Flipchip bonding of ultrahin Si dies onto PEN/PET substrates with low cost circuitry

Abstract

All-printed, cost effective, smart electronic products are expected to be used in a wide range of applications and in large quantities in our society. The substrate material for these applications will be low cost materials like PEN or PET foils. For the functionality of the printed electronics product it often will still be required to integrate a Si chip. To keep the flexibility of the package and not to add too much to the thickness, the chip needs to be integrated into the product as a bare, thinned die. Because of the low temperature stability of the PEN and PET and the use of printed conductors it is necessary to interconnect the chip using an adhesive. The current paper specifically addresses the challenges associated with this. Research efforts will be discussed on the flip chip bonding of ultrathin (i.e. thickness 20 µm) bare chips on printed circuitry on both PEN and PET foils using a typical anisotropic conductive adhesive (ACA). Based on the results it can be concluded that a reproducible, low contact resistance and a good lifetime and flexural durability can be achieved over a wide range of bonding forces and temperatures. IntroductionRecently a new class of flexible electronics is starting to emerge which is most effectively termed 'printed electronics'. This term often refers to all-printed, cost effective, smart electronic products that will find a wide range of applications in large quantities in our society. Examples include cheap sensor packages attached to food packaging to measure the ripeness of food, smart bandages that monitor the healing of wounds and smart active or passive RFID tags. These cost effective packages can include all-printed functional structures like conductive circuitry, OLED pixels, photodiodes and logics based on organic materials.The substrate material will in most cases not be polyimide as commonly used in the more traditional flexible electronics. By using polyesters like PEN (polyethylene naphtalate) or PET (polyethylene teraphtalate), the substrate costs can be reduced by a factor of 5-10 [1]. A disadvantage of polyesters is however that they are considerably less thermally stable. PEN has a glass transition temperature (T g ) of ~130o C and PET of ~85 o C while poly(imide) has a T g of ~350 o C) [1]. This limitation in thermal stability excludes many well established processes for making electronic products and/or renders the use of existing processes much more challenging.A tangible example of such a printed electronics product is the disposable smart pill blister that is under development at the Holst Centre and its industrial partners. A photograph of a first prototype of it is shown

Interactive UHF/UWB RFID tag for mass customization

“…14b. The pads of the chip are connected to the printed patten by the anisotropic conductive adhesive (ACA) which conducts only along one direction ( Van den Brand et al 2011. The flexible display is the new EC display sample in passive-matrix structure from Acreo and the flexible battery is the 3.0 V product from Enfucell.…”

Flexible and stretchable electronics for wearable health devices

“…Specifically work was performed on achieving a reliable and robust chip-foil interconnect with low temperature cure anisotropic conductive adhesive (ACA) [4,5]. Fig.…”

Feasibility Study of an Automated Assembly Process for Ultrathin Chips

“…After silicon thinning, the chips are detached from the wafer release foil and assembled either by face-down flip-chip technology [ 9 , 10 , 11 ] or face-up die-bonding with subsequent electrical contacting [ 12 , 13 , 14 , 15 ]. There are various disadvantages of flip-chip assembly in regard to ultrathin chips.…”