Choosing the Right Technology: Hot Bar Reflow Soldering vs ACF Bonding Part one of a three-part series on hot bar bonding INTRODUCTION As electronic assemblies continue to become increasingly smaller, lighter, and more complex, manufacturers are replacing traditional connectors – like ribbon cables – with thin, flat, fine-pitch flexible circuits. These flexible circuits are joined to other flexible circuits, rigid PCBs, glass displays, and sensors in products ranging from wearable electronics and medical devices to automotive displays and humanoid robotics. Drawing of wearable electronics with flexible circuit Historically, these kinds of connections would be made one-by-one using a soldering iron and wire solder to attach the wires to pads. It’s a highly operator-dependent process that has only moderately consistent results. To make multiple simultaneous connections quickly while improving repeatability and enabling automation, manufacturers are increasingly turning to thermode bonding technologies (see Table 1). Thermode bonding falls into two primary types: hot bar reflow soldering and anisotropic conductive film (ACF) bonding. The two processes share a common equipment platform but differ in how the electrical connection is created. Understanding the difference is essential when selecting the appropriate process for an application. Table 1 – hand soldering vs hot bar thermode soldering/bonding THE PROCESSES Hot Bar Reflow Soldering Hot bar reflow soldering is a process in which two pre-fluxed, solder coated parts are heated to a temperature sufficient to cause the solder to melt and flow, then cooled under pressure to form a permanent electro-mechanical bond. Because the process relies on localized heating, multiple fine-pitch solder joints can be formed simultaneously while minimizing thermal exposure to nearby components. This makes hot bar soldering well suited for compact electronic assemblies where conventional reflow methods may expose heat-sensitive components to unnecessary thermal stress. Reflow soldering flexible circuit to PCB ACF Bonding ACF bonding is similar to reflow soldering but replaces solder with an anisotropic conductive film containing conductive particles dispersed within a thermosetting adhesive. During bonding, heat and pressure cure the adhesive while compressing the conductive particles between opposing conductors. Electrical conduction occurs only through the thickness of the adhesive (the Z-axis), while adjacent conductors remain electrically insulated. This characteristic eliminates the possibility of solder bridging between closely spaced traces and enables extremely fine-pitch interconnections. Because the adhesive also provides mechanical attachment, ACF bonding is most widely used for attaching flexible circuits directly to glass substrates and displays. ACF bonding flexible circuit to glass What’s the difference? Hot bar reflow soldering and ACF bonding both work by applying heat and force to parts to create a mechanical and electrical connection. The difference is primarily in the parts being joined and the material used to create the bond (see Table 2). Table 2 – Hot bar reflow soldering vs ACF bonding A Common Equipment Platform Both processes utilize a pulsed heat power supply with closed-loop temperature feedback, a force-controlled pneumatic bonding head and an air-cooled thermode. These components are often integrated into single operator workstations complete with linear slides or rotary tables and customized tooling for part load and unload, or even into fully automated systems with conveyor motion and vision inspection. Typical desktop hot bar system DEVELOPING A SUCCESSFUL THERMODE BONDING PROCESS Successful bonding depends on balancing seven interrelated process variables: Temperature Time Force Contact (co-planarity) Alignment Materials Heat Sinks For both reflow soldering and ACF bonding, a typical heating profile looks like this: Typical hot bar heating profile This thermal profile optimizes the time and temperature relationship to create a reliable bond via flux activation, solder/adhesive wetting and cooling. Temperature The objective is to deliver enough thermal energy to achieve the process while minimizing thermal exposure to the assembly. For reflow soldering, the temperature should match the wetting temperature of the solder (183°-221° C). Similarly, for ACF bonding, the temperature needs to be sufficient to allow the conductive particles to create an electrical path and the adhesive to cure (160°C -220°C). Modern equipment like shown in Fig XX uses a thermocouple attached to the thermode to provide real-time feedback and maintain the programmed temperature. Thermode Time Time works in conjunction with temperature to deliver the energy needed for a reliable joint. Longer times can compensate for lower temperatures, but excessive heating increases the risk of component damage or material degradation. The goal is to use the shortest cycle that consistently produces optimal process results. Force Applied force ensures intimate contact between the thermode and the workpiece throughout the heating and cooling cycles. Too little force reduces heat transfer and solder wetting/particle crushing, while excessive force can deform components, squeeze solder from the joint, or damage delicate substrates. Contact The contact area between the thermode and the assembly directly affects heat transfer. Uniform contact (determined by the planarity of the thermode and part) distributes heat and pressure evenly across the bond line, while poor contact or inadequate planarity creates localized temperature differences that can result in incomplete solder reflow or inconsistent bond quality. Alignment Accurate alignment ensures that mating conductors remain properly positioned throughout the bonding cycle. Fine-pitch interconnections leave little margin for error, making fixture design and positional stability essential for achieving consistent electrical connections and preventing opens or shorts. Part design In addition to the process settings, it is critical to consider the part design and materials to achieve success. Materials Material properties, including substrate type, solder alloy, pad geometry and component construction, influence how heat flows into the joint. Different materials absorb and dissipate heat at different rates, requiring adjustments to the bonding profile to achieve repeatable results. Heat Sinks Heat sinks draw energy away from the bond area and are often overlooked and among the most significant factors affecting process development. Uneven heat sinking across the joint can create temperature variations between adjacent connections, often requiring changes to the thermal profile, dwell time, or tooling to achieve uniform solder reflow. Common design considerations can be found in our whitepaper: Part Design Guidelines for Reflow Soldering Flex to PCB. LOOKING AHEAD As electronics continue toward thinner substrates and increasingly compact wearable and robotic systems, thermode bonding technologies will continue to replace conventional connector-based assemblies in many applications. Whether the application requires hot bar reflow soldering or ACF bonding, successful implementation depends on understanding the interaction between materials, equipment, product design, and process control. Developing a robust process window early in product development remains one of the most effective ways to achieve reliable, repeatable interconnections in production. Want to know more? Check out the Hot Bar Reflow Soldering & Bonding Fundamentals Register for our on-demand webinar “Achieving Success with ACF Bonding” Category: Other Enjoyed this article? Share it!
