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Utilizing Limited Access Tools on ICT System

Case Studies

Modern electronic products are under very heavy pressure of losing further electrical test access at in-circuit test (ICT) due to product miniaturization and sophistication, and hence a means of assuring the quality of the product. This is ironic considering that this trend is driven by consumers’ insatiable need for better performance in acceptable form factors. Many of the tools available for limited access testing which were previously predominantly used in higher end products now ind themselves being applied even on consumer products. At the same time, new technologies have been developed to complement existing ones. This means users today have an arsenal of tools at their disposal. This article will introduce the seven most prominent and effective ones collectively known as the Keysight Medalist i3070 ICT Super 7 suite. 

In Circuit Testing (ICT) is synonymous to Bed of Nail testing where a printed circuit board assembly (PCBA) is being tested using a test probe and ixture (see igure 1). The test probe on the ixture will have to make contact on the PCBA through a test point. The ICT rule of thumb is that every device pin or node connection in the PCBA requires a test point (see igure 2). However this has always been a challenge. This is because current PCBA technologies like differential high speed signals and shrinking PCB sizes are making it harder to put test point in every nodes or device pin in the PCBA. Figure 1. In-circuit Test method showing a test point, test probe and a ixture (Bed of Nail) used to test a device or PCBA. 

 

 The Keysight Technologies, Inc. Medalist i3070 ICT platform’s most comprehensive limited access offers seven powerful test tools and methodologies to help users overcome challenges caused by limited access. This Super 7 suite consists of the following:

  •  Access Consultant
  • Bead Probe 
  • Cover Extend 
  • Drive Thru 
  • IEEE 1149.1 Boundary Scan 
  • IEEE 1149.6 Boundary Scan 
  • Silicon Nails 

This case study examines how, without sacrificing test coverage, the Super 7 suite on the Medalist i3070 can boost in-circuit test performance to help ensure better board quality. Two board types are used for this case study: 1. Board A – high-end low-volume network switch PCBA 2. Board B - high-volume consumer PCBA 

Cutting down on nodes 

In the first step, the boards are analyzed using the Keysight Access Consultant to identify nodes that can be removed without sacrificing test coverage. The Keysight Access Consultant analyze the PCBA board information and identifies nodes that will be able to removed test probes without sacrificing the test coverage (see figure 3). The following files are needed to run Keysight Access Consultant: 

  • “Board” - Keysight Medalist i3070 file format which contains the board device information and node connection. 
  • “board_xy” - Keysight Medalist i3070 file format which contains the board device xy coordinates and node accessible information. 
  • Config file - Keysight Medalist i3070 file format which contains the system configuration and enabled test techniques. 
  • Digital libraries – Digital device test library
  •  BSDL file for boundary scan device The Keysight Access Consultant analyzes probe requirements by selecting the analysis mode and test technique.

In this case, the analysis mode selected for both board A and B is “Update (Allow Change to Probe Access)” (see figure 3). This will analyze the test techniques installed and enabled in the board directory ‘config’ file. The ‘board_xy’ file contains the list of accessible nodes which you can use the Access Consultant to mark nodes for access removal by selecting them under “Probe Access”. When you click “Update”, the data in the Board Consultant will be updated. Upon selecting “Save Board Information”, the ‘board_xy’ file will show “REMOVE_PROBE” for the nodes you marked.  

 

The test techniques selected for boards A and B are as follows: 

  • Keysight Interconnect plus (1149.1 and 1149.6) 
  •  Keysight Silicon Nail 
  • Keysight Cover-Extended
  •  Keysight Drive Thru 

The Keysight Access Consultant probe analysis shows a nodes reduction of 1418 nodes for board A (see graph 1) and 166 nodes (See graph 2) for board B. See tables 2 and 3 for details of the suggested probe removal per test techniques. The high-end board A with a total accessible nodes of 3312 has the most number of nodes removed using Keysight Interconnect plus (Boundary Scan) with a combination of both IEEE 1149.1 and IEEE 1149.6 capabilities. There are a total of 14 devices for board A connected on a single boundary Scan chain which maximize the number of nodes in interconnect coverage (see table 2). The majority of differential nodes are still on the IEEE 1149.1 standard as the devices used on these differential signals are not compliant to IEEE 1149.6 standards (see graph 1). However, this is set to change as manufacturers use more and more 1149.6-compliant high speed differential devices to extend coverage of boundary scan interconnects on high speed differentials.  

 

 

Bead Probe 

One last dilemma for the test engineer is the case where a probe is really needed to test a device but a test point is not possible due to high speed signal limitation. The other case where bead probe will be able to help on fixture cost reduction is to replace testpoint that requires 50 or 39 mils probes in the fixture using bead probe which use 100 or 75 mil flat head test probes

 

Summary 

By using the Keysight Super 7 suite of tests for limited access boards, it can be seen that the total number of probes can be significantly reduced on both board A (high-end PCBA) and board B (low-cost high-volume PCBA). This will help to reduce fixture costs, digital requirements for ICT and also lower maintenance costs. These limited access solutions for incircuit testing of PCBAs will no longer be confined to only high-end PCBAs as more designers will be forced to adopt high-speed differential signals and work within the confines of smaller PCB footprints with embedded trace and components 

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