Basic Probe Fixturing

Spring contact probes are used as the interconnection medium between the unit under test (UUT) and the tester. Below is a general discussion limited to the most common types of probe fixturing.

Loaded Board and Functional Test

The test fixture environments that use spring probes can be grouped into two categories: loaded board test and functional test. Loaded board test refers to measuring all or key components installed on a printed circuit board to determine whether they are the correct components and/or within specified parameters or tolerances for the device. This type of test is commonly referred to as In-Circuit Test (ICT).

The second category of loaded board test may involve simple open and shorts tests on the board to verify manufacturability standards. During this test, spring probes contact test points and the tester performs a Manufacturing Defects Analysis (MDA) test.

In short, the goal is to find production deficiencies, such as unsoldered device leads, cold solder joints, bent connector leads not fully installed into plated through-holes, or solder defects like solder bridges between two traces. Both of these types of loaded board tests are supported by automated test equipment (ATE) used to perform these functions in very short periods, often in seconds.

Functional test, as the name implies, involves applying power to the loaded printed circuit board to verify whether the circuit performs the functions it was designed for. This test typically requires custom-built test equipment and custom test fixturing, which use cabled connectors in conjunction with test probes to apply power and ground. The probes also make the connections to specific areas of the board during testing.

Functional testing takes significantly longer than in-circuit testing, as the board components are actually exercised during the test. Most test fixtures have common components: probe plates to mount the probes, receptacle top or diaphragm plates to support the UUT, tooling to register the UUT to the probe pattern, and wiring to connect the probes to the test equipment.

Probe Plate

The most common types of test fixtures use spring contact probes combined with a receptacle. The receptacle is designed to press-fit into a drilled hole in the probe plate and serves as the attachment for the connecting wire. The probe plate must be made of a material suitable for precision drilling and rigid enough to support the combined force of the probes without flexing.

The most common probe plate material is Garolite, also known as G-10 or FR-4 in its fire-retardant grade. G-10 is a glass epoxy laminated material that allows for precise machining, has a very stable thermal coefficient of expansion (TCE), and is extremely rigid. Other materials include composites and laminates like Phenolic and Bakelite, but they are more difficult to machine. Lexan and acrylics are used but lack the strength and rigidity of G-10 and can fracture easily when stressed. Delrin and other thermoplastics should be avoided as they are thermally unstable and do not provide superior anchoring characteristics compared to G-10.

The probe plate should be thin enough to drill accurate holes but thick enough to provide adequate rigidity and sufficient guidance for the receptacle. For longer probes with 0.250 (6.35) travel, a thickness of 0.375 (9.53) to 0.500 (12.70) is recommended. For shorter probes with 0.050 (1.27) to 0.100 (2.54) travel, a thickness of 0.250 (6.35) is sufficient.

The receptacle is designed to press-fit into the probe plate. This is accomplished by drilling a hole slightly oversize for the receptacle diameter but undersized for the press ring. A recommended hole diameter is listed with each receptacle. The receptacle is dropped into the mounting hole and, if the hole size is correct, slides down until the press ring contacts the upper surface of the probe plate. An insertion tool should then be used in combination with a “soft” mallet (nylon, rolled leather, or paper) to drive the receptacle into place. Insertion tools leave the top of the receptacle at a fixed distance from the probe plate surface. A metal-headed hammer can impart too much striking force and may damage the receptacle.

Once the receptacles are installed, wiring is required. Receptacles are available in various forms to meet wiring needs: solder cups for larger gauge or multi-strand wire, crimp terminations for larger gauge or stranded wire, and wire wrapping, which involves twisting a single strand of wire around a square wire post. The most common post diameter is 0.025″ square, accommodating wire gauges from 30 to 26 AWG while allowing multiple wraps per pin.

The fixture interface can include discrete connectors, which are connected via cable, interface boards, or individual interface pins to connect to the tester. The tester platform dictates the interface method. Probes are then inserted into the receptacles to populate the fixture. Use a plastic piece to press on the probe tips when inserting them into the receptacles. Metal tools should be avoided to prevent damage to delicate probe tips.

Top or Diaphragm Plate

The guidance and tooling for the UUT depend on the type of test performed. Loaded circuit board assemblies may be mounted above the probes on a top or diaphragm plate. This plate is drilled to match the probe array on the probe plate. The drilled holes in the top plate guide the probes to the test targets on the board. Tooling pins pass from the probe plate, where the receptacles are installed, through the top plate to tooling holes on the UUT, ensuring accurate registration of the UUT to the probe array. The UUT may also be aligned with clips, but final alignment should depend on inserting a registering tooling pin into a precise datum hole on the printed circuit board.

Fixture Actuation

The application of force to drive the UUT down onto the probes depends on the tester design. Many loaded board testers use pneumatics to assist in forcing the UUT down or vacuum to draw the UUT down to the probes. ATE platforms commonly used in medium to high volume production environments use vacuum, mechanical, or pneumatic clamps to secure the test fixture to the tester and system vacuum to hold the UUT in place during the test cycle.

When vacuum is used, the fixture has a sealing gasket on the top plate that follows the outline of the UUT and ensures there are no open vias or holes that would hinder the evacuation of air between the probe plate and top plate. In cases with open vias or missing components, a vacuum box can be adapted to the top plate to hold the UUT in place during testing.

Pneumatic test fixtures use mechanical cylinders to move the board and top plate down onto the probe array or platen plate using nylon pressure fingers that push down on the UUT to make contact with the probes. Pneumatic test fixtures are used when precise registration is required, as they offer more control and are effective for staged actuation or non-vertical probe movement, such as when probing a side-access connector.

Mechanical fixtures, which rely on human intervention, are common when test equipment is custom and doesn’t have provisions for vacuum or pneumatics. These mechanical fixture kits provide labor-saving mechanisms, such as over-clamp gates with cam mechanisms to help minimize the work needed to force the UUT down onto the probe array. These kits are typically not suited for large numbers of test points or probes due to the amount of spring force required.

Whether performing in-circuit or functional testing, and whether using vacuum, pneumatic, or mechanical actuation, spring contact probes are the essential component for board testing.