What potentials cause make HeatSeal Connector peel off?

LCD use HSC solution and missed segment

Recently, there have been numerous instances of segment defects appearing on the display of the HSC (Heat Seal Connector) solution LCM, as reported from the field. Our experience has shown that the reliability of HSC bonding is not satisfactory, particularly for industrial products. This is primarily due to the HSC’s tendency to lose its adhesive properties over time, ultimately resulting in a loss of contact between the PCB and HSC after extended use.

To help people understand the reasons behind the weak bonding force of HSC, I have gathered valuable insights from HSC experts and suppliers. This information will be beneficial for those interested in investigating the root causes and addressing such defects.

Here are the important points I received from the HSC supplier and expert for your information:

  • The HSC is sensitive to temperature and humidity.
    Storing HSC parts in high-temperature or high-humidity environments can degrade their adhesive proprieties.

  • The HSC supplier recommends a bonding width of 3.0mm to ensure that the bonding force meets requirement.
    Unfortunately, many new product designs only have a bonding width of 1.5~2.0mm, which can result in lower bonding force and reduced reliability.

  • Why is the HSC often found to peel off on the PCB side rather than the glass side?
    – One potential reason is that the PCB gold finger may not be smooth enough for the HSC to bond effectively. It’s possible that the gold finger pad is higher than the space between gold finger pads, causing a rippling effect. This could occur if the hot melt resin doesn’t flow evenly into crevices, especially on the peaks (the finger pads).
    – Adding solder mask between the gold finger pads can improve the flatness of PCB gold finger.

  • The HSC supplier doesn’t recommend applying continuously stress or bending force to the bonding area, except for soft material directly pressing on the bonding area. Continuous stress can lead to bonding damage over time.

  • Misalignment issues between the HSC and substrate may occur since most HSCs are bonded by semi-automatic machines and aligned manually using CCD. Misalignment typically does not affect bonding force but may impact conductive resistance. The HSC supplier suggests maintaining 20-30 graphite pixels per trace on the bonding trace to address this.

  • The adhesive from the HSC should transfer to and remain on the substrate when it is peeled off by force to ensure good bonding. During defect sample inspection or adhesive strength testing, check whether the yellow adhesive has transferred to the substrate.
    The adhesive of HSC shall be transferred to and keep on the substrate once it is peeled off by force for the good bonding on HSC.

By the way, it’s essential to control most of the critical parameters during the HSC bonding process manufacturing:

  • Measure the temperature profile using real-time thermal couple on the substrate.
  • Utilize a pressure meter to verify the bonding pressure force, rather than relying solely on checking the air cylinder.
  • Ensure that the thermal wedge evenly and firmly presses onto the substrate before the bonding process..
  • The first bonding sample should undergo an HSC peel-off adhesive strength test and meet specifications. Additionally, check that the adhesive remains on the substrate during HSC removal..

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