Before and After Reflow Characterization of FCBGA Voiding
December 12, 2012 |Estimated reading time: 8 minutes
Editor's Note: This article originally appeared in the November 2012 issue of SMT Magazine.A joint project between Flextronics Inc. and North Star Imaging Inc. is being conducted to correlate current X-ray imaging and cross-section analysis of BGA voiding with state-of-the-art high-resolution CT scan imaging. The primary objective is to validate the void measurements obtained from non-destructive imaging techniques with the physically measured void measurements of cross-sectioning. A secondary goal is to characterize void properties before and after reflow.
Typical AXI equipment provides one to three horizontal planes of reference for BGA void measurements. CT scan imaging provides a full 3D volumetric representation of the BGA void, allowing for size, volume, and void position data. Information that can be used in failure analysis and process characterization projects without physical destruction of the PCB.
Five 50 mm FCBGA devices and five 52.5 mm FCBGA devices, with known voiding, are being used in the study. The voiding for each device has been measured on a 3D AXI machine (Figure 1), a 2D off-axis high-resolution X-ray machine (Figure 2), and CT scan system (Figure 3). The devices will then be placed and reflowed onto PCBs. After reflow, all voiding will be measured again using each piece of equipment. In addition, select voids will be cross-sectioned, polished, and measured using a high-magnification digital microscope and correlated to the other X-ray imaging tools.
Figure 1: A transmissive 2D X-ray image of BGA void.
Figure 2: A 3D AXI mid-ball image of BGA void.
Figure 3: A CT scan surface model with a partial cross-section of BGA void.
Introduction
As complex electronic assemblies become faster and faster, power with associated heat dissipation, signal integrity (SI), and reliability become more important than ever. Solder joint voiding can potentially impact all of these. With cost pressures on companies producing these types of products, properly diagnosing and characterizing voiding in a non-destructive fashion is crucial. Proper characterization will allow for adequate troubleshooting and the process development needed to minimize or eliminate voiding. In addition, non-destructive void analysis can be used in failure analysis cases.
Over time, X-ray technology used in the electronics industry has advanced from 2D transmissive, to 2D off axis, to 3D laminography, to 3D tomosynthesis. Resolution of X-ray tools has continued to advance along with the software required for automated analysis. Use of these tools has allowed identification and measurements of the voids in solder joints. Software has allowed for automated inspection of the solder joints to quickly identify and measure up 100% of the solder joints per component and per assembly in a timely manner. Typically, this software allows for measurement at a specific point in the solder joint (i.e., PCB level, mid joint, and package level).
While many improvements have been made in these tools (including resolution), smaller voids and the true position of these voids has been difficult to determine without actual cross-sectioning. Now, with the latest advancements in X-ray technology, a full high-resolution 3D image is available using CT scan technology. CT technology allows for infinite cross-sectioning in a non-destructive fashion.
The first objective of this work will be to correlate the most common X-ray technologies used by the electronics industry. Each technology will be correlated, not only to the newest CT scan technology, but also to actual cross-sections on a variety of void examples.
The second objective of this work will be to identify and characterize a variety of voids from incoming components through the SMT reflow process. Incoming components identified with solder voids will be subjected to a variety of reflow profile styles to determine what happens to them relative to size and position. Images and measurements will be taken before and after reflow using all the traditional X-ray tools along with CT scan. After all imaging has been completed, the actual cross-sections will be taken for comparison. In addition, components with incoming voids will be subjected to reflow under vacuum in an attempt to remove the voids prior to assembly.
Methodology
First was the design and fabrication of custom fixtures capable of holding 50 x 50 mm FCBGA and a 52.5 x 52.5 mm FCBGAs in a dead-bug position, for automated 3D inspection. Figure 4 shows the 10-up fixture while Figure 5 shows a close-up view.
Figure 4: Fixture for automated 3D X-ray inspection.
Figure 5: Close-up view of fixture for automated 3D X-ray inspection.
Next is the assembly of one SMT reflow profile board utilizing a large, complex PCB with 50 x 50 mm and 52.5 x 52.5 mm FCBGAs. Then, three different style profiles were created: Ramp to peak (Figure 6), long soak (Figure 7), and medium soak (Figure 8).
Figure 6: Ramp to peak SMT reflow profile.
Figure 7: Long soak SMT reflow profile.
Figure 8: Medium soak SMT reflow profile.
After that, follow the process flow diagram show in Figure 9.
Figure 9: Void experiment flow.
Void Detection Methodology Three typical tools will be used for the experiment, including 3D AXI (Figure 10), 2D X-ray (Figure 11), cross-sectioning (Figure 12), along with a fourth non-typical tool called high-resolution CT scan (Figure 13).
Figure 10: 3D AXI tool.
Figure 11: 2D X-ray tool.
Figure 12: Cross-sectioning tool.
Figure 13: High-resolution CT scan tool.
Results
Figures 14 and 15 show examples of the images collected from the experiment.
Figure 14: Example 1 of images collected using various tools.
Figure 15: Example 2 of images collected using various tools.
Table 1: Void growth analysis. Table 1 shows that:
- X-ray void measurements are not 100% driven by increase/decrease in void size.
- Measurements for X-ray are affected by void positioning within ball and changes in ball diameter.
- Full void characterization requires both parallel and perpendicular slices through void area.
Part of the experiment involved reflowing components in a vapor phase reflow machine and turning on vacuum.
Figure 16 shows the 2D X-ray images before and after reflowing in the vapor phase oven using vacuum. 3D AXI was first used to confirm there were no detectable voids. 2D X-ray images compare the same balls which confirm voids have mostly been removed beyond detection. CT scans were not taken based on these results.
Figure 16: Before and after results from vapor phase testing. Discussion
Multiple void studies demonstrate that a soak style profile can greatly reduce voiding. Figure 17 shows an example of a void study using data from 3D X-ray. This study was conducted on OSP PCB finish in nitrogen environment. While one vendor may work slightly better at a ramp-style profile, most tend to benefit from this style of profile. While SMT solder pastes are mostly designed to work in air, most work well in N2 and will survive a longer profile, which is what a soak style profile would represent. If running in air, perhaps a ramp or intermediate profile may work better so the vendor and part number of the SMT solder paste needs to be considered for the expected run environment.
In this figure, the Y axis represents number of voids. X axis represents void size bin.
Figure 17: Example of void study using size distribution.
Based on the consistency of results from a variety of void studies in both SnPb and Pb-free, we concluded that a soak style profile eliminates or greatly reduces voiding when compared to an intermediate style or ramp style reflow profile. It is from this position that this experiment was conducted.
Knowing and understanding the characteristics of voiding relative to a particular brand and part number of solder paste will point users to whether the reflow process and/or chemistry (SMT solder paste) is causing the voiding issue. Inspection of incoming components will determine if voids are present on incoming parts. 2D or 3D X-ray can easily be used to inspect for voids on incoming components.
Conclusions
Table 2 was created to summarize the key characteristics of each tool. A number was assigned to rank the various tools in these characteristics based on experience. Depending on each user and type of products/business, these may change slightly. Also, a weighting factor could be applied. The color (red, green, and yellow) is an added visual indicator. Table 2: Key characteristics of each tool summarized.
Findings include:
- 3D AXI is necessary to screen out significant quantities of components as data points, prior to further characterization.
- The combination of available void detection technologies is needed for complete characterization of process and components, especially for increased complexity (i.e., via-in-pad, finer pitch, etc.).
- PCB and component design, reflow profile parameters, as well as chemistry, can all affect growth and positioning of voids.
- High-resolution CT imaging allows for a complete analysis of components before and after assembly in non-destructive manner.
While IPC 7095B introduced tables for void process indicators and troubleshooting, and JEDEC Standard 217 has a guideline for component voids allowed (pre-reflow), a clear joint industry specification needs to be considered to create better linkage between component manufacturing and PCB assembly and inspection.
Acknowledgements
The authors would like to acknowledge the following for their contribution to this study: Jonathan Crilly, Eric Cruz, and Taylor Blair, Flextronics (Austin, Texas); Florian Wuest, IBL (Germany); and Jochen Lipp, IBL (USA). Gordon O’Hara is process engineering manager for Flextronics Austin. He has 22 years of experience in electronics manufacturing, with 19 of those years focused on PCBA manufacturing. O’Hara specializes in SMT pick-and-place technology and is registered professional engineer. Over the last several years, he has overseen a comprehensive chemistry set conversion from water wash to no-clean chemistries. In addition, O’Hara is currently leading the Global Flextronics team in the evaluation of various lead-free, no-clean chemistry sets.
Matthew Vandiver is structural test engineering manager for Flextronics Austin. He has 12 years of experience with AXI development and has worked closely with AXI suppliers on the implementation of X-ray inspection for high-reliability telecommunication products. Jon Crilly, process engineer at Flextronics, has been working in the high tech field for seven years with concentrations in SMT, IR reflow, and solder joint defect reduction. He is constantly looking into new technologies to help supply customers with the highest quality product possible.