How to Measure Strain Rate on Printed Circuit Boards (PCBs)

1. Why are strain measurements performed on PCBs?

In the course of our daily lives, we have to place trust on the reliablitiy of electronic components integrated in cars, smartphones, airplanes, and countless other devices. In many of these products printed circuit boards (PCBs) are integrated. The reliability of complex electronics and overall electrical systems is a result of experienced development and intensive testing.

PCBs are exposed to mechanical and thermal impacts not only during their manufacturing process, but also during their transport and in action (for example: deformation, misuse, vibration, shock, thermal exposure).

During the manufacture of PCBs, the following malfunctions and stresses may occur:

  • Bending strain during the installation of connectors, power rails, cooling plates, contact pins, solder terminations, or battery holders
  • Breakage during mounting of surface-mounted device (SMD), surface-mounted technology (SMT), and through-hole device (THD), through the hole (THT) and pin in hole (PIH) fitting
  • Stress cracks and dislodging of solder points with ball grid arrays (BGA)
  • Transient strain peaks during separation (determination of critical strains/shear strains during separation)
  • Elevated mechanical stress (strain) that occurs due to press fitting, screw tightening, or encapsulation processes in housings
  • Broken SMD capacitors due to high bending stress in other steps of the process
  • Hard touchdown of the test probes during the ICT test

During transport and operation, the following impacts could lead to a malfunction:

  • Mechanical load (static)
  • Vibration and splicing (dynamic)
  • Thermal effects resulting in cracks caused by thermal expansion (differing α values of housing, heat sink, printed circuit board, and electronic components)

All these effects can lead to a complete failure of components. If a systematic failure of a PCB is detected too late, the fulminating costs would be massive.The costs would continue to increase, as long as the detection of the malfunction is delayed. The rule of 10 shows that the later a systematic failure of a new product is detected, the costs per defect unit multiply by a factor of 10.

2. Extended Requirements and International Standards for PCB Testing

Based on the fact that detecting systematic failures at an early stage of development is absolutely essential, OEM manufacturers have increasingly begun to request their suppliers to check the mechanical quality of PCBs.

The utilization of PCBs has increased in recent years due to the following reasons:

  • Use of lead-free solder (RoHS conformity, EU guideline) that is more sensitive in relation to mechanical load and tends to crack earlier (flexure-induced damage)
  • More compact construction elements such as ball grid arrays (BGA) instead of surface-mounted devices (SMD)
  • Stiffer contacts that lead to higher mechanical tension

International associations such as IPC (Association Connecting Electronics Industries) and JEDEC (Joint Electron Device Engineering Council) – 9704 have been established, and these provide guidelines describing where, how and wherewith strain measurements on PCBs have to be performed.

Many companies have created their own test procedures to ensure that all manual handling steps are executed correctly during the assembly as well as developed test scenarios for PCB testing to cover all relevant cases.

3. How to Measure Strain on PCBs

Numerical simulation methods such as FEA are limited in their scope since they are based on mathematical model approaches. Therefore, physical tests on real PCBs are at least additionally required to test the real strain behavior of the board. Other test methods such as CTs and X-rays are not sufficiently adequate to check the influence of the mechanical impact and are, on top of that, expensive methods to employ.

Strain values are the only reliable calculations for measuring the mechanical deformation of PCBs. Therefore, strain gauges are designated to measure the deformation of the PCBs to an extremely accurate degree. PCBs are usually small in dimension, and the challenge is to install strain gauges in the limited space available. 

HBM offers over 2000 different strain gauges for special applications, alongside some specialized strain gauges for PCB strain measurement. The RF91 three-grid miniature rosette, for example, is an excellent product to measure strain on miniature components such as PCB since the direction of principal strain is unknown. Moreover, it is only 5mm in diameter and can, therefore, be mounted easily on PCBs. It is available in different variants: prewired and with integrated solder pads. Other strain gauges, such as the RY31-3/120 (6.9mm diameter), can also be used for PCB testing.

Key facts about the HBM RF91 Miniature Rosette

  • Only 5mm diameter for miniature applications deliverable ex stock
  • 120 Ω resistance with three-stacked measurement grids
  • Measurement of two-axis tension state when the principal stress direction is unknown
  • Temperature compensation for austenitic and ferritic steel as well as aluminum
  • Prewired (0.5m) or with solder pads - No soldering on strain gauge
  • For two-, three-, and patented HBM four-wire configuration usable
  • Different colors of paint-insulated cooper wire

New: PCB Test Kit

Start your measurement right away with the PCB Test Kit. From the matching strain gauges to the measuring bridge amplifier and the data acquisition software, the kit includes everything you need for strain measurements on PCBs in compliance with IPC/JEDEC 9704.

From the sensor to the result, simply “plug and measure”:

  • Strain Gauges in compliance with IPC/JEDEC 9704 and their installation aids
  • QuantumX bridge amplifier module MX1615B
  • Ready-to-use measurement project in catman DAQ Software

To PCB Test Kit

PCB Test Kit Key Facts:

  • Portable and compact case includes all materials
  • Fast, easy and flexible setup – also in different locations
  • Optimal support for every measurement task
  • Fully compatible with all QuantumX modules and catman software packages
  • All products available off-the-shelf for reordering

 

4. Where to Measure Strain on PCBs

The tension status on PCBs is mostly unknown and mechanically complex. Strain situations result in the deformation of a plate. Plate deformation does not follow the classic models of beam deformation or torsion of a shaft, which are quite accurately described by linear electrostatics. Furthermore, it needs to be considered that an assembled PCB contains several single components that are soldered or connected in different ways to the PCB. This means that a PCB is quite heterogeneous in its material properties.

It is neither useful nor possible with respect to cost and time to check every single section of a PCB according to the strain properties and behavior. Therefore, measurements on PCBs are set at areas where the risk of failure is estimated to be especially high such as:

  • Corners: Corners can be mechanically critical if they are fixed.
  • Stiff regions of the board (e.g. the ones close to capacitors): Bigger elements lead to increased stiffness of PCBs.
  • Regions close to interconnects (solder-joint failures): Solder points are weak points in terms of yield strength.

5. How to Install the RF91 Miniature Rosette on PCBs

1. PCB Preparation for Installation

 

First, the printed circuit board must be prepared for the installation of a foil strain gauge. This video shows the required steps.

2. Bonding of a RF91 Rosette to a PCB

 

This video illustrates how a rosette is glued to a circuit board using the Z70 quick-curing adhesive.

3. Data Acquisition with a PCB

 

Finally, the implementation will be illustrated by means of a PCB using a QuantumX MX1615B and the catman software.

QuantumX MX1615B Measurement Amplifier

QuantumX is a modular, freely scalable and distributable data acquisition system from HBM for measurement and testing purposes allowing quicker innovation. All modules offer an Ethernet interface and can be freely combined with each other. All channels work completely time synchronized - module to module with < 1 µs. 

Every channel can be individually parameterized via software, supporting the following:

  • Strain gauges in full-, half- or quarter-bridge (120 or 350 Ω)
  • Standard voltage, PT100, resistor, potentiometer
  • Individual data rates up to 20kS/s per channel, active low pass filter

To QuantumX MX1615B

QuantumX MX1615B Key Facts: 

  • Bridge input, PT100/RTD, voltage, potentiometer selectable for each of the 16 individual 24 bit ADCs
  • DC or carrier frequency (CF) for maximum noise suppression
  • Internal 120 and 350 Ω quarter bridge completion resistor
  • Six-wire technology for full bridge and five-wire technology for half bridge
  • Three- and four-wire technology for strain gauge quarter bridges
  • 20kS/s datarate, 3kHz bandwidth
  • Galvanically isolated (channel to channel, to supply, to network)

6. How to Set Up a Strain (Rate) Measurement in catman®AP

With the HBM DAQ software catman AP, it is easy to set up a PCB board strain measurement. A quick and easy visualization of data is one of the strengths of catman. Data recording can be performed differently using trigger or special time points.

The three measurement grids of the RF91 rosette allow the calculation of maximum and minimum principal strain (rates) as well as the corresponding angles. The latest versions of catman also support strain rate measurement (the strain is derived from the time).

In the subsequent steps, the way to set up a strain rate measurement in catman is shown:

1. Open the catman software and check the relevant channel of the strain gauge. The green lights indicate that the channel is detected and is ready for measurement. In this example, the three grids of the rosette are connected with Channels 1, 2, 3.

catman channel settings

 

2. Use the sensor database to assign the channels to the sensor application. In this case, drag and drop the 3 wire 120 Ω strain gauge to each of the three active strain channels.

Assigning sensors to channels in catman

1. Now, the sensor specifications need to be set. Set the correct parameters by using the k-factor displayed on the datasheet of each HBM strain gauge package. Enter the excitation voltage, the bridge factor, and the measurement range. Also ensure that you look at the temperature compensation polynomial if you want to correctly consider the temperature fluctuation material properties.

 

2. Set the sample rate (classic or decimal) and filters correctly before beginning measurements. Click on ‘Create new sensor’ and activate ‘Update in sensor data base’ to save your parameters in the database.

 

3. Select all channels and zero the offset of the strain channels of the rosette. The zeroed strain channels appear.

1. Now, we have to set up the rosette calculation channel. A new channel needs to be created, and catman makes it easy for the user to create different setups for rosette calculations.

 

2. Add all three channels in a, b, c and define the material properties and the transverse sensitivity of the gauges. Choose the right rosette type (0/45 or 60/120 for the three grid rosettes) and select the relevant strains (principal strain, shear strain).

 

3. Click on ‘Create calculation’. Now, the calculated channels appear in the channel list.

 

4. Set a name and click on ‘Apply changes’

 

5. The strain rate channels will appear in the ‘computation channel’ list at the end.

Go to ‘Visualization’ and configure your own GUI

catman®AP DAQ Software

catman®AP from HBM is a powerful software package for PC based data acquisition and data analysis. Thanks to an intuitive user interface, you are only a few clicks away from starting your measurement. Simply configure the amplifier using TEDS, the transducer electronic data sheet, or the extendible sensor database – and the test can start.

Many options for graphical data analysis and versatile export options make catman®AP a reliable and indispensable tool for every measurement technician.

To catman AP

catman®AP Key Facts:

  • Quick and easy measurement results including automated rest procedures
  • Durability testing (rainflow analysis)
  • Modular, free, scalable with regard to channels
  • Real-time and post-processing
  • Maths for rosette calculation
  • Strain and strain rate measurement
  • Report generation and Export Data

7. Data Analysis in catman®

The aim of the analysis is to check if the measured data meets the acceptable criteria for the PCB strain. The following diagram illustrates the boundary lines as a function of the strain rate and the board thickness according to IPC / JEDEC-9704A (2012).

The idea is that the maximum principal strain (Y-axis) should not exceed a certain value. With increased PCB thickness higher principal strains are acceptable. Additionally, another criterion needs to be considered – the strain rate. This means that the lifespan of a PCB is impacted not only by the pure value of the maximum principal strain but also by the speed of changing the strain (impulse). Fast changes in material usually result in earlier micro cracks and material damage.

1. To analyze the test data, open a new ‘Analyze’ project in catman.

 

2. Search for the test data and drag & drop it to the column on the right hand side.

1. Change to the ‘Visualization’ panel. Create a graph by dragging & dropping the test data of gage 1 to the empty surface.

 

2. Then choose the corresponding strain rate 1 and drag it onto the text gage 1[01] in the graph explanations. Use strain rate 1 as x-axis.

 

3. The following graph will appear:

 

4. As this is not the desired shape of the graph, configure the plot as follows (dotted-style).

 

5. Adjust the x-axis by choosing a manual logarithmic scaling.

1. To picture the limit line, define the following data series function (according to IPC/JEDEC-9704):

Max. allowable strain = sqrt[2.35/(PWB thickness)]*[1900-300*log(strain rate)]

 

and x as function

 

1. Finally visualize the computed functions. Drag & drop the ‘Limits’ computation to the displayed graph. Then drag & drop the ‘x_channel’ computation onto the Limits in the graph explanations and use as x-axis.

 

2. The resulting graph looks as follows:

According to IPC/JEDEC-9704 Printed Wiring Board Strain Gage Test Guideline, the measured strain is within the acceptable strain range. The tested PCB is, then, not damaged during the manufacturing process.

8. Glossary

  • BGA: Ball Grid Array
  • FEA: Finite Element Analysis
  • ICT: In-Circuit Test
  • JEDEC: Joint Electron Device Engineering Council
  • PCB: Printed Circuit Board
  • SMT: Surface-Mounted Technology

 

Legal Disclaimer: TECH Notes are designed to provide a quick overview. TECH Notes are continuously improved and therefore, change frequently. HBM assumes no responsibility for the correctness and/or completeness of the descriptions. We reserve the right to make changes to the features and/or the description at any time without any prior notice.

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