The SMT and PCB industries have faced several concerns due to shifting from tin-lead to lead-free materials. These concerns are still drawing the interest of many fields, including consumer electronics, aerospace, military, and telecommunications.
Pad cratering is a mode of failure in lead-free assemblies due to lifting away of the contact pad on a packaging substrate or PCB. It is more common in lead-free assemblies than in tin-lead assemblies due to the following factors:
- A higher amount of strain is transferred to the assembly by lead-free solders as they are mechanically stiffer when compared to tin-lead solders.
- Unlike tin-lead, lead-free solders demand higher cooling rates and reflow temperatures, increasing strain on the assembly.
Significance of Isolating Failure Modes
SMTs and PCBs are generally subjected to mechanical testing, including shock and bend testing, to corroborate product design, and ensure the quality and longevity of final products.
Mechanical testing involves application of mechanical stresses to assess a product’s susceptibility to failure. Here, the entire assembly is being tested, causing difficulty in distinguishing between the different types of failure modes taking place over the course of the test. The following factors induce different failure modes:
- PBA materials
- Solder metallurgy
- Reflow conditions
For that reason, isolating the failure modes is crucial to determine the weak points in an assembly. It is a key step in the development and production stage to ensure appropriate product design, and control manufacturing process and product quality.
Hot Bump Pull
According to IPC-9708 pad cratering standard, hot bump pull is an approach of assessing the sensitivity of PCB designs and materials to cohesive dielectric failure of underneath SMT attach pads.
The new IPC-9708- and JEITA ET-7407A-complaint 4000Plus HBP system from Nordson DAGE is completely coupled to a single load cartridge on a standard 4000Plus system.
The single load cartridge (Figure 1) is equipped with pin clamping mechanism and heating and cooling stages. The test is configured by means of the Paragon™ software which offers an easy to use interface with temperature-time profiles.
Simple straight test pins can be used, thanks to the innovative design of the cartridge. This enables transferring optimum force and offering an affordable consumable for testing.
Successful and reproducible testing is the result of the new straight pin design. The pin has to be pulled directly by the instrument in a vertical fashion devoid of bending moments.
Figure 1. Hot Bump Pull load cartridge.
The Hot Bump Pull Test Procedure
Setting up the test parameters and positioning the pin over a solder bump are the two parts of the hot bump pull test procedure.
Creating a temperature profile is the first step to enable users to input time and temperature parameters for reflow and test conditions. The desired temperature and the time taken to achieve this temperature are the only requirements for this simple interface.
The hardware and software of the system automatically handle the heating and cooling rates and perform the test.
The following are the stages of a temperature-time profile (Figure 2) in Paragon™:
- Rate of rise
- Cool down
- Test execution
Figure 2. Temperature-time profile in Paragon™.
The preheat region is T1 set by defining the desired temperature and the time taken to reach this temperature. T1 to T2 is the soak period, which is user selectable. T2 to T3 is the rate of rise, whereas T3 to T4 is referred as the reflow period, which is user selectable. The cooling period is between T4 and T5. T6 represents the temperature at which the test is performed.
A straight pin is introduced in a vertical fashion into the cartridge and securely placed. Motorized horizontal and vertical stages then lower the pin onto the solder bump and set to make contact with the solder bump. The test is then performed by increasing the pin temperature in accordance with the defined temperature profile (Figure 3).
Figure 3. Close up of copper probe on BGA.
At the point of reflow (Figure 4), the pin is lowered to a desired level to ensure a good solder joint. The pin is then clamped by the clamping mechanism to be pulled off. Cooling is carried out internally at the defined temperature profile by passing compressed air through the pin onto the test specimen. The test is automatically performed on the onset of reaching the test temperature, storing the energy, force-displacement and force-time values (Figure 5).
Figure 4. Close up of solder ball reflow.
Figure 5. Test results in Paragon™.
Failure Mode Inspection
As part of the test, a HBP test is conducted to generate a failure mode, followed by recording the visual data and storing it in a convenient manner without delaying the test procedure.
The 4000Plus can take in-depth images in between tests using the optional built in image capture camera, which can be configured to capture a high resolution image of the region being tests and save it for further analysis or reporting (Figures 6 and 7).
Figure 6. Failure mode image capture via integrated camera.
Figure 7. Image analysis within Paragon™.
The 4000Plus allows users to allocate appropriate failure modes through an easy to operate GUI, a key capability for failure analysis. Reports can be produced to present force, failure mode, data and image.
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.