The connection between the ICT and the unit under test (UUT) is made through a test fixture commonly referred to as a ‘Bed of Nails’. The nails (see Figure 1) are spring-loaded probes that make contact with the test point (unmasked vias, THT components, test pads, and gold fingers). The reliability of the contact is directly related to the fixture type, size of the UUT, size of the contact targets, target registration, and the placement of the targets.
Figure 1: A detailed view of a spring-loaded probe in a test nail
Bare board test fixtures
Test points for BBT are the lands of components and a fixture is usually universal where the flexible probe position is adjusted through alignment plates. Custom fixtures are also possible.
Universal fixtures
Figure 2: Universal test fixture showing flexible probes and translator plates guiding probes
This gets its name from the fact that the probe pin ends that go into the tester mate into a fixed size grid of contact points and consequently one fixture can be used for many boards. Most universal fixtures are on 2.5 mm (100 thou) grids and an example is shown in Figure 2.
Probe points can be anywhere on a board and are not usually in a grid layout. To get around this problem flexible probes are used. They are angled to the correct points as in Figure 2. To make the fixture three plates are drilled to support and bend the pin to the probe point on the board.
In a 2.5 mm grid the density of pins is limited to approximately a 5×5 matrix or 25 pins per cm2. This poses a problem for designs with high lead count fine pitch or densely spaced components. Angled probes would be used to gain tighter spacings and increase the pin density but where this isn’t possible a custom fixture will be required.
Universal grid testers
Universal Grid Testers (UGT), as the name implies, are a standardized form of test equipment, originally developed in the 1970s. The universal grid consists of a uniform arrangement of test probes which are used to contact with specified test points on the PCB. The original grid specification is for test points equally spaced on a 0.1" grid to give a test point (tp) density of 16tp·cm−2. However, as PCB technology as progressed and track and component spacing have shrunk, double (0.07" spacing and 32tp·cm−2) and quad density grids (97cm−2) have been produced. As the grid becomes increasingly dense the cost of the equipment increases due to the increasingly high manufacturing tolerances required.
Figure 4: Universal Grid Probe Layout
The test grid points which are connected to the PCB test points are controlled using an interface stage known as a test fixture. This fixture mat takes a number of different forms, varying from a mask which simply blanks off undesired connections whilst letting desired connections pass through, to a translation fixture which consists of pins or probes from the grid points in use to the PCB test points. The probes are generally spring loaded and are applied to the board using mechanical pressure from a loading system which pushes the fixture against the PCB which is held in place against the probes using a vacuum seal or alternatively a piston arrangement
Figure 5: Universal grid probe structure
Figure 6: UGT and mask or personality test fixture
The advantage of the UGT is that measurements may be made directly, so no reliance needs to be placed on conversions from measurements that use other characteristics. The UTT test method is also fast; as all test points are accessed at the same time, the speed of test is limited only by the measurement hardware and not by the mechanical of the test fixture.
A serious disadvantage of UGTs is that the cost of manufacture of test fixtures is high; because of this UGTs are not usually applied to low volume production runs when the fixture expenditure cannot be justified.
UGTs have difficulty in testing fine pitch boards as the pitch is often below the spacing of the probes. With the increase use of this kind of technology the UGT becomes less useful.
Custom fixtures
Each board has a fixture built for its test process including test head and plates. These fixtures have the advantage of coping with high probe densities but are more expensive and at present there aren’t many bare board test systems available that will accept them.
Single-sided boards
Figure 3: Single-sided test fixture
The single sided fixture for bare board is designed to probe the component side. This side must be tested because it contains both the leaded pads and SMT lands to be probed. A vacuum is used to force pin contact to the board and tooling holes are necessary for accuracy registration with the fixture. These will be discussed later in the section on board design issues.
Double-sided boards
Where components have been designed on both sides of the PCB double-sided probing is available. There are three options:
- Double-pass testing This uses the single-sided probing twice but with different fixtures for each side. Testing one side only causes problems for the board’s vias and leaded components.
- Via-only testing Strictly speaking this is not double-sided because the equipment only probes THT leads and vias on the bottom-side of the board. This reduces the testability of the board and is not usually recommended.
- Double-sided clam shell fixture This provides 100% testing of the board because all the lands and pads are probed simultaneously and all routing and plated through holes are checked. Figure 4 and Figure 5 show diagrams of the double-sided clamshell fixture.
Figure 4: Double side bare board testing
Figure 5: Double-sided clamshell fixture
Probes
Figure 6 shows some of the common probe types used for BBT.
The spear probe is used for probing pads where solder mask residue may be present. It has a fine point that could leave a mark after probing. Some companies use this as a check for probing.
The blunt tip probe avoids the piercing of fragile lands and has more contact area that improves probing accuracy.
The pyramid head probe is used to probe plated through-holes. It comes in 3 and 6-sided varieties.
Probe diameters are typically 0.25 mm (10 thou) larger than the finished hole and can be between 30 mm and 100 mm long.
Figure 6: Common probe types for BBT
Assembly test fixtures
The fixture for assembly ICT is similar to BBT but different test equipment and fixtures are used. Test points are test pads, THT pins and vias. In BBT, the Universal fixture has a universal test head and several alignment plates are used to direct the probe to the test point. In contrast, the assembly fixture is custom built and has the pins mounted in two plates and wired to the test head of the test system. The most common fixture style is the single sided fixture as in Figure 7. The double-sided clamshell fixture is the same layout as the BBT clamshell in Figure 5.
Figure 7: Single-sided ICT test fixture
Probe types
Figure 8 shows the common probe types. Assembly probes are chosen in the same way BBT has different probes for different uses. The serrated tip is used for non-via pads or plated via pads. The crown tip probe is used on components because it twists as it is compressed, creating a scrubbing action that makes a good contact through flux or oxides on the tip, although this can cause surface damage to copper on a board. The spear tip probe is less commonly used now because it can slip off the probe pad, pierce pads and is susceptible to damage.
Figure 8: Common probe types for testing assemblies
Requirements of a test fixture manufacturer
In order to provide a PCB test application (program and fixture) test fixture manufacturers require certain items, the deliverables, which must be of the latest revision. Incorrect or out-of-date deliverables will cause extra debug time and possibly extra time on the client’s site. The following list clarifies what is needed and why.
- Gerber files: This means top and bottom layer files, pad files, aperture or match file, solder mask and silkscreen or component identification.
- Drill file: The XY coordinates used by the PCB manufacturer to drill the PCB.
- CAD schematics: These can usually be imported into the fixture manufacturers CAD files and it makes component identification much quicker.
- All PCB and document revision numbers: It is vital that to know that the correct information is being used.
- A blank PCB: Sometimes it is necessary to electrically trace a particular track and the blank PCB allows this to be done.
- A built PCB: This is used in manufacturing the fixture, mostly to check for component heights. It does not need to be a working board.
- An agreed number of tested PCBs: These must be manufactured using the same process as the real PCBs. Hand soldered samples are not acceptable where wave solder will be used. These are used both to choose the correct probe types and to test the program. Thus if the manufacture is different, wrong probes may be used.
- Fresh copy of the schematics: Often schematics supplied for quotation purposes are initial versions.
- Test specification: Used for functional testing.
