Solder resists for flexible wiring boards “SN Series”

Large TVs and PC LCDs use COF (chip on film) with driver ICs packaged on wiring films, and solder resists are coated to protect the insulation. Recently, higher resolution and bezel-less displays have boosted the demand for solder resists with high insulation reliability responding to the fine pitch and folding of COF.

Conventional solder resists (“competing product” in the charts below) pose such challenges as low resistance to obstructive factors and high leakage currents under conditions involving cleaners and salt water.

Resonac recommends its solder resist SN series for COF as a solution to these challenges. Using resins with both insulation and flexible properties, our product offers excellent insulation reliability and resistance to cleaners and salt water.

COF and insulation protection resins used for large TVs

Innovative Solution

High reliability achieved with polyimide resins containing both soft and hard segments

Insulation protection films for COF require reliability with consideration given to factors ranging from yields in the manufacturing and transportation processes to the end users' operating environments.

For example, when an end user sprays and wipes a screen, solder resists with low cleaner resistance may cause defects due to the cleaning agent soaking through the edges. As another example, solder resists with low salt water resistance may fail to protect displays used in coastal areas. A high leakage current may also generate heat that can cause short circuits or even fire.

To overcome these challenges, Showa Denko Materials' solder resist SN series for COF adopts the structure of its original, modified synthetic polyimide resins that combine insulation and flexibility by incorporating soft segments into heat-resistant hard segments, while utilizing compounding technologies for ion trap and other materials. These approaches enable the SN series to achieve excellent insulation reliability and resistance to cleaners and salt water, as well as low leakage currents.

Product Features

Excellent resistance to cleaners and salt water

We compared the insulation reliability of our latest SN series grade SN-10000 with its competing product by directly exposing wiring substrates with solder resist-coated L/S=10/10μm pitches to glass cleaners or a 5 percent NaCl solution. The results are shown in the charts below.

Comparison of resistance to cleaners and salt water

Low leakage currents

We compared the leakage currents of SN-10000 with its competing product when applying a voltage of 20V to test pieces with solder resists coated and cured on flexible wiring substrates and the pieces heated at 120 degrees Celsius. The results are shown in the chart below.

Comparison of leakage currents

Excellent insulation reliability

SN-10000 achieves excellent reliability under higher temperature and humidity conditions, as shown in the charts below.

Comparison of insulation reliability under severe conditions (N=3)

Conditions: Substrate: 20μm pitch (L/S=10/10, Upilex), Chamber: EHS-412M (ESPEC corporation), Standard condition: 130℃/85%RH, High humidity condition: 130℃/90〜95%RH

For other detailed reliability data, please refer to our technical materials.

• ※The data shown above are representative figures. They are not guaranteed values.