IML Technique and EM Simulation Software Move Antenna Design Forward
By Hao Weidong and Xiong Ye, Foxconn International Holding Group • Foxconn Antenna Division

In-molding label (IML) is a new type of in-molding decoration technique, incorporating screen printing, forming and injection molding, that is used in molding insert injection. While today this technique is widely used in decorating products, its ability to save space and simplify the production process makes it a viable contender for antenna design as well. The IML technique not only improves product quality, but by leaving room for product innovation, raises a product’s added value.

One possible application of the IML technique is to inlay an antenna in the back case of a mobile phone. Doing so gives the antenna stability and wear resistance, but also greatly inconveniences its design. Use of appropriate electromagnetic (EM) simulation software to design and optimize the antenna within the handheld device can be used to overcome this difficulty. It can also find the antenna’s sensitive area, helping to reduce and even avoid antenna performance deterioration because of tensile deformation and assure compliance with wireless acceptance tests like specific absorption rate (SAR), hearing aid compatibility (HAC). The overall result is a reduction in design cycles, risk and costs, as well as shortened development time.

Understanding Antenna Design
Traditionally, in the early stages of designing an antenna, a printed circuit board (PCB), antenna frame and copper sheet are used to make a prototype, which is then tested using a network analyzer (Figure 1). The idea is to stick the copper sheet on the frame and optimize the antenna’s performance by changing its form and size. Unfortunately, this “cut and paste” method cannot be used to optimize an antenna in the back case of a mobile phone, nor can it measure such antenna parameters as SAR and HAC to comply with existing wireless acceptance tests.

A Better Approach
The IML technique offers an alternate approach to antenna design. Rather than relying on optimization of a prototype, it integrates the antenna into the back case of the mobile phones. The antenna is pressed into the film of the back case, shaped by a former and placed in an injection mold for molding after cutting.

There are a number of advantages to employing the IML technique in antenna design. Since the antenna is integrated into the back case of the mobile phone, the relative distance between the antenna, speaker and other RF modules is increased. As a result, mutual interference is reduced and antenna performance is enhanced. During mass production, the IML technique simplifies the overall process, reducing bad yield due to human factors, lowering production cost and enabling production of highly consistent antennas.

Despite its advantages, the IML technique has its limitations, namely that the IML antenna is susceptible to deformation because of the tensile strain that occurs during high-pressure molding. This deformation affects the performance of the antenna. And, since the antenna is inlayed in the back case of the mobile phone, the case’s thickness and material have a great impact on antenna performance. A final limitation is that the antenna simply cannot be designed using traditional cut and paste methods.

Proving the Method’s Viability
While the idea of employing the IML technique to design an antenna for a mobile phone may seem like a good idea, proof that this method works is the only true way to be certain of its viability. As previously mentioned, an antenna based on this technique cannot be tested in isolation with traditional methods. Luckily this problem can be solved using electromagnetic (EM) software to design and optimize the antenna within the handheld device.

Consider that a dual-frequency antenna is produced based on the IML technique. EMPro simulation software from Agilent Technologies is used to analyze and simulate the IML antenna. As is obvious from Figure 2, tensile deformation is greatest where the curvature is high. In order to reduce the impact of deformation on the antenna’s performance, it is critical to design the sensitive area of the antenna on a plane. This area can be easily identified through simulation with EMPro.

The EMPro software can clearly show the form and size of the antenna in the mobile phone case and vividly prototype the real-world environment of the antenna after meshing. The simulation result is accurate, efficient and rapid, so the design and development cycle can be greatly shortened, significantly reducing design cost.

Designing an IML Antenna through Simulation
Designing an IML antenna using EM simulation software involves a number of critical steps, including creation of a simulation model and debug of the antenna’s sensitive area. To establish the simulation model, the designer inputs the 3D model into the EMPro software, sets up the parameters and priority class of materials, and meshes the models. Next, the feeder port and other parameters are set up and simulation is run. Finally, the design is optimized.

The next critical step is to debug the antenna’s sensitive area on the curved surface. For this task, the mesh division in the overall model is 1 mm by 1 mm by 1 mm. The antenna’s mesh division is 0.3 mm by 0.2 mm by 0.2 mm. From Figure 3, note that the curvatures of parts 1 and 2 of the antenna are high and therefore will deform more after high-pressure forming. The sensitivity of these two parts must therefore be evaluated.

Begin by parameterizing part 1 to reduce its size. Starting from the edge, parameterize it once every 4 mm, for a total of five times. After every cut, calculate its return loss characteristic curve. Repeat the process for part 2 of the antenna. Note that whenever a part is changed, the other parts of the antenna should remain unchanged. The return loss characteristic curve obtained from simulation of the antenna is shown in Figure 4.

In this figure, the curve of part 1 changes substantially and is the sensitive area. This area must be reduced as much as possible during the design phase in order to minimize deformation of the antenna’s shape in high-pressure forming. The antenna’s performance remains basically unchanged after adjusting the position of part 2 to the position of part 3 (the plane area), and after mockup and simulation, but the deformation sensitive area of the antenna inside the mobile phone is decreased.

After further optimization, the antenna in Figure 5 is obtained. The sensitive area of the antenna on the curved surface is shifted to the planar surface, greatly reducing deformation of the antenna caused by printing and pressing. Using the EM software to design the antenna therefore solves the biggest problem associated with IML antennas and, in the process, lays a good foundation for future mass production.

Examining the Test and Simulation Results
Following the optimal design of the dual-frequency antenna, a DELL490 desktop computer with GPU accelerator was used to simulate the design by broadband, a process that took 58 minutes. The return loss of the antenna resulted.

Figure 6 provides a comparison of the IML antenna’s tested and simulated return loss. As is apparent, the test and simulation results are basically consistent, proving the feasibility of this antenna design scheme.

Single frequency simulations of the antenna efficiency are shown in Table 1. Here, the total simulation time was 32 minutes.

Table 2 illustrates the two key benefits that arise from using the simulation technique to design an IML antenna. It allows the engineer to find sensitive areas in the antenna design and also eliminates the need for a hardware prototype. These benefits are crucial to minimizing the multiple design iterations that are often required for design optimization using the traditional antenna design technique. The result is a reduction in design cost and a significantly shortened product development cycle.

The EMPro software can clearly show the form and size of the antenna in the mobile phone case and vividly prototype the real-world environment of the antenna after meshing. The simulation result is accurate, efficient and rapid, so the design and development cycle can be greatly shortened, significantly reducing design cost.

Compliance with Wireless Acceptance Tests
A critical concern in the mobile phone industry today is the impact of antenna radiation on the human brain. The SAR value is closely connected with the radiation power of mobile phones and therefore must be minimized in an antenna design in order for it to pass the corresponding standard.

The EMPro software uses the SAM (head) model for SAR measurements, adjusting the mobile phone and SAM to suitable positions. SAR is calculated for the key frequencies. The relative dielectric constant and conductivity of tissue fluid are frequency dependent and are automatically taken into account when simulating SAR. The simulation results compared to measurements are shown in Table 3 (on page 16). As with the return loss, both the simulation and test results for SAR are consistent.

HAC is another critical concern for the mobile phone industry. Many mobile phones entering the US are required to undergo an HAC acceptance test and pass the corresponding standard. EMPro supports the HAC compliance test. A comparison of HAC simulation and test results is provided in Table 4.

The Bottom Line
The IML technique holds great potential for antennas used in applications like mobile phones. Its benefits though, come at the cost of making the traditional cut and paste antenna design method impossible. A new and highly efficient design method for IML antennas now offers a solution to this dilemma. It uses EM simulation software like EMPro to design and debug the antenna. Using this technique, a dual-frequency mobile phone antenna was produced that was suitable for IML production. The simulation and test results for this design were highly consistent, proving the feasibility of the design. Adoption of this new technique and process for designing antennas can therefore save cost, while also shortening the development cycle and enhancing product competitiveness.

WeiDong Hao and Xiong Ye are antenna engineers at Foxconn's Antenna Division. Their responsibilities include antenna concept design and simulation. WeiDong can be reached at haoweidong2414@163.com.

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