Tom
11-03-2002, 11:50 AM
IPC Advanced Study Guide Page Reference: Pages 44 & 45, Section 1.6
To ensure precise alignment of the probe contact pins with the printed board assembly, the exact probe position and the specific networks must be furnished to the fixture developer. The designer also must correlate the characteristics of the design to those areas that need to be tested. Identifying the test location in the CAD database will allow for easy transfer of data used to drill the fixtures. This data will reduce fixture development time and eliminate the drilling of excessive, non-functional holes in the fixture base.
Bare board testing must be done from both sides. The printed board fabricator must test all circuits at the end of the net. Bare printed boards are checked for continuity (no opens, no shorts). The pins in the bare board test fixture are pointed wires, thus allowing for easy bending of the pin through translation plates to provide access to either side of the board. The test fixtures for bare board testing are usually grid-based. They are on the inch-base system with locations at 0.100 inch (regular), 0.050 inch (staggered dual grid), and 0.050 inch (quad every location).
In-circuit testing of assemblies can be done from one or both sides depending on design access. Testing from both sides requires a clamshell fixture. Clamshell fixtures are more expensive and take longer to fabricate. They must provide the accuracy for probe pins that are spring-loaded and usually require larger test lands to protect against registration problems due to tolerance build-up. Clamshell fixtures tend to lose registration and are difficult to maintain. In-circuit testers whether using clamshell fixtures or one-sided probe fixtures must have access to at least one node per net.
The minimum probing land or via should be 0.9 to 1.0 mm [0.035 to 0.040 inch]. A square shape is recommended for The 0.9 mm land to provide more material; the 1.0 mm diameter land can be round. This minimum land size will ensure less than 3 misses per 1000 probes. The use of the square land provides a larger target zone for the probe to contact. If room permits, it is a better approach to use a larger land of 1.25 mm [0.050 in.] in order to optimize the testing process. All test lands should be on 2.5 mm [0.100 inch] or 1.25 mm [0.050 inch] minimum pitch (spacing).
The decision of lands size, the amount of probing on any particular board or panel, in many instances is dependent on the fixture design and whether clamshell fixtures (testing from both sides) is a requirement. It should be noted in some applications, especially coming only from one side, the size of the probing land could go down to 0.8 mm [0.032 inch] provided the spacing between lands is accommodated, and that the land is square rather than round. Designers make their decisions based on the testing strategy and will thus call for land sizes from 0.8 to 1.25 mm.
Design for testability normally involves system level testability issues. In most applications there are system level fault isolation and recovery requirements that consider the amount of time it takes to repair a particular assembly. To meet contractual requirements, the system design may include testability features and many times these features can be used to increase testability of the assembly. It should be understood that the test philosophy in working with different assembly companies will vary based on their capability,their test equipment, and their operator proficiency.
The major concern when doing functional testing is the manner in which the board layout occurs, and the characteristics of the test connector. The use of the card edge or two-part connector is not without its problems. Initialization and synchronization, long counter chains, self-diagnostics, and physical testing are a few of the conditions that need to be considered in evaluating a design for meeting a test philosophy. Disabling functions on the printed board assembly (such as disabling a free-running oscillator and adding signal step capability via the test connector)is another consideration.
If strategic signals are brought out to a test connector or an area on the board where the signals can be probed (test points), fault isolation may be much improved. This lowers the cost of detection, isolation, and correction. Some engineers are able to correlate the I/Os at the connector edge so that a self-test or fault isolation application program can help determine which part of the circuit is not functioning properly. If properly designed, the test connector can be used to stimulate the circuit, such as taking over a data bus via the test connector.
To ensure precise alignment of the probe contact pins with the printed board assembly, the exact probe position and the specific networks must be furnished to the fixture developer. The designer also must correlate the characteristics of the design to those areas that need to be tested. Identifying the test location in the CAD database will allow for easy transfer of data used to drill the fixtures. This data will reduce fixture development time and eliminate the drilling of excessive, non-functional holes in the fixture base.
Bare board testing must be done from both sides. The printed board fabricator must test all circuits at the end of the net. Bare printed boards are checked for continuity (no opens, no shorts). The pins in the bare board test fixture are pointed wires, thus allowing for easy bending of the pin through translation plates to provide access to either side of the board. The test fixtures for bare board testing are usually grid-based. They are on the inch-base system with locations at 0.100 inch (regular), 0.050 inch (staggered dual grid), and 0.050 inch (quad every location).
In-circuit testing of assemblies can be done from one or both sides depending on design access. Testing from both sides requires a clamshell fixture. Clamshell fixtures are more expensive and take longer to fabricate. They must provide the accuracy for probe pins that are spring-loaded and usually require larger test lands to protect against registration problems due to tolerance build-up. Clamshell fixtures tend to lose registration and are difficult to maintain. In-circuit testers whether using clamshell fixtures or one-sided probe fixtures must have access to at least one node per net.
The minimum probing land or via should be 0.9 to 1.0 mm [0.035 to 0.040 inch]. A square shape is recommended for The 0.9 mm land to provide more material; the 1.0 mm diameter land can be round. This minimum land size will ensure less than 3 misses per 1000 probes. The use of the square land provides a larger target zone for the probe to contact. If room permits, it is a better approach to use a larger land of 1.25 mm [0.050 in.] in order to optimize the testing process. All test lands should be on 2.5 mm [0.100 inch] or 1.25 mm [0.050 inch] minimum pitch (spacing).
The decision of lands size, the amount of probing on any particular board or panel, in many instances is dependent on the fixture design and whether clamshell fixtures (testing from both sides) is a requirement. It should be noted in some applications, especially coming only from one side, the size of the probing land could go down to 0.8 mm [0.032 inch] provided the spacing between lands is accommodated, and that the land is square rather than round. Designers make their decisions based on the testing strategy and will thus call for land sizes from 0.8 to 1.25 mm.
Design for testability normally involves system level testability issues. In most applications there are system level fault isolation and recovery requirements that consider the amount of time it takes to repair a particular assembly. To meet contractual requirements, the system design may include testability features and many times these features can be used to increase testability of the assembly. It should be understood that the test philosophy in working with different assembly companies will vary based on their capability,their test equipment, and their operator proficiency.
The major concern when doing functional testing is the manner in which the board layout occurs, and the characteristics of the test connector. The use of the card edge or two-part connector is not without its problems. Initialization and synchronization, long counter chains, self-diagnostics, and physical testing are a few of the conditions that need to be considered in evaluating a design for meeting a test philosophy. Disabling functions on the printed board assembly (such as disabling a free-running oscillator and adding signal step capability via the test connector)is another consideration.
If strategic signals are brought out to a test connector or an area on the board where the signals can be probed (test points), fault isolation may be much improved. This lowers the cost of detection, isolation, and correction. Some engineers are able to correlate the I/Os at the connector edge so that a self-test or fault isolation application program can help determine which part of the circuit is not functioning properly. If properly designed, the test connector can be used to stimulate the circuit, such as taking over a data bus via the test connector.