Bar Soldering and Thermode Bonding
What Hot Bar Soldering Is
Hot bar soldering uses a heated bar, called a thermode, pressed onto a row of joints to reflow them all at once. It is used where the joints cannot be reached by reflow, such as a flex cable bonded to a board, a fine pitch connector, a display’s tail or a shielded component. The thermode conducts heat into the joint through the parts, so the process is a localised reflow with a controlled force, and its quality depends on the thermode’s geometry, the force and the pulse profile as much as on the solder.
How the Heat Reaches the Joint
The thermode is heated by a pulse of current and cooled by the same mechanism, and the temperature is measured by a thermocouple or inferred from the pulse’s parameters. The heat has to travel through the flex or the component to the solder, so the materials between the thermode and the joint determine the profile: a thick polyimide or a large copper plane delays the heat and requires a longer pulse or a higher set point. The thermode’s surface has to contact the joint area evenly, which means the parts have to be flat and the force has to be distributed. A thermode that touches one end before the other creates a temperature gradient along the row and produces open joints at one end.
Tooling and Alignment
The tooling holds the parts during the bond and defines the alignment. The board and the flex each need support, and the alignment has to be repeatable to the pitch of the connection, which for a fine pitch flex is a fraction of a millimetre. The tooling should also control the force, because the process needs pressure to ensure contact and too much pressure deforms the parts or squeezes the solder out. Where the parts have a fiducial, the alignment can be done optically; where they do not, a mechanical stop is used and its wear has to be monitored. The tooling is the difference between a process that works and one that depends on the operator.
The Profile
The profile has a ramp, a dwell at the bonding temperature and a cooling phase. The ramp is limited by the parts’ tolerance for heat and by the thermode’s thermal mass. The dwell has to be long enough for the solder to melt and coalesce, and short enough to avoid damaging the flex or the component. The cooling is controlled by the thermode’s own cooling or by a gas jet, and the joint must not be disturbed until it has solidified. Because the heat reaches the joint by conduction through the parts, the set point on the equipment is not the joint’s temperature, and the profile should be verified with a thermocouple or a trial rather than taken from the display.
Materials and the Joint
The joint itself is usually made with solder that is already present, as a coating on the flex’s pads, a preform or a paste. The solder’s alloy determines the temperature needed, and a lower temperature alloy reduces the thermal load on the parts. Where the flex has been coated with a finish, its condition and its thickness affect the bonding, and a flex that has oxidised will not wet. The joint should be inspected after bonding, and because the joint is under the flex, the inspection is usually optical from the side, a visual check of the fillet where it is visible, or an electrical test. A process that cannot be inspected has to be controlled by its parameters.
Defects and Their Causes
The common defects are an open joint, which usually follows an uneven thermode contact or an inadequate dwell, and a bridged pair, which follows too much solder or too much pressure. A joint with a cold appearance follows a dwell that was too short or a set point that was too low for the thermal path. A damaged flex or component follows a set point or a dwell that was too high, and the damage may be a delamination or a burnt coverlay. A misalignment follows the tooling rather than the profile. Reading the defect against the cause shortens the setup and reduces the scrap during a product change.
Where It Fits
Hot bar soldering is used where a connector would take too much space, where the pitch is too fine for a conventional process, or where the assembly cannot be reflowed again. It replaces a connector and its mating cycle with a permanent joint, which improves the reliability and reduces the cost, at the price of a process that is harder to rework. Where the product allows a connector, the connector is often the better choice; where the space or the reliability demands a direct bond, the hot bar process is the practical answer. The design should provide the alignment features and the support that the process needs, since a bond that is hard to align is a bond that will be unreliable.
FAQ
What is hot bar soldering for? Making a row of joints at once where reflow cannot reach, such as bonding a flex to a board.
Why is the set point not the joint temperature? The heat travels through the parts by conduction, so the joint reaches a lower temperature than the thermode.
What causes an open joint at one end? An uneven thermode contact or a temperature gradient along the row.
How is the joint inspected? Optically from the side or by an electrical test, since it is hidden under the flex.
What does the design have to provide? Alignment features, support for both parts and a flat area for the thermode to contact.
Conclusion
Hot bar soldering is a localised reflow whose result depends on the thermode, the tooling and the profile, so control all three and verify with a trial. Give the design the alignment it needs. Fine pitch bonding sits within SMT PCB assembly, the assembly it becomes part of belongs to PCB assembly, and the verification is part of PCBA testing. Hot bar bonds are first made during prototype PCB assembly in 2026.
