Intelligent flip-chip technology for smart logistics – smart labels
Dipl.-Ing. (FH) Florian Hierl, Business Development Manager, DELO Industrial Adhesives Dr. Michael Stumbeck, Research & Development, DELO Industrial Adhesives The rapid progress in the field of semiconductors and integrated chips as regards efficiency and miniaturization has paved the way for the technology of Radio Frequency Identification (RFID). This development has already started in the sixties and has meanwhile established itself especially in the automotive sector. By means of increasingly sophisticated technologies and production methods, RFID applications have progressed out of a niche into a feature of consumers’ everyday use. No matter if cash registers in supermarkets, luggage labels at airports or labels of parcels or letters are concerned – if encrypted information such as prices or addresses are to be read, RFID is more and more replacing the previously ubiquitous barcodes. Due to RFID, mainly logistics companies benefit from a decisive acceleration and facilitation of storage management. Thus, the information saved on the chip can be read without contact by means of radio waves and extended by further contents. Orientation of the label towards a scanner system is omitted: the individual piece goods of a whole pallet can be read in a bulk and clearly identified as being original by the serial number of the chip. Corresponding system integration allows companies to gain a detailed overview of their current stock at any time as automatic booking of the goods in stock is ensured. This is enabled by the use of a so-called smart label, a label with semiconductor chip and antenna, communicating with surrounding readers. Normally, the smart label’s data are transmitted in the HF (high frequency; 13.56 MHz) or UHF (ultra high frequency; 860-930 MHz) range.
Illustration 1: Flip-chip bonded to an RFID
Due to a variety of implemented application examples, RFID technology is finding increasing favor in logistics processes. Therefore, the demand for smart labels will increase significantly in the near future as mainly in the medium-term the entire value and production chain will be displayed by means of RFID technology and low-price products will be furnished with the respective labels. Thus, the demand for economical and efficient production methods is becoming more and more important. Currently, semiconductor manufacturers are in the focus as the chip is the greatest expense up to now. However, the manufacturers of inlays and labels are also under increasing pressure to reduce the unit costs for smart labels through optimized process engineering and reduced costs. Bonding of the RFID chip to the tag antenna is a crucial and essential production step. On the one hand, this connection permanently ensures the electrical contact (which is indispensable for communication), on the other hand, this step considerable contributes to the highest possible yield in volume production. In this field, the so-called NCA (non-conductive adhesives) and ACA (anisotropic conductive adhesives) adhesives offered by DELO Industrial Adhesives offer an effective solution already successfully realized millionfold in practice due to their reliable connection of semiconductor chip and antenna in seconds.
From COB process to flip-chip technology For numerous applications in the smart card sector as well as for smart labels initially, the so-called COB (chip on board) or COF (chip on flex) technologies represented the preferred process for the mechanical and electrical connection of semiconductor and substrate. With this method, the chip is mechanically fixed to a substrate and electrically contacted in a so-called wire bond process with gold wires. The individual contact surfaces must be processed successively. Then, the contacts are protected against environmental influences by encapsulating the chip and the wire bonds. Standard encapsulation compounds are cured by light or heat. The curing times range from 30 seconds to 2 hours at temperatures from room temperature up to 200 °C. For contacting bare semiconductor chips in smart label applications, however, this complex and relatively slow process is not an adequate solution. Considering the long curing times, the production of several billions of RFID labels per year would be unthinkable. Here, the advantages of the flip-chip technology come into play enabling easy, economical and fast contacting of chips on miscellaneous substrates. In the flip-chip technology, a semiconductor chip is already equipped with so-called bumps on its structured, active side by the manufacturer. For
contacting, the chip refined with bumps is pressed into the substrate metallization with its active side facing the substrate (see illustration 2a).
Bump
Chip
Contacts, e. g., of an RFID antenna Substrate
Illustration 2a: Illustration of a chip with bumps pressed to a substrate with contacts.
In theory, any number of electrical contacts can be established in one process step. The adhesive previously applied to the active surfaces can be cured when placing the flip-chip, for example, with a heated plunger (thermode). Thus, mechanical fixing and electrical contacting are effected in only one process step. The contact surfaces are already protected against environmental influences by being embedded in adhesive (see illustration 2b).
Bump
Adhesive
Illustration 2b:
Chip
Substrate
Illustration of a chip contacted by means of a NCA/ACA adhesive
The use of either an NCA or an ACA depends on the available bump variants of the chip (see illustration 3). If the bumps are very flat and have a regular shape (Ni/Au, Au), ACA adhesives are normally used. If geometrically irregular stud bumps or palladium bumps are used, NCA adhesives
are used. Depending on the substrate material (PET, paper, PI) and/or metalization (Cu, Al, Ag), adhesives which are accordingly adjusted are available.
Flip-chip connection
Anisotropic electrically conductive adhesive
Chip Au- or Ni/Aubump Substrate
Non-conductive adhesive
Chip Stud- or Pd bump Substrate
Abb. 3: Schematic setups of flip-chip connections produced with DELO-MONOPOX or DELO-MONOPOX AC.
As can be seen in illustration 3, the filler ensures the electrical contact to the antenna when using Au or Ni/Au bumps. As the system is contacted in only one spatial direction (z-axis) by pressing the chip, the adhesive is called anisotropic. NCA adhesives are not filled. The contact is established by the rough bump surface penetrating the metalization layer. The adhesive ensures that this condition is maintained during the lifetime of the inlay.
Fast curing and short cycle times Indeed, there is no generally valid specification for smart labels, however, the requirements on performance and service life are often adjusted in accordance in relation to the application. However, some general requirements the bonded connection has to meet can be named. Curing depends on type of substrate and system at 6 – 10 seconds and temperatures of 170 °C up to 190 °C at the adhesive Availability of ACA and NCA products for different chip/bump types Preferably comprehensive adhesion to different antenna materials Resistance of the bonding during storage at 85 °C and 85 % relative humidity
temperature change tests in the range of -40 to +85 °C bending test (bending of the smart label over a radius of 2 cm) continuous temperature storage at -40 °C and / or +85 °C In practice, reliability tests for storage times of 168 h to 504 h for humidity and temperature tests as well as 50 – 500 cycles in temperature change tests are requested frequently. Besides the NCA/ACA adhesive used, the performance of the substrate material used is decisive for the reliability of the flip-chip bonding. Driven by cost pressure, this market segment is developing rapidly in the RFID sector. Standard antennas are mainly produced on PET-and paper-based foils using different methods (etching, electroplating, printing processes, PVD). The quality and the surface properties can be very different from manufacturer to manufacturer and, therefore, also directly influences the total performance of the smart label. In order to be able to support customers effectively, DELO closely cooperates with leading substrate, chip and system manufacturers. Therefore, an optimized adjustment of DELO adhesives with today’s and future requirements is ensured in order to be able to obey new trends immediately.
From theory to practice – DELO sets standards In order to guarantee permanent process stability and a high reliability, DELO adhesives are subject to numerous investigations and tests. At first, some product-specific data are listed below (chart 1):
Properties
DELO-MONOPOX AC163
DELO-MONOPOX AC265
DELO-MONOPOX AC VE42878
Ion content Na+[ppm] K+[ppm] Cl-[ppm] F- [ppm] Curing with a thermode <10 <10 <10 <10 8 s at 190 (at the adhesive) <10 <10 <10 <10 °C 8 s at 190 (at the adhesive) <10 <10 <10 <10 °C 6s at180 (at the adhesive) °C
Water absorption Storage life at 5 °C Storage life at rt (20 °C to 25 °C)
0.4 % 6 months 2 weeks
0.4 % 6 months 2 weeks
0.4 % 4 months 1 week
Chart 1: Excerpt from the product properties
The following results for the ACA products DELO-MONOPOX AC265 and DELO-MONOPOX AC VE42878 exemplarily show the tested adhesive properties. Moreover, further standard and development products are available as NCA types with similar properties. For the tests stipulated, PET substrates with aluminum metallization were used. As chip, a silicon semiconductor with test structure for resistance determination with Au bumps was chosen. The following shear strengths were determined for a chip with a surface of 1 x 1 mm² (tested with DAGE Series 4000, according to DELO Standard 31).
Adhesive DELO-MONOPOX AC265
Shear strength on PET/Al
23 N DELO-MONOPOX AC VE42878
25N
Chart 2: Shear strength on a 1 x 1 mm² chip.
The high strengths reached guarantee an excellent mechanical anchorage of the chip and, therefore, form the starting point for successful temperature and humidity storages. Here, storage at 85 °C and 85 % relative humidity proved to be the most critical reliability test, as adhesive properties, such as
low humidity absorption, high adhesion, low subsurface migration tendency on the relevant substrate and high strengths are especially stressed in this test.
85/85 Performance on PET-Al / Au-bumped Chip
1000 900
e c a re is n e / m h le tric l s ta c Om
800 700 600 500 400 300 200 100 0 directly after curing 24h after curing 168h 334h 500h 668h
AC VE42878 AC265
836h
1000h
Illustration 4a: Tests results of storage at 85 °C and 85 % relative humidity
Moreover, the behavior at temperature changes is decisive for the practical use. Having a glance at RFID applications in goods and vehicle trade with most different ambient conditions this becomes quite clear. Here, temperature ranges between -40 °C and +85 °C emerged as relevant for the application.
-40/85 performance on PET-Al / Au-bumped Chip
1000,00 900,00 800,00
e c a re is n e m h le tric l s ta c / O m
700,00 600,00 500,00 400,00 300,00 200,00 100,00 0,00 directly after curing after 166 cycles after 332 cycles after 500 cycles
AC VE42878 AC265
Illustration 4b: Results of the temperature shock test at -40 / +85 °C During the two tests, an excellent performance of the ACA products could be observed. The slight increases in the resistance values after temperature shock and humidity storage can primarily be attributed to substrate aging, i. e., oxidation of the antenna metallization, swelling effects in the basis laminate as well as a marginal relaxation of the adhesive in the first hours of the storage tests. Practice showed that these moderate changes in the resistance do not carry weight for HF and UHF applications.
Conclusion NCA and ACA products by DELO Industrial Adhesives make it possible to keep shortest cycle times and, therefore, to reach highest throughputs. Thus, more than 10000 uph were made possible for smart label applications, depending on substrate dimensions and flip-chip mounter. At the same time the flip-chip method allows for the call of efficiency, i. e., in the meantime 20 million chips can be contacted per kilogram of adhesive. Therefore, the NCA/ACA products present the most efficient method for the bonding of chip and substrate. Due to the high reliabilities reachable in the field of the manufacturing process and the service life, the systems conducted with NCA/ACA adhesives offer a stable and efficient way in the production of RFID labels.
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