Hot Bar Reflow Soldering Guide

Hot bar soldering joins a flexible circuit, a flat cable or a fine pitch connector to a board by pressing a heated bar against the joint. The heat is conducted through the joint itself rather than applied to the whole assembly, so the process suits a connection that has to be made without reflowing everything around it. It is a standard method for attaching an fpc to a rigid board, and its quality depends on the tooling as much as on the heating.

How the Joint Is Made

The two surfaces are aligned, a solder deposit or a pre tinned pad is present on one or both, and the thermode is lowered onto the joint with a controlled force. The thermode heats the joint to the melting point, holds it for a defined time and then cools it, usually with an active cooler or an air jet, before the pressure is released.

The joint has to be held still during the whole cycle. A movement before the solder has solidified will produce a disturbed joint with a rough surface and a poor bond, and the movement is often caused by the tooling rather than by the operator.

The thermode is a replaceable tool with a defined tip geometry. The tip width sets the joint width, the flatness of the tip sets the pressure distribution and the condition of the tip determines whether the joint is heated evenly along its length.

The Pulsing Profile

The heater is controlled by a pulsing profile rather than by a fixed temperature. The profile has a preheat, a heating ramp, a dwell at the peak and a cooling stage, and the stages are timed rather than measured at the joint.

The dwell is what determines the joint temperature, and it has to be set with the thermal mass of the thermode and of the parts in mind. A short dwell gives a joint that has not fully melted, while a long one allows the heat to spread into the flex and into the board beneath, which risks delamination and damage.

The cooling stage is often underestimated. Forcing the joint to cool before the pressure is released gives a fine grain structure and a smooth surface, while releasing the pressure while the solder is still liquid produces a disturbed joint.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/pcb4.jpg" alt="Hot bar thermode bonding a flex circuit to a board” />

Tooling and Alignment

The board is held in a fixture with the flex positioned over it. The alignment is set by the fixture and by the fiducials on the flex, and the accuracy required is high because the pitch of a fine flex pattern is small.

The thermode must be parallel to the joint. A tilt of a fraction of a degree produces a pressure gradient along the joint, with one end starved of heat and the other over pressed, and the result is a joint that is good at one end and open at the other.

The part underneath the joint needs support. A flex that is bonded over an unsupported area of a thin board will deflect under the thermode force, which changes the contact and the heat transfer. A support block or a dedicated anvil is part of the tooling.

Materials and Solder Systems

The joint can be made with a solder paste, with a pre tinned surface or with an anisotropic conductive film. The choice is made from the pitch and from the requirement for electrical and mechanical connection.

Pasteless bonding uses the tin on the flex and on the board to form the joint, which removes the need for a paste deposit and its variability. It requires the two coatings to be sufficient and to be clean, and it produces a thinner joint than a paste process.

A hot bar process that uses an adhesive between the conductors provides a mechanical bond while the solder provides the electrical one. The adhesive has to cure without interfering with the solder joint, and the two processes have to be sequenced correctly.

Defects and Their Causes

An open joint appears when the heat, the pressure or the alignment is wrong. A joint that is open at one end of the thermode often follows a tip that is not flat, and one that is open in the middle follows a flex that is not held flat.

A solder bridge between adjacent conductors comes from too much solder, from a thermode that is too wide, or from a joint that was compressed before the solder solidified. The bridge may be invisible from above if the joint is under a cover layer.

Delamination and discolouration of the flex around the joint indicate that too much heat has been applied for too long. The flex substrate has a limited tolerance, and the damage is often seen as a lifting cover layer rather than as a visible burn.

Inspection

Visual inspection from the side shows the fillet at the edge of the joint, but the interior of the joint under a cover layer is not visible. The method is therefore limited to confirming the outer row.

X ray is the standard method for a fine pitch hot bar joint. It reveals the presence of the joint, the bridging and the void content, and it works through the flex. The interpretation requires experience because the image includes the copper pattern of the flex.

A pull test on a sample is used for qualification rather than for production. The value of the test is in the failure mode, which distinguishes a joint that failed in the solder from one that pulled the pad off the board.

Process Control and Records

The parameters of a hot bar process are the temperature profile, the force, the dwell, the cooling and the thermode condition. Each of them can be recorded and each of them changes with the tooling, so the record should be tied to a specific fixture and thermode.

The thermode should be checked for flatness and for wear on a schedule. A worn tip produces a joint that looks correct and fails in test, and the failure is usually attributed to the flex or to the paste until the tip is inspected.

The first article of each run should be inspected by X ray rather than visually, and the result recorded. A hot bar joint that passes a visual check and fails in a thermal cycle is a joint that was never properly formed.

Practical Rules

Control the force, the profile and the parallelism of the thermode, support the assembly behind the joint and cool before releasing the pressure. Check the tip on a schedule and inspect the first article by X ray.

Record the profile and the tooling data with the build records and the defect history, and review the flex processing rules and the flexible material data when a new flex design is introduced.

FAQ

Why is the cooling stage important? Releasing the pressure while the solder is still liquid produces a disturbed joint. Cooling under pressure gives a fine grain structure and a smooth surface.

How is a hot bar joint inspected? By X ray. A visual check shows only the outer fillet, and the joint under a flex cover layer is not visible from outside.

What causes a joint that is open at one end? A thermode tip that is not flat, which produces a pressure and heat gradient along the joint. The flex not being held flat gives the same result in the middle.