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#1822, August 26th, 2018 (English Edition)

(Micro Electronics & Packaging)

 

Thinner & Lighter Printed Circuits

 Manufacturers from the electronics industry consistently compete to create higher density circuits. Cutting edge semiconductor manufacturers can generate less than ten nanometer traces on silicon chips.  Compared to IC chips, the fine pattern capabilities from printed circuit manufacturers are three orders lower, while flex circuit manufacturers are generating patterns finer than 15 micron on polyimide films using a reel-to-reel process.

Flexible circuits have some advantage when considering thickness and weight of circuit devices, and wafer manufacturers are ready to produce 20 micron thick silicon wafers (chip manufacturer are not ready for thin wafers). The industry standard in America and Europe for polyimide film thickness use to be 50 microns because of the reliable handling. However, Asian manufacturers are using 25 micron thick polyimide films because of the lower cost.  Over the last decade, the thickness of polyimide film got thinner and thinner during even though cost remained the same.  The reason for this is the end users are demanding thinner flex circuits that are earmarked for the tight spaces in smart phones and smart watches. Now, 12.5 micron thick polyimide film is the predominant size for the industry.

 Thickness of the copper foils has been getting thinner. One ounce copper foil (35 micron thick) is no longer as the standard thickness of the flex circuit industry. The majority of the industry is consuming 18 micron thick and 12 micron thick copper foils for the major laminates.

Consumer electronics continue to evolve, and circuit customers are requiring thinner circuits. Polyimide film manufacturers have commercialized 9 micron thick, 7.5 micron thick and 5 micron thick films. Copper foil suppliers have been also developing thinner foils down to 2 microns with carrier sheets.

DKN Research conducted several trials with ultra thin polyimide films.

We found no issues with micro hole generation with UV laser. Metallization by sputtering or elecro/electroless plating was successful in creating conductor layers thinner than 0.2 microns while maintaining bond strength. Standard laser scribing and photolithography are available to generate circuit patterns on thin polyimide film. Lastly, the total thickness of the flexible circuit was about 5.5 microns, and the unit weight of the thin circuits was less than 10 milligrams per square centimeter.

We learned how difficult it is to handle thin flexible circuits during our trials.  A couple things we learned with 7.5 microns and 5 microns thick films: the 7.5 micron thick films are manageable, but the 5 micron thick polyimide films require special tools and equipment.  You must also be aware of the static electricity from the plastic films during manufacturing and assembling. It is almost impossible to keep the thin film materials flat processing by traditional equipment, so the manufacturing yield and assembling yield are quite low.

 

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