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Direct Bonding Applications on OLED and EPD

Direct Bonding Application Trends

Direct bonding has been applied for almost 2 decades on TFT displays for consumer electronic products, offering optical, mechanical, and electrical performance benefits. With the wide application of new display technologies like OLED(Organic Light Emitting Diode) and EPD(electronic Paper Display), direct bonding processing is brought with new challenges for the thin fragile glass and soft surface of new material.

 

Reliability of the Direct Bonding

OCA (Optically Clear Adhesive) is commonly used to bond the OLED or EPD displays. Reliability and durability of the bonding with OCA in elevated temperatures and humidity can have an impact on optical properties, so it’s often tested for reliability and durability at elevated temperatures and humidity (commonly 65°C/90% RH) during the processing. As some products requires more adhesive force, some manufacturers apply UV curing during the processing to reinforce the bond strength.  A common failure mode under this condition is yellowing, and too much UV exposure will also hurt the display screen, although the polarizer on OLED can reduce some UV transmission to the OLED panel, not to mention that EPD doesn’t have polarizer on the surface, so the UV blocking functionality on OCA and proper processing parameters during bonding are required to protect the display screen.

 

Acrylic Bonding

Some products require the cover to be acrylic material, or with additonal acrylic sheet to be bonded under the glass cover. This makes the bonding more complicated and difficult because of big expansion coefficient difference between glass and acrylic. The expansion coefficient could be 10 times different depending on the material of the glass. This will result in bonding lamination or conglomerate bubbles between glass surface and the acrylic sheet. Stronger cohesive strength and thicker OCA can reduce these failure modes, but still cannot completely avoid it from happening, specially when the bonded devices are put under high temperatures and humidity (Common Storage Reliability Testing Temperatures: OCA: 70°C/-20°C, OLED: 85°C/-40°C, EPD: 70°C/-25°C). Considering the materials and the layers of the bonded devices, people usually test them under (60-70)°C/-20°C for temperature cycles in the engineering phase and (60 -70)°C/90%RH for high temperature in the mass production phase.

Direct Bonding Applications in SWI

With the rich experience of bonding solutions since 2005, SWI is bonding the covers in different materials and shapes to displays including LCD, OLED and EPD, using OCA, SCA or OCR. (LCD: Liquid Crystal Display, OLED: Organic Light Emitting Diode Display, EPD: Electronic Paper Display, OCA: Optically Clear Adhesive, SCA: Solid Clear Adhesive, OCR: Optically Clear Resin, also called Liquid Optically Clear Adhesive).

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