Printed Circuit Boards (PCBs) are used widely across modern industries to incorporate electronic components into larger systems and subsystems. These include (but are not limited to) vehicle systems such as automobile and aerospace, industrial systems such as machinery, and commercial systems such as smartphones. In these applications PCBs are often subjected to harsh environments and expected to function under challenging conditions. Strain gauge testing allows for the objective analysis of the strain and strain rate levels that PCB components may be subjected to throughout their lifecycle. The purpose of this testing is to characterize strain-induced failures and failure modes.
The Standard: IPC/JEDEC-9704A
IPC/JEDEC-9704A is a guideline for PCB strain gauge testing. This standard defines a methodology for strain gauge placement on PCBs during testing, as well as test procedures throughout the manufacturing, population, and system integration processes. The guideline allows for independent strain gauge testing at any PCB manufacturer, or assembler, to quantitatively characterize board flexure and assess the risk of damage to the PCBs. The focus areas covered in this standard are:
- Test Setup and Equipment
- Strain Measurements
The methodology defined in IPC/JEDEC-9704A is used for qualification and acceptance testing against strain levels specified by the component supplier, the customer, or internal best practices. The standard is designed to improve PCB manufacturing operations by identifying steps in the manufacturing, assembly, and handling processes where strain limits are exceeded, and corrective actions need to be taken.
1. Test Setup and Equipment
Printed Circuit Boards
PCBs experience minimal strain before SMT (Surface Mount Technology) reflow, therefore strain testing and characterization is only necessary following the SMT reflow process. The minimum requirements for testing stipulate that at least two PCBs are instrumented with strain gauges:
- PCB with surface mount components after reflow soldering
- PCB with surface mount and through hole components after wave soldering
Components and Devices to be Tested
It is up to the manufacturer as well as the customer as to what components must be strain gauged and tested. A list of recommendations is made in the standard:
- Area Array Components with a package size greater or equal to 27mm x 27mm OR finer pitch components with a pitch size lesser or equal to 0.8mm, that has a package size greater or equal to 10mm
- Non-Area Array Components such as multi-layer ceramic capacitors
The recommended strain gauge parameters for this type of testing are:
- Stacked rosette strain gauges (0/45/90)
- 1 to 2 mm2 nominal gauge sensor area
- 120- or 350-ohm strain gauges
- Prewired strain gauges or all solder pads located on one side of the strain gauge
The RF9 Strain Gauge Rosette from HBM is a specialty strain gauge, designed for use in PCB testing and is compliant as per the standard. This rosette helps determine the biaxial stress states with unknown principle stress directions on small surfaces.
Strain gauge placement for Area Array Components, as well as Non-Area Array components, should follow the preferred placement locations mentioned in the standard unless otherwise agreed upon by the manufacturer and consumer.
Before a strain gauge is attached to the PCB under test, it is important that the board is prepared for strain gauging. A suitably smooth and even surface is required where the strain gauge will be positioned. Once the surface has been found then the following steps can be followed for attachment:
- Clean surface with non-aggressive solvent
- Apply adhesive to the surface
- Attach the strain gauge to the surface
- Use Teflon tape to apply even pressure to the strain gauge as per the adhesive instructions
- Remove Teflon tape
- Apply strain relief to the strain gauge wires
HBM’s RMS 1 is a suitable cleaning agent for this application and Z70 is their recommended cold curing adhesive for bonding strain gauges to PCBs.
2. Strain Measurements
Multiple strain gauges must be sampled simultaneously during PCB testing, as sequential sampling may lead to erroneous strain measurements. When sampling, it is important to consider the signal conditioning, sample frequency, as well as the bit width of the recorded data. The following recommendations are made in the standard:
- Sample at 2kHz for high strain rate events
- Sample at 500Hz minimum for low strain rate events
- A sampling resolution of 12 to 16 bits
- Condition the signal to prevent clipping while maximizing the dynamic range
- Use a data acquisition system (DAQ) that has built in low pass filtering to reduce measurement noise
- Multiple measurement channels should be available, with a minimum of three for strain gauge rosettes
- The excitation voltage for the strain gauges should be adjustable to reduce their heating due to electrical current, but in most cases 2V excitation should provide adequate performance
The QuantumX MX1615B from HBM is a compact strain gauge measurement amplifier and DAQ that meets the above recommendations for both dynamic and static testing. It is robust, accurate and boasts individually configurable sample rates up to 20 kHz, on each of its 16 channels, at a 24-bit A/D conversion per channel.
The strain gauge measurement equipment must be calibrated as per the manufacturer’s recommendations.
HBM’s QuantumX MX1615B comes with a working standard calibration certificate stored in the module as per ISO 10012.
Manual simulation is required to characterize the assembly process. This is done by replicating the assembly processes on a strain gauged PCB in a twofold simulation as defined by the standard.
Firstly, observed handling is used to represent the usual handling a PCB would experience during assembly and testing at the PCB manufacturer. The strains experienced by the PCB would represent the nominal loading at the manufacturer.
Secondly, extreme handling is used to represent the worst-case in situ handling of a PCB.
After the measurements have been completed, a test report must be generated. The test report must include an Abstract, Introduction, Test Apparatus and Setup chapter, Results chapter, Conclusions chapter and Future Studies chapter as per the template provided in section 4 of IPC/JEDEC-9704A.
The strain limit criteria must be agreed upon by the PCB manufacturer and customer. Once a criterion has been set, at least the maximum and minimum principal, or diagonal strain, should be reported for each testing step. Strain rate calculations may be required; however, these can be computed within the catmanEasy/AP software when used in conjunction with the MX1615B.
For assembly operations, a time history graph should be plotted, to evaluate the strain experienced in this process, against the strain limit criteria.
Strain gauge testing should be conducted at the following intervals:
- Before a PCB is released for commercial use
- If there are modifications to the PCB fixture
- If there are modifications to the PCB assembly process
- During the fixture design process
- During the enclosure design process
The testing, as defined in the standard should form part of the manufacturers Preventative Maintenance (PM) schedule.
Tools, such as nCode and ReliaSoft, are purpose built to perform durability and fatigue post processing, manage your recorded data and optimize maintenance planning, based on historical data. These tools will allow the manufacturer to analyze and collate data in a meaningful way and then use this data to define a PM schedule.
PCB testing with strain gauges is a necessity to ensure that PCBs can withstand manufacturing and assembly processes before being passed on to the consumer.
TANDM can provide a unified set of tools that aids with the instrumentation, data acquisition and reporting required by IPC/JEDEC-9704A.
Contact us for all your PCB testing needs!