Hot Bar Soldering / ACF Equipment for Flex Circuits

Hot Bar Soldering / ACF Equipment is used in making flex circuits. These circuits are used in several types of products, such as car assemblies, cell phones, digital cameras, flat-screen TVs and printers. The current market for flex circuits is more than $63 billion. It has grown annually at a rate of 13.5%.

The enhancement in technologies allows manufacturers to produce slimmer and lighter products that are in high demand. Due to the changing tastes of buyers, manufacturers look for ideal solutions to their manufacturing needs. ACF bonding allows them to produce slicker and sleeker models. The film has small, spherical particles that work like conductors and allows for easy circuit buildings. This type of bonding is commonly used in bonding flexible and rigid circuit boards, glass panel displays and flex foils. ACF Equipment allows one to use foil, flex or paste for binding purposes.

Hot-bar soldering also known as pulsed-heat bonding is used to attach flex circuits to printed circuit boards (PCBs), glass and ceramics. It is used to bond battery packs, surface-mount edge connectors, small coaxial cables to PCBs and ribbon cables. All parts that are sensitive to heat are joined using this type of soldering. Hot-bar soldering is also used to bond leaded components. The process itself is semi-automatic and the equipment looks like a bench top resistance welder.

A titanium thermode is used for bonding two surfaces. It is mounted to a pneumatic cylinder and works using an electric current. The control system of the hot bar soldering machine has a microprocessor. This allows the machine operators to pre-program all their settings and create a soldering profile. This profile can be used repeatedly to get the same identical results. Operators can program the amount of pressure to be used during the welding process, the temperature to be used, the amount of heating time, cool down time and the reflow time.

In a typical welding scenario, the machine operator will first load the PCB into the fixture and then apply the flux to it. The flex is then loaded into the fixture and slid into the work area and then the machine is turned on. The thermode that is used for the welding process is then pushed by the cylinder against the parts to be joined with a desirable force. This pressure is required throughout the heating and cooling cycles and also the reflow. Commonly used pressure is between the ranges of .25 to .50 lbs. When the PCB is flat this degree of force is ample but when it is not flat more force needs to be used to join surfaces.

Once a thermode wears out you can always take it out and load another one. Custom made thermodes are also available online. A typical thermode is not more than 4” long and less than 0.08 inch wide. Long thermodes work better on pads but there is no need to match the width of the pads with that of the thermode. It can be three times smaller in width. If the bonds are not coming out well, try changing the width of the thermode. Getting professional advice from an experienced hot bar soldering contractor is also useful.

After the right hot-bar is loaded, you can start the bonding process. The thermode will make contact with the parts to be joined and will heat up. It takes a hot-bar at least 2 seconds to heat up for the reflow. You can set different temperatures for various solder alloys and parts. For soldering unexposed leads of a flex, a higher temperature is used, and the heat is then transferred via the Kapton. This can take longer than 3 seconds to complete the solder. When the solder reflows, the current to the thermode will stop, allowing it to cool down. It can take 2-3 seconds for the cooling down process and to speed it up some blowing air helps a lot. After the cooling process, the solder is solid, and this is when the cylinder lifts the thermode while freeing up the parts.


Good operators keep an eye on three parameters during the soldering process, temperature, time and force. Ample heating time is required to reflow the solder. Applying unnecessary pressure may push the solder out of the joint.


A vital part of the hot-bar soldering equipment is the fixture that holds the two parts during the joining process. It aligns both parts, so the joints come out accurate. It has an aluminum base and heat-resistant thermoplastic nest that is static-free. Some machines also have a vacuum system that pulls and keeps the flex down and still on the substrate. The use of video cameras also helps with alignment of two parts. Operators are able to fine-tune the position of the two parts easily.

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