For more than half a century, the semiconductor industry has been governed by a commonly known principle described as Moore’s Law. This “law” predicts that through technological advancement a doubling of the number of transistors per integrated circuit will occur within a given geometric area on regular 18 month intervals. The realization of this doubling effect over time has resulted in an ever-widening range of semiconductor (IC) devices exhibiting increases in functionality and processing speed, combined with an increased demand for power and effective thermal management. This doubling effect has also driven a matching rapid evolution in IC package types (microprocessors, LEDs, memory packages, etc.) and I/O interface configurations.
LEDs are in high demand lately due to their established position in backlighting of portable devices and their increasing use in mainstream backlighting for large-sized LCDs. LEDs gained their market position due to features like energy-efficiency, environmental friendliness, long life, and low maintenance. LED driver circuits play a major role in this area, depending on the application, as they have to supply constant current with minimal fluctuations as well as low power consumption. LED controllers/drivers come in standard IC packaging formats, and these ICs have to be tested for functionality before being incorporated into the final production systems.
The typical life of an IC starts with the concept/prototype phase, moves to the design validation phase, leaps into the application development phase, soars into the marketing/sales phase, rides into the production phase and ends with the upgrade/replacement phase (Figure 1). During these phases innovative interconnects have kept pace with the rapid evolution in semiconductor technology. IC sockets have been developed for a complete range of performance requirements and I/O configurations for each of those phases. This article will describe the form, fit and function requirements of IC sockets in each of those phases.
The important step in this phase is to verify whether the prototype functions as per the design intent. Since there can be revisions to the IC, an IC socket in the development board helps to avoid the routine of soldering and de-soldering devices.
The IC socket plays a major role in determining whether the device meets design intent or not. Because of this, the IC socket has to be carefully selected. The number one factor is bandwidth. Because the IC has to perform certain functions at specific speed, the signal loss has to be minimal.
Since additional socket interface introduces losses in the signal loop, either the socket has to have sufficient bandwidth to pass signals without insertion/return losses, or the socket specifics have to be de-embedded in the functional verification. Since it is very complex to de-embed specific parameters, a safer solution is to find a socket with higher bandwidth.
Next to bandwidth, DC series resistance plays an important role. Socket technology that can provide low and consistent contact resistance is preferable to avoid false failures.
The next factor is current carrying capability. An image sensor may require a low current such as 100mA to 200mA per ball whereas a power management device may require 3A to 5A per ball. Socket contact technology that accommodates this requirement has to be selected properly.
The next critical factor is contact technology compliance. Because the devices have wide co-planarity, the contact technology chosen has to accommodate the flatness variations.
One more factor that plays a significant role in prototype stage is temperature requirement. Various socket contact technologies include embedded wire-on elastomers, plated flex circuit capped elastomers, diamond particle interconnects, silver particle matrix elastomer (Figure 2), stamped contacts, spring contacts, hybrid contacts and other variations. Also socket features such as small footprint, easy chip replacement, easy mounting methodology, moving socket from board-to-board,and low cost are a must for this market.
Design Validation Phase
In this phase, a typical IC goes through many kinds of environmental tests. Typical tests include thermal shock, temperature cycling, HTOL (High Temperature Operating Life), humidity exposure, vibration test, salt spray and other tests based on end usage. Validation is required to verify whether it performs in all environments. Socket contact requirements such as an air-tight connection (low and consistent contact resistance), repeated cycles, temperature extremes, wear, and cleaning play a top role. Socket features which are used in the prototype stage are not relevant for this validation stage. In this stage, robust lid, tight latching, permanent mounting and extremely low cost (because of one time usage) are factors that satisfy this market.
Application Development Phase
Now that IC function has been proven and it is capable of withstanding end user environments, the next step is to develop application software for the IC. Sockets that are used in the prototype stage will satisfy the requirements of the application development stage as the need is to simply swap devices during the software development process. Since the performance is verified by executing software routines, the socket contact technology should accommodate high bandwidth, low signal loss, low contact resistance and appropriate current flow capability. Since the devices are not swapped frequently, socket lids can be screw mounted as opposed to easy open latch to reduce overall cost of test.
After developing applications, a critical stage of IC life is selling it to end users who use them in their systems. Typical steps include producing system boards with sockets and convincing end users of their various capabilities. Since very minimal device change in the socket is required, the socket technology used in the application development stage satisfies the requirement of sales phase.
When the ICs are fabricated, they need to be tested before being shipped to the customer. Various tests such as burn-in, functional, fuse test, failure analysis, etc., happen in this production phase. All ICs go through burn-in and functional test. A typical IC’s life is exemplified by a bathtub curve. Because of the inherent nature of IC manufacturing processes, only a small percentage of ICs fail very early in their life and the failure is very minimal during their lifetime. Then failure percentage goes up at the end of their life.
Burn-in tests are conducted to screen those early failures. Typical burn-in test includes testing IC devices at 125°C for 8 hrs. If the IC passes this test, it is ready for primetime. Typical socket requirements are high temperature and high insertion/extraction cycles. Since millions of manufactured ICs go through this burn-in test, more sockets are needed which drives the cost down. Socket technology using molded housings and low cost stamped contacts satisfies the burn-in test market.
After passing burn-in test, ICs go through functional test, which is often called production test. Since function is verified at this stage, bandwidth and current capacity requirements ranked high. Since millions of devices are being tested, higher the insertion/ extraction cycle count, lower the overall cost of test. Spring pin sockets (Figure 3) dominate the production test market.
Upgrade/Field Replacement Phase
After production test, the devices are either soldered to the board or sometimes placed inside the socket for replacement with future IC. These sockets are very similar to validation stage sockets, which are more robust with few insertion/extraction requirements and costs fraction of a dollar in volume. Sockets (Figure 4) made out of FR4 with screw-machined contacts are prevalent in this market.
A single contact technology cannot satisfy all requirements for IC verification. Selecting a socket that has replaceable modules of different contact technologies to accommodate all the test requirements of IC life cycle is a fruitful solution. Socket footprint defines component proximity to IC. Series resistors, capacitors, tuning inductors and others need to be placed minimal distance from IC in GHz applications. Socket footprint is important in concept/prototype, design validation, application development, marketing/sales and upgrade/field replacement phases if the bandwidth requirements are in the GHz range. Also socket footprint standardization will eventually lower the overall cost of test per IC life cycle.
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