Test Equipment Load Boards / Interface Boards


A load board, interface board, or DUT board is a circuit board designed to serve as an 'interface' circuit between the automatic test equipment (ATE) and the device under test (DUT).  Load boards (see Fig. 1) contain the necessary components to: 1) set up the DUT for correct testing by the ATE; 2) route the test and response signals between the DUT and the ATE; and 3) provide additional test capabilities that the ATE may not be able to provide. There are also load boards designed for the purpose of testing or calibrating the ATE itself (see Fig. 2).


Fig. 1.  Photos of ATE Interface Boards or Load Boards



An ideal load board introduces no distortion, noise, delays, nor errors to the testing process of the DUT. This means that an ideal load board is one that doesn't seem to exist at all, i.e., as if the DUT were directly connected to the ATE.  To come out with a load board as close as possible to this ideal one is the challenge to every engineer who designs load boards.


Load boards are often customized to a specific device or group of devices.  As such, complete, ready-to-use load boards are not normally available off-the-shelf.  The usual way to acquire a load board for a new product is to have it designed and fabricated. Thus, most test engineering groups have a certain level of expertise in designing and fabricating a load board.


A load board consists of a PCB with a test socket or handler interface as well as a variety of components (IC's, resistors, capacitors, inductors, relays, connectors, etc.) that make up the load board's test circuits. The typical laminate for the load board PCB is the FR4 (Flame Retardant 4 fiber glass). The number of layers of a load board PCB also varies, depending on the complexity of the design. Some load boards for complex devices may even have more than 20 layers.


Load board design takes into consideration many factors, one of which is power supply routing.  It is good practice to assign a separate power plane for every supply voltage needed by the DUT, even if two or more supplies will be tied up to the same nominal voltage.  This has two advantages: 1) noise immunity between power supplies and 2) the ability to assign each power supply to a different voltage later on. Adding sense lines as close as possible to the DUT to each power plane for output monitoring would also be helpful. Put decoupling capacitors between each power supply pland and the ground plane to reduce power supply noise.  Note that the values of the capacitors must be chosen based on the operation frequencies of the DUT.


Signal routing is another consideration in the design of a load board.  Avoid overlaying power supply planes over signal planes. There are two types of DUT signals, and guidelines for handling them in a load board differ. The first type, the low-speed digital signal, doesn't require much as far as load board design goes. Digital signals can share the same plane, but they should have their own plane. The lengths of their traces should be the same. 


Fig. 2.  Photos of load boards used for calibration

and engineering purposes only


The second type of DUT signal is the high-performance signal.  This signal type requires high-performance instrumentation for measurement, because the speed and accuracy of these signals could not be handled by the ATE. Keep the length of the cable connecting the DUT to the load board as short as possible.  Avoid parallel runs of mixed signals as well, to avoid noise coupling. Needless to say, high-performance signals need a signal plane of their own.


Load boards require connectors for cables that run to the test site.  High-speed applications commonly employ SMA and SSMB connectors. Higher bandwidth applications are better off using the SMA connector, since this screw-on/screw-off type of connector is larger and designed for higher bandwidth than the SSMB connector, which is a push/pull type of connector. SMA is also sturdier on the load board and can withstand a greater deal of mechanical stresses.


Decoupling capacitors must be placed as close to the DUT as possible - preferably directly under the socket. AC coupling capacitors and termination resistors must likewise be as close to the DUT as possible. The use of surface-mounted components is also recommended for load boards. Power supplies used for load boards must have enough power to energize all relays on the board.


See also:   Test Accessories;   Test Equipment;   Electrical Testing;   Semiconductor Manufacturing




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