Assembly Testing (PCB)

Related articles: Fabrication Testing, Testpoint System

Assembly testing relates to the testing of a printed circuit board at the post-assembly phase of manufacture, after the board has been populated with all components specified in its associated Bill of Materials (BOM). As such, it is often referred to as in-circuit or electrical testing. This method of testing typically involves (but is in no way limited to!) the use of a manually-configured bed-of-nails fixture. Depending on the type of testing being performed using the fixture, the board may or may not be powered up.

Two common test scenarios for in-circuit assembly testing are:

1. Simultaneously probe one dedicated testpoint for each net (or net of interest). This is done by pulsing a signal through each probe individually and detecting signal reception results by all other probes.
2. Test specific components/buses to ensure that the device itself is operating properly. These are specialized tests that can be conducted using a variety of testing methods. In the simplest case, a bed-of-nails fixture can be used to test across component pads.

Testpoint Location Constraints

• Testpoint locations typically need to conform to a minimum spacing requirement. Test equipment, such as bed-of-nails fixture probe heads, have a body size that constrains their proximity to each other. Unless a custom fixture is used, testpoint locations are often required to be on a specific grid. Fabricating a custom fixture is more costly and time-consuming than configuring a reusable bed-of-nails fixture.
• Testpoints can be required to be located according to a minimum clearance with respect to component bodies. The probe heads of most test fixtures tend to be larger nearest the fixture, tapering down to a smaller size as they approach the shank of the probe. Taller components require more clearance. It is therefore best to consider the maximum component height on the board and define an overall body clearance that factors in this worst-case scenario.
• Testpoint locations can be required to be positioned according to a minimum clearance with respect to the board edge. The process of contacting the board with a flying probe pair or a bed-of-nails array of probes can be tough on the board itself, and the board may flex to some extent under the strain of automated testing. For this reason damage to thin, fragile boards, is minimized when testpoints are located at a distance from the board edges.
• A dedicated probe-accessible testpoint pad/via is usually required to be added to each net during design for several reasons:
  1. Bare copper component pads are usually rendered inaccessible to the presence of placed components.
  2. Unless the board is specifically designed with testing in mind, component pads and vias are not likely to be located on the required testpoint grid.
  3. Even if a component pad itself seems suitable to be used as a testpoint location, clearance constraints to the component body often prevent its use.

Exceptions and other Considerations

• While bed-of-nails fixtures are a common method of assembly testing, it is by no means the exclusive method. Among the variety of alternatives are cases where flying probe devices are used – with care, so as not to collide with placed components.
• When performing an electrical test on a specific component, the component pads might be accessible by a test probe to some degree. However, it is important to note that the component body often obscures the center of such pads. In other words after the board has been assembled, the accessible portion of the pad rarely includes the pad center location.