Cradle & Layers
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4 minute read
You can think of a test jig as a giant connector, bringing signals between the device under test (DUT) and the test engine. The primary mechanical component of a JigsApp bed-of-nails is a highly dimensionally stable printed cradle, designed specifically for each board. Its printed features and support pins together align the board precisely in both position and angle. Replaceable spring-loaded probes embedded in the cradle contact designated test pad positions on the DUT. The probes are connected to a probe PCBA, which has mounting features that line it up with the probes.
Connecting the probes through a probe board and connector makes for a more reliable system that is much easier and faster to assemble than the old-school hand-wired ones. Connectors also help improve signal integrity.
When the device under test (DUT) is first inserted into the jig, it sits stably on the spring-loaded support/alignment pins without touching the probes. (This helps eliminate damage due to fat fingers.) Contact is made by lowering the cover plate, which has pressure pins to distribute the force evenly. As the cover is moved into place, the probes make contact with the test pads, all at once or (optionally) in a specified sequence. See Generating a Test Point Report.
The cradle is supported and attached to the chassis by means of a support plate. The support plate includes an actuator (typically an over-center lever) that provides the clamping force to the cover, compressing the spring-loaded contacts until the DUT rests on the top of the cradle’s support layer.
The cradle has several such designated layers, and contains embedded hardware including the probes and support pins. The layers allow for a wide range of customization to fit the needs of a given DUT, including reliefs for large components and overhanging connectors, access to the other surface of the board, and accommodating auxiliary test hardware. The cradle has the contacts and other hardware placed at separately configurable heights which allows connections to occur in a specified order. A common example is connecting all the grounds, then the power, and then the signals. This gives extra ESD protection as well as protection against accidental hot plugging. (Probes are not designed for use as switch contacts, so repeated hot plugging may lead to premature probe wear.)
Our input method fits seamlessly into your existing work flow. The design layers are input as vector files (Gerbers), so you can specify the fixture at layout time, using the same PCB EDA software. This makes correct alignment a breeze, and saves a lot of work. As our method is fully parameterized, if there’s a board revision, the jig can be automatically updated to match, with no extra work. Here’s how it goes.
| Layer | Description | Suggested File Name | Type | Required? |
|---|---|---|---|---|
| Outline / Board Cuts | Outline of the DUT. This is used to set the coordinates of all the other inputs and is used for DRC. | outline.gbr | Meta Layer | yes |
| Pressure Pins | Positions of the pressure pins. Circles placed in this Gerber are extracted and used to generate the pressure plate. This way you can see easily the positions when designing and place them like any other footprint. | pressure_pins.gbr | Indirect Layer | yes |
| Clearance | The clearance layer defined the top of the cradle. The DUT passes through the clearance layer without obstruction. The height of the clearance layer is set to protect the probes from damage and its outline hugs the DUT closely, providing one source of in-plane alingment in XY and angle. The DUT is placed through the clearance layer and onto spring loaded support/alignement pins whose height is set to hold the DUT above the probes. | clearance.gbr | cradle layer | no |
| Support | The DUT comes to rest on the top of the support layer when the cover is down. (The spring pins don’t bottom out.) The support layer is designed to keep the DUT parallel to the probe layer during testing, provide vertical support, and minimize deflection in the DUT. The support layer is generally sparse to avoid short bottom-side components. | support.gbr | cradle layer | no |
| Probe | The probes are friction-fit into the probe layer and base layer. The support pins and probes can be set at different heights within all layers to support different DUT requirements. | |||
| Bottom-side components taller than the probe travel length require reliefs to allow them to pass through. | probe.gbr | cradle layer | no | |
| Base | This layer defines the bottom of the cradle. In order to provide mechanical rigidity, this layer is normally featureless except for reliefs for very tall components and access to the surface of the PCB, for example to allow optical access. | base.gbr | Cradle layer | no |
| Probe Positions | This layer allows the position of the probes to be set using the same Gerber method as the pressure pins. As this method lacks essential information (see Generating a Test Point Report), it can only be used with our Arc Extraction Tool as the first step of generating a test point report. | Indirect Layer | no, unused in project entry |