[Introduction]Generally, a bed of needles is used to test a circuit board that is not powered on, and technologies such as direct digital synthesis (DDS) and discrete Fourier transform (DFT) are used to generate stimulation signals for analog measurement and analysis, so as to allow the online tester (ICA) to measure Actual data such as inductance, capacitance, impedance, and resistance to confirm that all device-under-test (DUT) test node results are within tolerance and open, short, wrong, or reverse polarity. These are all measured without powering up. Relay multiplexers can be used to connect probe contacts to the board’s analog channels or digital drivers/sensors (D/S) (Figure 1).
In-Circuit Testing (ICT) is a method of analyzing Electronic products in production.
Generally, a bed of needles is used to test a circuit board that is not powered on, and technologies such as direct digital synthesis (DDS) and discrete Fourier transform (DFT) are used to generate stimulation signals for analog measurement and analysis, so that the in-line tester (ICA) can measure inductance and capacitance. , impedance, and resistance to confirm that all device-under-test (DUT) test node results are within tolerance and for open circuits, shorts, wrong parts, or reverse polarity. These are all measured without powering up. Relay multiplexers can be used to connect probe contacts to the board’s analog channels or digital drivers/sensors (D/S) (Figure 1).
Figure 1: Typical 2 x 16 bed of needles relay multiplexer (only one channel shown)
In some more advanced systems, the ICA module can also be used to do part of the device functional test (FCT) by powering up the circuit and measuring the input and output characteristics with load. This test is usually done separately using another test adapter. The reasons for this are as follows:
First, the probes of the ICT bed of needles cannot carry the required supply voltage or load current to fully functionally test powered devices. The heavy-duty probes of a dedicated FCT test bench must be able to withstand high current or high voltage without overheating, arcing or excessive wear. The downside is that these overloaded probes take up more space, so FCT test adapters typically only check one DUT at a time.
Second, the programmable power supplies, relays and electronic loads inside the ICA are also not suitable for high current testing. If simply switching to a larger power supply, the higher current can seriously interfere with sensitive ICT measured analog quantities causing errors, including ground bounce, line voltage drops, and inductive load switching transients. Measurements made with dedicated FCT adapters typically have lower resolution and larger filters and are therefore less sensitive to interference. Additionally, the power and relay contacts are more robust and therefore capable of switching currents in excess of 1 amp.
Third, the relay interface hardware and software control is typically through a parallel input output (PIO) controller and relay driver to change the relay configuration (Figure 2). Switching speed of relays is usually not an issue in ICT applications because relays are reconfigured by multiplexing connections from one set of pins to the next after each DUT test. In the case of FCT test adapters, the relays are used to change the functional test settings of each DUT with each test, so the control data throughput of the relays is higher. In a dedicated FCT setup this would not pose a problem as only one DUT is checked at a time, but if multiple devices are to be tested with an ICT/FCT adapter then the speed limit of the relay control can be a bottleneck.
Figure 2: Test System Diagram
Finally, while ICT can complete measurements in milliseconds, FCT procedures cannot take measurements immediately when the device is powered on, so it is much slower than ICT, so it is necessary to make sure that the FCT procedure has output before taking measurements to get reliable data. Typically, FCT procedures take five to ten times as long as ICT to measure the same product. If testing is consolidated into one ICT/FCT platform, the FCT part may hinder production. If the two programs are separated, one ICT instrument can feed multiple FCT test benches to increase throughput and reduce blocking.
However, for RECOM Power’s newly developed DC/DC product line, the extra cost and test time brought by two independent test adapters was not acceptable, and a way had to be found to combine the speed advantages of ICT with the 100% of functional testing. % Quality Assurance (all integrated in one test adapter) combined. This is a complex technical challenge: the product family covers devices with up to 6A output current and 60V input voltage. Each PCB board contains 40 semi-finished Modules, which means parallel testing with powerful and durable power supplies. So not only is the data throughput high, but any timing errors can be a problem. RECOM has contracted with Elmatest in the Czech Republic to jointly build a combined ICT/FCT test adapter for Teledyne Teststation LH used by EMS providers.
Zdenek Martinek, an application engineer at Elmatest, realized from the beginning that this was no ordinary project. There are several important problems to be solved: how to combine ICT/FCT into one connection board; how to handle such high relay control data throughput; how to speed up the FCT procedure, and how to not damage sensitive probes at high power. Working closely with Markus Stöger from RECOM’s R&D department, they found a solution to these problems.
The first problem to be solved is how to combine ICT/FCT in the product’s board design. Each PCB contains 40 individual circuits. These modules are not parts but a finished product that has been produced, packaged and silk-screened, so not all internal nodes can be connected to the ICT pin panel. This is intentional. The DC/DC converter switches at a high internal frequency, and the metal case and multilayer PCB form a complete six-sided Faraday cage to avoid EMI problems. Any external connection to the internal high-frequency switching node will create a path for EMI to pass through the EMC shield and emit radiation, which can cause errors in measurements.
The solution to “How to do ICT testing of sealed products” is to make a test module for each connection board. The test module can verify that each connection board is normal by connecting to all necessary ICT nodes. Once the test module has passed the regular ICT procedure, the remaining modules only need to be checked for FCT.
Figure 3: Front and back views of the PCB connector board, with the ICT test module in the corner
The code required to execute a test and measurement procedure is called a test vector. The configuration of input, output and analog channels required for the measurement is transmitted in “burst data”. These configurations are loaded into local on-board memory and then activated simultaneously by timed triggers. The configuration is latched until the test is complete and the measurement data is transferred back to the CPU. At the same time, the next burst data will be loaded into the register in advance to wait for the next trigger signal. This approach enables ICT to achieve extremely high data throughput rates of about 4µs per vector.
But the standard relay drivers used by the GenRad Teststation are driven by a parallel input/output port (PIO) controller that receives commands from the control PC via MXIbus (Figure 2). This configuration is too slow for our project as we want to use a high speed system controller to control the relay configuration, handling different FCT measurements in one test vector. To increase relay switching rates, RECOM’s test adapters use an “active burst” technique to implement a new relay driver topology.
When performing an active burst, some relays are not driven by the PIO control card but are driven directly by the D/S outputs, which remain active until the ICA measurement is complete. Each D/S can be programmed with 9 independent functions (idle, drive low or high, sense low or high, hold, drive deep serial memory, sense deep serial memory and collect CRC data). In this example, we used the drive loop to power the relay. The D/S driver output is limited to TTL voltage and current levels, which is usually insufficient to drive a relay without a separate drive loop, but if a Darlington transistor current amplifier relay coil is used to make a test adapter, the D/S module can be wound around Directly operate the relay through the PIO controller, which not only makes relay control instant but also makes coding easier.
The second problem that needs to be solved is how to speed up the test of the FCT, because waiting for the analog level to stabilize can make the whole test time too long. The trick is to take advantage of the existing processing power of the ICA system and use waveform generation and analysis techniques like Direct Digital Synthesis (DDS) and Discrete Fourier Transform (DFT) because they are inherently faster than any analog bridge balance measurement technique. This breakthrough made us realize that these advanced techniques can be used to determine power-on functional tests. Rather than applying a steady load and waiting for the output to settle before measuring the input and output currents and voltages, the output load pulses are delayed by a few milliseconds and the processing results are used to derive the final output characteristics. This reduces measurement time by 80%.
Figure 4: 6-terminal impedance measurement
A significant development problem was matching this dynamic load and power switching to the old Spaghetti software used by GenRad’s test equipment, which consisted of Pascal, Assembler, and Basic. Although GenRad ceased to exist as an independent company back in 2003, it deserves credit for its durable design, which allows the use of the latest operating system on its original hardware even today.
The solution to the second problem also solves the third problem: how to avoid damaging the sensitive probe. Since the load current is only pulsed for a short period of time, even if 6A peak current flows through a probe rated at only two amps, there will be no significant localized heating at this very small contact point. The on/off time ratio can also be programmed so that even with sequential measurements, the probe tip is cooled between pulses to avoid burnout or charring. This pulsed load technique also ensures that the power supply is not overloaded.
The ICT can also measure the resistance of the internal voltage divider used to pre-set the output voltage, enabling the test system to automatically derive the output voltage, output current and input voltage range from the ICT, and then transfer this data to the FCT test program for proper execution. function test. This eliminates the possibility of damage to the product, expensive pin board or programmable power supply due to the operator mistakenly setting the FCT variable out of range.
The result of using these techniques is a combined ICT/FCT test time of about 1.8 to 1.9 seconds per DC/DC module, which means that 100% of the entire PCB connection can be tested in less than 80 seconds, including To remove the PCB that has been tested and put the next PCB to be tested into the test adapter. The minimum number of runs is 5,000, and the cumulative time savings contributes to the success of the entire product line. As such, the design of RPM modules has expanded from a single series of eight models to now a total of twenty-two models in three series, all with the same packaging and test adapters.
Figure 5: Finished adapter under test