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Reliable Joining with Hot Bar Reflow Soldering

Thermodes for Desktop Hot Bar Bonding Systems

You’ve heard us say it before, and hot bar reflow soldering is no exception: success is found at the intersection of material selection, an optimized process and proper equipment. Today, we’re going to talk about optimizing your hot bar reflow soldering process.


Let’s start with the basics. Soldering is a manufacturing process that uses a filler material called solder to join different types of metals in order to create an electro-mechanical joint. Traditionally, it’s a single point process achieved using a hand-held soldering iron, but as parts/terminals continue to get smaller and importance of consistency increases, the demand for a controlled process also increases. Enter hot bar reflow soldering. Hot bar reflow soldering (emphasis on the “hot bar” part) enables multiple connections simultaneously by applying heat and force to pre-fluxed and pre-tinned parts using a thermode (hot bar). The electrical current required to heat the thermode is supplied by a pulsed heat power supply. Both the heat and the pressure are programmable, providing the desired control. When the thermode reaches sufficient temperature, the tin (solder) melts and flows and is then cooled down to solidify – while still under pressure – forming an entire array of joints. You can see how this would be useful.


So what is the real-world application of hot bar reflow soldering? It’s used just about anywhere electronics are employed using a flexible circuit; notably medical and commercial wearable devices that monitor heart rate, blood oxygen levels, respiration, movement patterns and sleep and more. In the automotive industry, it can be used in the assembly of advanced driver assist systems (ADAS) and 5G connected devices including cameras, sensors, displays, USB ports, electrical connections and much, much more!


Just a few of the many reasons to employ a hot bar process:

  • You need to achieve higher throughput
  • You need to make multiple connections simultaneously
  • You need a more reliable and repeatable process
  • You are having trouble accurately positioning parts with a manual production process
  • You want to shave a little cost of the process


Mentioned earlier, solder is a filler material used in traditional soldering which creates the electromechanical joint between two metal parts. Flux is a catalyst; the chemical agent which removes and prevents oxide formation during the soldering process and also improves the wetting action of solder. Both are critical components in hot bar reflow soldering.

Selecting the right solder and flux for your application requires knowledge not only of their melting and activation temperatures but also “soak time.” This information is typically included in the material data sheet provided by the supplier. To comply with ISO manufacturing safety standards, lead free solder and no-clean flux are commonly used in the industry today. The melting temperature of most lead free solders ranges from 220 – 230°C and no-clean flux activates between 80 – 100°C.

Solder deposition – the amount of solder applied to the PCB – is a critical part of the process. Too much solder and it will overflow and “bridge” to adjacent bonding pads, causing shorts. Similarly, insufficient solder may result to weak or poor connections which may generate high resistance or even arcing at the joint area leading to premature electrical connection failure.


When designing parts for hot bar reflow soldering and bonding, consider both heat migration and heat sinking and use thermal dams to isolate the bonding pad tracks from the main circuitry. Be sure to incorporate even spacing and uniform pad dimensions in the design.

Fig 1: PCB Circuits Design

Proper dimensional relationships between the thermode face and the PCB and flex tracks design and through-hole alignment features will

  • Make part loading easier
  • Prevent misalignment
  • Prevent solder bridging
  • Promote efficient heat transfer

All resulting in a more reliable hot bar reflow process. Note that the thermode face must overhang by at least 1 mm on each side of the PCB tracks and should cover 50-60% of the track length.

Fig. 2 Thermode Face, PCB and Flex Pad Design


All hot bar reflow soldering processes require:

  • A pulse heated power supply to heat and cool the thermode. The power supply should be capable of preheating in order to activate the flux and remove oxides, solder wetting to reflow the solder, and cooling to allow the solder to solidify without bridging to the other joints.
  • A bonding head capable of delivering sufficient bonding force.
  • A thermode. The thermode transforms energy from the power supply into heat and must be not only the correct size for the parts to be bonded but also maintain co-planarity to ensure even bonding results. Even heat distribution is critical.


Here’s a quick overview the hot bar reflow soldering process:

  • Load the PCB to the part fixture. Tip: consider including alignment features in the part and fixture design to simplify this process.
  • Apply flux to the PCB’s bonding pads
  • Load the flex, making sure it is properly aligned and secure. Use a high magnification microscope or camera to verify alignment.
  • Activate the bond head with the properly sized thermode face for the application. Make sure the thermode face is co-planar with the part to ensure even heat transmission. A pressure indicating film can be used during setup to verify planarity.
  • Be sure to use sufficient bonding force, typically this is within 0.5 – 1.5 N/mm2 (72 – 217 lb/in2) of the thermode face area in contact with the flex.
  • As soon as the bonding force is achieved, heat can be applied. The solder will melt, reflow and re-solidify before the bond head retracts.
Things to Look At: Alignment, Temperature, Force, Heat Sinking, Heat Barriers


How do you know if your hot bar reflow soldering process is successful?

Visual indicators

    • Look for shiny solder fillets
    • No solder bridging
    • An even thermode witness mark
    • No scorching

Destructive testing

    • Peel the flex away from the PCB. Take note of any voids, uneven force imprinting or cold solder joints
    • Perform a DOE to establish pull force process control specifications. Solder joint should remain intact

Non-destructive testing

    • Utilize an x-ray vision system to verify presence or absence of voids, uneven force imprinting or cold solder joints.

Once the process is dialed in, we recommend the use a hot bar process monitor to ensure it remains in control.

For more detailed information, watch our recent webinar Achieving Success with Hot Bar Reflow Soldering and contact our applications lab for free feasibility testing; you’ll find an evaluation request form here.

Also learn about the benefits of interposer tape and the hot bar reflow soldering process.