Dongho Ha

Analysis of external force dependent lumped RLGC model of high-bandwidth and high-density silicone rubber socket

In this paper, structural and electrical characteristic deviations of silicone rubber sockets are analyzed depending on external force based on its lumped RLGC model. Electrical performances heavily depend on compression rate of the silicone rubber socket. Thus, study on the structural and electrical characteristic deviations of the silicone rubber socket depending on external force is needed for reliable tests. The deviations are analyzed based on an assumption based simplified equivalent model of the silicone rubber socket. For verification, a 3D electromagnetic (EM) simulation program is used and its results are compared with that of proposed RLGC models in the frequency domain. The electrical performance of the RLGC model is nearly identical to that of 3D EM simulation result. In addition, the electrical performance of the compressed silicone rubber socket is also nearly identical to that of a solder ball when it is compressed at different compression ratio.

Modeling and analysis of a conductive rubber contactor for package test

For high-speed package testing, conductive rubber contactors become more and more promising because of its excellent RF performance. For electrical characteristic analysis of the conductive rubber contactor, the lumped equivalent circuit model of conductive rubber contactor based on its physical dimension and structure is proposed and verified with 3D field and SPICE simulation.

Signal Integrity Design and Analysis of a Multilayer Test Interposer for LPDDR4 Memory Test With Silicone Rubber-Based Sheet Contact

As the data rate of Low Power Double Data Rate 4 (LPDDR4) memory now exceeds 3.2 Gb/s, it is becoming more difficult to meet the target specifications. While testing has become of utmost importance, it is not viable to have a direct access to the signal pins in a package on package configuration due to the densely located array of solder balls; instead, a test interposer with an excellent electrical performance needs to be adopted to provide test access. In this paper, we first propose a novel test interposer scheme for testing LPDDR4 memory packages. For accurate testing without significant influence on the intrinsic signal path, the proposed test interposer is designed considering a number of signal integrity issues such as intersymbol interference, jitter, impedance matching, and crosstalk. Furthermore, by adopting silicone rubber sheet in place of soldering, the proposed test interposer enhances reusability of the packages with a fast setup time. Moreover, a reconstruction method is proposed that can reconstruct the voltage at application processor using the waveform captured on the test interposer, instead of probing at the ball gray array directly. Through a series of simulations and measurements, we experimentally verified the proposed test interposer. The proposed test interposer scheme can be widely adopted for testing of high-performance packages with its high accuracy and practicality.

Design and analysis of silicone rubber-based TERAPOSER for LPDDR4 memory test

Motivated by the increasing market demand for the high performance mobile devices, both the data rate and the number of pin of mixed-signal systems keep on increasing to realize multifunctional yet compact system designs. With this ascending technical trend, many signal integrity (SI) and power integrity (PI) problems such as ISI, jitter, crosstalk, simultaneous switching noise (SSN) have arisen that need to be thoroughly analyzed and tested. In order to accurately monitor the data signals of LPDDR4, a carefully designed test interposer considering such factors as impedance matching, minimization of skew and crosstalk needs to be proposed. In this paper, a novel test interposer scheme composed of a test interposer and silicone rubber sheets is proposed for LPDDR4 memory test. Through a series of simulations and measurements, we experimentally verify the proposed structures in time and frequency domains, and prove their accuracy and practicality.

High-Frequency Modeling and Signal Integrity Analysis of a Silicone Rubber Socket for High-Performance Package

As a demand for electrical systems with a wide data bandwidth has increased, high-performance packages ensuring high data rates, such as low power double data rate series, have become common. The need for high-performance test sockets has also emerged to test these packages. However, a conventional pogo pin socket has a limited test bandwidth due to the parasitic components arising from its spring. On the other hand, a silicone rubber socket satisfies the wide bandwidth requirement because it has low parasitic components due to high-density conductive metal powders in an elastic silicone rubber. In this paper, we propose an RLGC equivalent circuit model of a silicone rubber socket and first experimentally verify it. The proposed model is experimentally verified in the frequency domain by comparing the insertion loss obtained from the proposed model to the measurement up to 20 GHz. The proposed model is experimentally verified in the time domain by comparing the eye diagrams obtained from the proposed model to the measurement at a data rate of 12.5 Gb/s. Also, the insertion loss of the sockets with varied height, diameter, and pitch is analyzed using the proposed model. The proposed model provides physical insight of a silicone rubber socket, and it allows to determine whether a socket is reliable for testing high performance packages in a short time. It also gives us the idea how to design a high-performance test socket. Furthermore, we discuss the current capacity and life cycle of the silicone rubber socket in terms of signal integrity as well.

Eye diagram estimation of 8B/10B encoded high-speed serial link for signal integrity test using silicone rubber socket

In this paper, an eye diagram of the 8B/10B encoded signal is estimated by using the single bit response (SBR) of a silicone rubber socket. The 8B/10B encoding scheme has been used in the HDMI. Because the eye diagram heavily depends on the bit patterns, the encoded PRBS patterns should be considered when the resultant eye diagram is obtained. Likewise, the 8B/10B encoding scheme must also considered when the eye diagram is estimated. An eye diagram estimation method finds the worst contour of the eye diagram using partial information. This can be done in significantly reduced testing time by using a SBR, as opposed to all of PRBS patterns. In high-speed IC tests, the ICs are interconnected with an automated test environment (ATE) by test sockets. Since the resultant eye diagram consists of 8B/10B encoded signal, the eye diagram should be estimated considering the encoding scheme. This paper considers the mapping scheme of Lattice RD 1012. In conclusion, the eye diagram of the 8B/10B encoded signal by Lattice RD 1012 is estimated using the SBR of the silicone rubber socket under the various compression effect. The estimated eye diagram and simulated eye diagram of the encoded PBRS patterns are nearly identical. Therefore, the proposed eye diagram estimation including the conventional 8B/10B encoding scheme is verified by comparing with that of PRBS patterns.

Measurement of High-Bandwidth and High-Density Silicone Rubber Socket up to 110GHz

In this paper, electrical performances of a silicone rubber socket are experimentally verified using several measurement set-ups. Silicone rubber socket is a promising test socket that can be adopted in high-speed testing environments in place of pogo type sockets, due to it relatively shorter length and lower inductance. However, the electrical performance of silicone rubber socket is dependent on an external force because its substrate is an elastic polymer. Accordingly, the compression effect causes reliability problems of testing environments, which needs to be thoroughly evaluated to ensure reliability. In this paper, two measurement set-ups for evaluating the performance of silicone rubber sockets are introduced and compared. Firstly, double-sided measurement set-up is introduced. However, it is unsuitable for high-speed testing environments due to the coupling effects between the closely placed microwave probes. For this reason, this paper introduces another measurement set-up which uses an additional printed circuit board (PCB). The second measurement set-up evaluates the electrical performance of the silicone rubber socket by comparing the performances of the PCB only and the PCB with the silicone rubber socket. While the measured electrical performance of the second set-up is much worse because of the parasitic elements of the PCB, use of an additional PCB is more reliable than the double-sided measurement result. Using this configuration, the electrical performance of the silicone rubber socket is evaluated up to 110GHz.

High-frequency modeling and signal integrity analysis of high-density silicone rubber socket

In this paper, we propose and analyze an equivalent RLGC model of silicone rubber socket with single-ended signaling. A silicone rubber socket consists of highly dense metal powders in elastic silicone rubber. When the silicone rubber is compressed, metal powders form a column which corresponds to a pad of package. Thus, it can be modeled as a pair of cylinders. We have successfully verified the proposed model using a 3D electromagnetic (EM) solver in frequency domain and the eye diagram measurement in time domain. As a result, the verified model can be used to determine whether socket is applicable to the test system in the simulation level concurrently with offering physical insight and reducing time spent 3D EM simulating socket.

Modeling and measurement of high-bandwidth and high-density silicone rubber socket for 100Gbps transceiver IC test

In this paper, an RLGC model of a silicone rubber socket is proposed and compared with simulation results. Due to the reusability of the silicone rubber socket, it is suitable for a reliable test. In addition, its electrical performance is compared with that of conventional solder balls. The performance of the two are compared using measurements in both frequency and time domains. Their eye diagrams are nearly identical at the data rate of 28Gbps, which is the data rate of a single channel in a 100Gbps transceiver IC that consists of 4 parallel channels.