In printed circuit boards (PCBs), the solder joint is very weak and this can be further compounded by cyclic strain from vibration, thermal expansion and bending.
One major cause of solder bond failure is attributed to fatigue. Due to the small size of the solder bond, it is difficult to assess its fatigue resistance. To this end, mechanical tests carried out isothermally are much different from those performed by thermal cycling, which strains the connections and leads to major changes in microstructure.
Isothermal fatigue testing can be performed to gain a better understanding of the effect of aging, solder composition, pad materials on bond lifetime, and reflow parameters.
Bi-directional Shear Loading
A solder joint can be easily strained by bending a test board on which the part or component has been joined. In order to monitor joint failure, the resistance of two or more connections should be determined. Nevertheless, solder fatigue is a complicated issue and hence test can be performed on individual bonds to achieve the desired solution.
One simple way to apply an alternating shear load to a separate joint is bi-directional shear loading. However, the shape of the bond makes it more complicated to interpret the data, but for comparative reasons, this drawback is compensated by the effortlessness of the test.
Fatigue lifetime at high plastic strains can be from tens to thousands of cycles. As and when the solder deforms, significantly changes when displacement can be seen.
Using an S5KG (5Kg) or S250 (250g) cartridge, bi-directional shear fatigue, the cartridge detects the surface of the substrate and using the Z servo equipped in the 4000Plus micro materials test instrument, sets shear heights to below 1µm. Figure 1 shows the fatigue life for two lead free solders.
Figure 1. Fatigue life for two lead free solders.
In order to oscillate the sample below the tool, a direct drive XY stage is used with the ParagonTM test software (Figure 2). This helps in controlling the hold time and ramp rates at peak load. Similar to all kinds of small scale testing, displacements are extremely small and tool deflection is taken into account. When higher accuracy is needed, the external interface of Nordson DAGE’s 4000Plus system can be utilized to enter measurements from an external transducer.
Figure 2. All measurement parameters can be saved to a Test Group.
Results and Discussion
In order to assess the extent of crack propagation, tests can be stopped and a static shear test /dye penetration can be utilized, or alternatively cycling can be concluded to failure of the bond.
Dye penetration enables direct observation of the shape, size and location of the crack. At any condition, fatigue lifetimes differ significantly and data analysis must consider the mathematical nature of this method, often testing different samples for each test load. Fatigue data is generally plotted as strain or load against the log of the number of cycles to failure.
Figure 3. Work per cycle determined from the load vs. displacement plot for three different test loads.
Data related to load displacement is obtained for each cycle and also for the region which is enclosed by the plot utilized to determine the energy lost in plastic deformation (Figure 3).
This data is used by certain models to forecast fatigue life under loading conditions that are more complex and challenging. Moreover, as and when the crack grows the bond becomes more pliable. The slope of the load-displacement plot subsequently measures this change.
Nordson DAGE’s 4000Plus micro materials test instrument with ParagonTM test software offers a fully integrated solution to meet the challenges of small scale testing. The system is suitable for shear testing of solder joints on PCBs.
About Nordson DAGE
Dage was founded in 1961 and is a market leader in its chosen markets of Semiconductor and PCBA Manufacture. It has an award winning portfolio of Bondtester and X-ray Inspection Systems for destructive and non-destructive mechanical testing and inspection of electronic components.
Dage was acquired by the Nordson Corporation in 2006.
Nordson DAGE has a strong portfolio of award winning products for destructive and non-destructive mechanical testing and inspection of electronic components. It has an excellent, wholly owned distribution and support network of seven offices covering Europe, Japan, China, Singapore, and the USA, and maintains representative offices in other territories.
With its self-contained R&D facilities, Nordson DAGE has developed world-leading products for testing wire bonds on semiconductor packages such as BGA, Chip Scale Packages (CSP) and other electronic components. More recently Nordson DAGE has been heavily involved in the testing of the newest technology 300 mm wafer bump shear.
Nordson DAGE has developed an excellent suite of award-winning X-ray inspection equipment targeted at both the Semiconductor and PCBA markets.
Control of the patented core technology of X-ray tube manufacture ensures that the high-resolution X-ray will detect, identify and measure even the smallest of features. Nordson DAGE's high precision, state-of-the-art inspection equipment when joined with their sophisticated software offerings, ensure that the equipment is simple to use.
This information has been sourced, reviewed and adapted from materials provided by Nordson DAGE.
For more information on this source, please visit Nordson DAGE.