Voiding calculations help in making accurate quantitative measurement of the total voiding within the solder joints. However, this kind of measurement does not give data regarding the actual location of the voiding within the joint. So far, the effect of the void location on the joint consistency has not been examined thoroughly.
Calculation of interfacial voiding percentage is a quality assurance process that is followed within the testing system of PCBs and microelectronics. It is regulated by IPC-A-610, which states that voiding below 25% as seen by top down view provided by 2D X-ray inspection is acceptable for some class of BGA devices (Figure 1).
Figure 1. 2D voiding calculation of a BGA device as per IPC-A-610.
Figure 2. Total voiding vs. interfacial voiding of BGA device as per study. Very week correlation between Interfacial voiding and Total Voiding (IPC-610) is evident- (R2 =0.12).
The interfacial voiding can be critical for the joint quality and strength, and does not match well with the total voiding as determined by IPC-610 (Figure 2). One way to analyze interfacial voiding in a non-destructive manner is to use the Large Board CT technique (also called PCT). The basic principle of this method is illustrated in Figures 3 and 4.
Figure 3. µCT limitations for larger samples.
Figure 4. Basic principle of Large Board CT also called limited angle CT or PCT.
It can be seen from Figure 3 that the µCT technique is not ideal for large PCB assemblies. Since the sample has to be rotated between the detector and X-ray source, the bulk size of the PCB places the BGA device of interest far away from the X-ray source, and thus leads to low resolution and magnification. This is applicable for all X-ray CT devices. To overcome this issue, the Large Board CT technique ensures that the PCB is flat and near to the X-ray source and simultaneously turns the detector at an angle. This set up helps in obtaining high-resolution images sans the need for cutting down the PCB (Figure 4).
This article describes a method that integrates the mechanical testing capabilities of a typical Bondtester machine with the strengths of the Large Board CT. Bond strength is also determined using mechanical shear testing and the results obtained are correlated to interfacial voiding as observed by Large Board CT.
Using the Large Board CT technique, it was possible to study a large number of PCBs to search for a suitable BGA device for the experimental tests. This was achieved non-destructively since the technique allows the use of a large PCB sample. Ultimately, a BGA device was identified that included a large number of pins, which had 6% to 10% interfacial voiding, and also pins that did not exhibit voiding or had low levels of interfacial voiding. The total number of joints was 374, pitch was 1mm, and average ball diameter was 0.65mm.
To create a complete map of the interfacial voiding percentage at the PCB interface, the device was carefully scanned using Large Board CT. Then, the device was polished to expose the solder joints and prepare for the shear testing. The entire process has to be performed carefully so that the integrity of the joint is not disturbed.
Before the commencement of shear testing, the pins were categorized into two groups as follows:
- Group 1 - pins with 6% to 10% interfacial voiding
- Group 2 - pins with up to 1% on average interfacial voiding
This grouping was made by using the electronic cross section data achieved through Large Board CT. Figure 5 depicts an electronic cross section of the interfacial area of a BGA device; Figure 5a shows voids that emerge as black oval areas within the joint represented in white. Figure 5b shows a voiding calculation performed on an electronic cross section at the interfacial area of the BGA device.
Figure 5. Electronic cross sections (e-sections) of interfacial area of a BGA device. Black oval areas represent the voiding (a), (b) BGA voiding calculation on an e-section at the PCB interfacial area. These sections are obtained in a completely non destructive way.
The shearing of the bonds was carried out by means of a Dage 4000 Plus bondtester. Modern bondtesters are not only versatile, but also accurate. They are capable of performing a wide range of mechanical tests both in a destructive and non-destructive fashion. These comprise pull, shear, peel and a large set of material tests such as 3 and 4 point bend tests. In some testing conditions, these machines can be automated to acquire improved precision, speed and productivity. Figure 6 shows the typical shear test results. When compared to the joints in Group 1, the joints in Group 2 reveal more reliable and higher results for break force.
Figure 6. Typical shear results for Groups 1 and 2 solder joints. Group 2 joints (less than 1% voiding) show better joint strength.
Due to the shear testing, two types of failure mechanisms were seen such as pad cratering and ductile, with the ductile failure being more proliferated. Figure 7 shows a ductile failure that corresponds to a failure occurring in the solder bulk. Pad crater was the other type of failure seen during the shear testing (Figure 8). In this type of failure, the break occurs in the PCB material and appears like a crater.
Figure 7. Example of a ductile or solder failure due to shear test. The dark area in the middle is an interfacial void.
Figure 8. Pad crater failure due to shear test. The break occurs in the PCB material; (a) side view, (b) top view.
Once the shear testing was completed, the results were averaged which revealed that the average value for break force for Group 1 joints was 1192 g, while the result for Group 2 joints was 1317 g. This shows that the joints of Group 2 display 9% to 10% higher values for break force on average. This result correlates well with the theory that interfacial voiding has a negative impact on the bond strength. The device employed for testing had moderate levels of interfacial voiding and still a negative impact on the solder strength was observed. In addition, pad crater failures were found to be the weakest link for this device and occur around 800 g shear force.
This article shows a testing process, which integrates non destructive X-ray analysis and destructive shear testing to explore the effect of interfacial voiding on joint strength of BGA devices. An X-ray Large board CT method was used that makes it possible to take a virtual electronic cross section at the BGA to PCB interface and exposed the interfacial voiding. The shear experiments were performed by means of a multi-purpose Bondtester system. It was observed that interfacial voiding negatively affects joint strength by up to 10% for a moderate level of interfacial voiding, i.e., 6% to 10%.
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.
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