1. Introduction to RFID antenna manufacturing methods

Antenna manufacturing technology is mainly coil winding in the low frequency range. The general UHF and high frequency antenna manufacturing methods mainly include etching method, electroplating method and printing method.

1.1 etching method

First, a resist ink is printed on the metal foil-coated PET film to protect the antenna line pattern from being eroded during etching, and then baked, etched, and cleaned to obtain the antenna pattern we need.

The advantages of this method are: the process is mature, the yield of antenna production is high, and the performance of the antenna is very consistent; the disadvantage is that the etching process is slow, resulting in slow antenna production; due to the use of the reduction process, Most of the copper foil is etched away, which results in a relatively high cost.

1.2 Printing method

The antenna pattern is printed on a PET substrate by a conductive silver paste, and then baked and cured to obtain an antenna manufacturing process. The advantages of this method are: fast production speed and flexible production, which can be applied to small batch production.

The disadvantages of this method are: 1 The conductivity of the conductive silver paste is far less than that of the copper foil (about 1/20 of it), the conductor loss of the antenna is relatively large, resulting in the antenna efficiency is not as good as the etching antenna; 2 conductive silver paste on the PET base The adhesion of the material is not good and it is easy to fall off, resulting in low reliability of the antenna. 3 Recently, the price of silver has risen sharply, which has led to a significant increase in the cost of conductive silver paste, which has weakened its cost advantage.

1.3 Electroplating method

First, the antenna pattern is directly printed on the PET substrate by conductive silver paste (thinness thinner than the printing method) or other electroplated seed layer, baked and then plated thickened to obtain the finished antenna product. The advantage of this method is that the production speed is fast, the antenna conductor loss is small, and the performance of the antenna is good. The downside is that the initial equipment investment is large and it is only suitable for mass production.

1.4 Vacuum coating method

First, the Masking is printed on the PET substrate to form the reverse pattern of the RFID antenna, and then the aluminum layer or the copper layer is plated by vacuum coating. Finally, the RFID antenna is formed through the De-masking process.

The advantage of this method is that the production speed is fast and the cost is relatively low; the disadvantage is that the deposited film is about 2 μm, which is far lower than the 18 μm of etching and plating. The performance of the antenna is between etching and printing. The vacuum coating equipment is about one million US dollars, and the equipment investment is very large. Similar to electroplating, it is suitable for mass production.

It has also been attempted to first print a platinum-containing ink onto a PET substrate to form an antenna pattern as a seed layer, followed by electroless copper plating. It has the advantage that platinum-containing inks are less expensive than conductive inks. However, electroless copper plating is slower and has a deposition thickness of a few microns.

In addition, there is also a wiring method for the high-frequency antenna, that is, the enameled wire (about 0.25 mm) is passed through the ultrasonic head, and the ultrasonic head is routed according to the designed pattern; during the routing, the enameled wire is ultrasonically connected with the PVC substrate. The antenna performance of this method is very good, and the reliability is also high, that is, the cost is more expensive than the etching method.



Figure 1 (a) wiring method ultrasonic bonding head; (b) high frequency antenna manufactured by wiring method

2. Introduction to die cutting technology

Because the mainstream etching process is slow, wastes materials, and pollutes the environment; the cost of printed silver paste is high and the antenna reliability is not high; all this has led to the development of new low-cost, high-performance antennas. method. Therefore, we have used die-cutting technology to process self-adhesive structural materials to produce RFID antennas.

2.1 Principle of die cutting technology

Die-cutting technology is actually a kind of cutting process. The self-adhesive material is placed on the die-cutting table of the die-cutting machine, and then the die-cutting knife plate made according to the pre-designed pattern is applied with pressure to make the blade corresponding to the blade. The force is broken and separated to obtain the desired shape, as shown in Fig. 2. Die-cutting of the self-adhesive material generally only cuts through the facestock and the adhesive layer, that is, half cut through, retaining the silicone oil coating on the backing paper and its surface; and finally leaving the die-cut label on the backing paper.



2.2 Die cutting materials

The R F I D antenna is typically constructed of a layer of 18 μm thick aluminum or copper plus a 10 μm thick release liner. The aluminum or copper layer acts as a functional layer on which the pattern shape of the RFID antenna is formed; PET is a carrier layer as an antenna pattern, mainly serving as a mechanical support, and in addition, the dielectric constant and thickness of the PET substrate are also affected. The resonant frequency of the antenna. This structure is very similar to the traditional self-adhesive structure, except that there is a layer of reinforcement in the middle of the adhesive; so we use the antenna to form a self-adhesive structure. The material used for die cutting has a three-layer structure: release paper with silicone oil or PET (about 100 μm), adhesive layer (about 20 μm), aluminum foil with reinforcement layer (about 35 μm), as shown in the figure:



Among them, silicone oil is mainly for the purpose of separating waste materials, and the reinforcing layer is mainly for strengthening aluminum foil, which is convenient for waste disposal.

2.3 Die cutting machine

The die-cutting machine mainly performs die-cutting by controlling the pressure. The working principle is to use a die cutter, a steel knife, a metal mold, a steel wire (or a template carved from a steel plate), and apply a certain pressure through the plate to cut the material into the shape you need.

According to the die-cutting plate and the press-cutting mechanism, the die-cutting machine can be divided into three types: flat flat, round flat and round press.

3. RFID antenna die cutting solution

3.1 Analysis of Die Cutting Characteristics of RFID Antenna

Mold requirements:

Although we use a self-adhesive structure to make our antennas, our facestock is metallic aluminum or copper. Metals are more prone to wear out of the die. For non-metallic materials, the etching die can be die-cut 200,000 times. For metal, it must be repaired or discarded about 20,000 times. Therefore, we can choose a better mold material to heat the blade to improve the hardness of the blade.

The RFID antenna pattern is relatively complicated and the spacing is relatively small, and the general line width is about 1 mm.

Therefore, we choose high-precision etching knives or engraving dies, and generally choose a single-peak dies, with beveled faces facing outwards and no sloping faces inward, thus ensuring that the cut line width is 1mm and straight. As shown below:



Figure 4 Single peak knife die cutting effect diagram

Die cutting material requirements:

The strength of the facestock mentioned above has a great influence on the discharge. The aluminum foil we use is generally around 18 μm. At this time, its strength is very weak, and it is basically broken by hand. Directly using a single layer of aluminum foil or copper foil as the face material, the strength is obviously insufficient. To this end, we added a layer of reinforcement on the back side of the aluminum foil. Here we chose PET with a thickness of 10 μm, as shown in Figure 3.

In order to save costs, we chose release paper as the antenna substrate. Adhesives For the convenience of waste and die cutting, we chose water-based glue as our adhesive layer. The thickness of the glue layer is about 20 μm.

Difficulties in exhausting waste:

The RFID overclocking antenna pattern is so complicated that it is extremely difficult to discharge the die cutting process. This is also the difficulty of die-cutting antennas. Specifically, there are the following characteristics:

There is a closed loop. In order to adjust the impedance to match the chip, the dipole antenna has a T-type matching structure or an inductive coupling structure in the antenna structure; these impedance matching structures are basically a closed ring. Direct discharge is basically impossible.

In order to adjust the real part of the antenna in the antenna structure, the T-type matching structure is only connected to the intermediate portion of the antenna radiating portion. The other part of the T-shaped structure has a gap with the radiating portion of the antenna. This gap is perpendicular to the bending line and the normal layout direction, and is generally not good for disposal.

In order to miniaturize the dipole antenna, a bending line technique is generally employed. The pitch of the bending line is generally about 1 mm to 2 mm. The bending height is about 8mm. These slender bend lines are more difficult to dispose of. After adding the reinforcement layer, we found that the bend line gap at one end can be directly drained, and the bend line gap at the other end is not well drained.

Also for miniaturization, there is sometimes a folded structure at the end of the antenna, which is equivalent to a larger half of the closed loop, which brings greater difficulty to waste disposal.



Figure 5 Several exhaust points of the RFID UHF antenna

3.2 Adhesive die cutting and waste disposal scheme

In view of the delicate and complex situation of the R F I D antenna, we propose an antenna manufacturing scheme in which two die-cuts are used twice. We divide the antenna into two parts: the inner pattern and the frame pattern. The frame pattern is a very regular pattern that can be directly discharged; while the interior is a more difficult pattern, we divide it into separate patterns, which are glued to remove waste. See below:



Figure 6 Die-cutting waste map (black line is the outer frame pattern, green line is the inner pattern)

The principle of glue discharge:

Viscose discharge is mainly based on the relative size of the adhesive to achieve the purpose of waste. As shown in Fig. 7, the purple portion is the portion to be discharged, and they are separated "islands".

The portions of the pattern to be retained are integrally joined together. Adhesive tape is attached to the pattern to be discarded. When the adhesive is lifted through the “island”, because the “island” part is relatively small, the adhesive force of the “island” part of the adhesive tape team is greater than the adhesive force of the “island” part and the release paper, “ The "island" part was transferred to the adhesive tape. When the adhesive tape passes through the pattern to be retained, the area of the pattern to be retained is large, and the adhesive force of the adhesive tape to the "island" portion is smaller than the adhesive force of the portion to be retained and the release paper, so it is necessary to retain The pattern continues to remain on the release paper. In this case, the separated "island" duty is carried out by the adhesive tape, and the patterned layer to be retained is left on the release paper, thereby achieving the purpose of waste disposal.



Figure 7 Schematic diagram of bonding and discharging

Discharge waste flow chart and pattern process change chart, the specific process is shown in the following figure:



Taking the reference antenna provided by N X P as an example, FIG. 9 is a stepwise change diagram of the antenna pattern during die cutting.



Figure 9 Pattern change diagram of the reference antenna waste process provided by NXP

Specific implementation process:

According to the glue discharge scheme, we chose two 300 mm wide knife die cutters. Two laminating machines, one laminating machine and one stripping machine.

The bonding is responsible for attaching the tape to the aluminum foil on the sticker, and the stripping machine is responsible for recovering the tape from which the scrap is stuck. Due to the silicone oil on the release paper, the adhesion of the aluminum foil to the release paper is small (50g even if the super-stripping force), the general tape adhesion can reach more than 100g, so the adhesion is generally no problem. The width of the adhesive tape is generally narrower than the width of the largest waste.

According to the process mentioned above, we opened two pairs of molds according to the inner and outer frames. In order to increase the output rate, we made three rows of patterns on a pair of molds, as shown in the following figure:





Figure 10 (a) Internal pattern die-cutting mold; (b) Outer frame die-cutting mold

Die cutting process physical map:



3.3 Comparison of die-cut antenna and etched antenna performance

(1) Edge alignability: Since the die-cut antenna is mechanically cut, its edges are very flat. In the antenna manufactured by the etching method, the edge is uneven due to the side etching of chemical corrosion. See the picture below for details:



(2) Production speed: the speed of the die-cutting machine is 3 times per second. Assuming that the die has 3 patterns and the machine works for 12 hours a day, then we can produce 400,000 antennas per day, which is not only much higher than etching. The speed of the law is faster than the printing method.

(3) Fine pattern: The accuracy of the etching method can reach 0.2 mm, which is suitable for flipping the chip directly on the antenna; the accuracy of the die-cutting method is about 0.5 mm, so the antenna must be transferred by means of module transfer. Interconnection with the chip.

(4) Determinacy of the pattern: the etching antenna pattern is firmly adhered to the P E T substrate, and the die-cutting antenna is not fixed due to the silicone oil on the release paper, and is easily slid to cause pattern distortion. This requires as much manual intervention as possible during the production process.