One billion national ID cards in China, bank cards, LED flashlights, contactless smartcards and labels, RFID transponders, or identification cards for access control to high security areas all have one thing in common - they contain flexible substrates as carriers for electronic components such as mini chips. Flexible substrates can be made of stamped, plated and, in many cases, laminated thin tapes that customers can use for a variety of purposes. Flexible substrate technology permits compact design, complex arrangements of fine structures and optimum heat management. The components are mass produced, low cost products. However, specialists are necessary for the cost-effective and customer oriented production of these small but complex components.
Heraeus is a market leader for these flexible substrates in the field of RFID and has more than 15 years experience in this technology for mass production of standard 35 mm tapes for a growing number of applications, for example in documents such as ID cards which may include features such as biometric data or fingerprint information. Flexible substrates are in demand as adhesive and stabilising material in all areas where tiny chips need a carrier. Due to its freedom of design and its high accuracy without entailing high costs, the stamping and laminating technology has great potential for meeting the future demands of progressive technologies.
Each application presents a new challenge
Despite mass production - special stamping machines typically process up to 350 pieces per minute - flexible substrates are not an off-the-shelf product. For each application and requirement they are individually conceived and newly developed. This means that there are many challenges which have to be met, such as different structures with various surfaces including three-dimensional textures, several metallurgical and electrical features of the laminated alloy layers and also the progressive miniaturisation of interconnect technologies.
Using materials with varying thermal expansion coefficients (eg, metal and plastic coatings), which must be pressed with high precision under pressure, presents a particular challenge. The super-thin and flexible substrates consist of a data link and an insulation layer. For the construction of flexible substrates different kinds of base materials can be used, such as Cu and Cu alloys, Fe and Fe alloys or stainless steel with material thicknesses from 100 μm (or more) down to 40 μm (depending on the base material) and also various types of insulation layer, made of glass fibre reinforced epoxy, acrylate, PEN, Pet or paper with material thicknesses from 100 μm (or more) down to 25 μm. Heraeus has developed and established several reel-to-reel manufacturing technologies, such as high precision stamping, high precision laminating, electroplating and also etching technologies for samples, qualification and prototypes. Another standard product is flip chip carriers, in which a chip is mounted directly to the substrate (active bonding side down) without additional leads.
For most applications, extremely accurate stamping tools must be developed. This usually takes place in close cooperation and agreement with the customer. For the manufacture of prototypes and qualification samples there is the possibility of acquiring economical etching parts before investing in a stamping tool. These have the same properties and design as the stamped, mass produced part. For instance, this also applies to the substrates which are needed for the production of new, uniform identity cards for the population of China.
LED flashlights a new field of application
A typical new field of application is light emitting diodes or LEDs, which are being used more and more in everyday lighting applications. A well-known LED manufacturer launched a high-power LED module for the flash on digital cameras and mobile phones in October 2006. Technical expertise from Heraeus is in each and every one of them: the flexible substrate, together with the LED chip based on gallium nitride, forms an LED module that the precious metals and technology company manufactures on a mass production scale. The flexible substrates plated with nickel-gold or palladium are so conceived that customers can mount the two mini-chips needed for operation on one tiny area in addition to the LED lens, thereby saving space. What is more, the thermal energy produced when the LED is illuminated or flashes, is readily transferred along the substrate to the metal layer. These optimal thermal properties of LED modules together with their quick recovery times permit higher flash frequencies and brighter operation.
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