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Cover-Extend Technology for In-Circuit Test

Case Studies

The computer industry is facing technical challenges in their motherboard manufacturing test due to the introduction of a new generation of central  processing unit (CPU) sockets and ball grid array (BGA) devices with higher pin  counts. This case study illustrates how the Keysight Technologies, Inc. Cover- Extend technology, which is part of the Medalist VTEP v2.0 Powered vectorless  test suite, can help to enable test access for situations where test access  becomes increasingly limited with usage of high complexity components on  computer motherboards. This article was first published in Circuits Assembly, January 2009, and is  republished here with kind permission from UP Media Group.  The higher pin counts on CPU sockets and BGA devices along with high speed  differential signals are posing new challenges to existing In-Circuit Test of  motherboard printed circuit board assemblies) (PCBA) on the   manufacturing floor.  

The following are the manufacturing test challenges:  

1.  The new generation CPU sockets consist of about 55% signal pins. Most  of these signal pins are high speed differential signals that are no longer  accessible for ICT probing (see Figure 1).

2.  The demand for smaller, low cost computers is also driving down the size of  the motherboard, causing constraint in the PCB size and loss of access for  ICT.

3. Too many ICT probes under the BGA devices or CPU socket can result in  solder ball crack (Figure 2).  

4. How to lower the cost of test without sacrificing test coverage.  Keysight’s Cover-Extend Technology is a hybrid between VTEP and Boundary Scan. It draws the best from what each technology offers and enhances the overall capability of Keysight in-circuit test systems. In short, VTEP and Boundary Scan are the main building blocks providing coverage extension for manufacturers even as fewer test points are accessible these days. The Keysight VTEP unpowered vectorless test method uses a stimulus signal that is driven by the in-circuit probe using a sensor plate to measure the capacitance between device pins or BGA balls and the printed circuit board (PCB) pads. Figure 3 shows the number of pins tested on a circuit using VTEP test. The VTEP methodology requires physical test access (i.e. test probes) to deliver this stimulus signal. With Cover-Extend, however, the stimulus signal is delivered via a Boundary Scan device  Boundary Scan is a world-wide standardized test methodology (IEEE 1149.x standard). It provides limited-access capability – i.e., the ability to control the I/O functions of individual pins through the use of only four pins on the test access port.  

 

Cover-Extend works as follows: 

1. The VTEP sensor, which is able to capacitively pick up stimulus signals, is placed on the component to be tested (e.g. a connector). 

2. The Boundary Scan device does not require test probes on every pin. 

3. As per the IEEE 1149.x standard, using only the test access port, users can deliver the necessary stimulus signal to the connector. 

4. A defect (e.g. an open) on the path between the Boundary Scan device and the VTEP sensor will affect the stimulus signal that is bound for the sensor.

 5. The result is captured and diagnosed by the ICT system and thus, the defect is detected.  The new generation of motherboards consists mainly of CPU sockets, Input/ Output (I/O), BGA devices and power circuitry where in about 50% of the total targeted testable pins and solder balls are CPU sockets and connectors. 

Traditionally, CPU sockets and connectors are tested at ICT using vectorless testing. However, these test strategies are rapidly eroding as the designers are not able to place test points at every signal pin and solder ball on the motherboard, forcing the test engineers to look for an alternative test strategy. Results from a study using Cover-Extend on a Keysight Medalist i3070 In-Circuit Test system are shown in Figure 7. The data shows the difference between good signal pins and open pins. New generation notebook motherboards are seeing dramatic changes in their design, driven by cost pressure, size reduction, as well as demands for longer battery life and increased performance. Notebook motherboard PCBs will continue to shrink in size even as they need to be able to accommodate the new generation of CPU and BGA devices and meet the various demands mentioned above. 

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