-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
3-D or 2-D - Choosing with Today's Technology
December 31, 1969 |Estimated reading time: 12 minutes
UNTIL VERY RECENTLY, COSTLY 3-D SYSTEMS WERE NEEDED TO FIND NUMEROUS SUBTLE DEFECTS. TODAY, NEW 2-D WITH OVHM SYSTEMS OFFER AN INEXPENSIVE ALTERNATIVE IN MANY INSTANCES.
By Holger Roth
Recently, this question arose during a presentation: "Competitors are pushing three-dimensional (3-D) X-ray inspection as the best way to inspect ball grid arrays (BGA) and chip scale packages (CSP). Do I need to spend the extra money to ensure my production quality?" My answer, as usual, was yes and no. That decision depends greatly on what is being manufactured. For more than 95 percent of manufacturers, two-dimensional (2-D) X-ray actually is a better solution. The question then follows: "Are we one of the 5 percent who need 3-D?" This article offers information to help ascertain the answer to that question.
Test Criteria and RequirementsX-ray inspection, whether offline manual or inline automated, verifies the functional integrity of assemblies after the soldering process. X-ray inspection finds:
- Solder bridges (shorts)
- Missing or undersized solder joints (voids)
- Misalignments
- Open solder joints.
Figure 1. Interpretation of the X-ray image of a BGA solder joint.
In practice, BGA bridges, missing joints and misalignments can be found and recognized by either an operator or an automated software package, using a top-down view without rotating or tilting the board. Open solder joints, however, are in contrast to these rough solder joints, and typically require an inspection in oblique view at higher magnification. Under such inspection, either a gap in the solder joint can be seen directly, or the shape of a solder mass will indicate insufficient pad wetting (the board pad, in most cases).
Using the oblique view to inspect smaller solder joints such as fine-pitch ball grid array (FBGA), microBGA CSPs and flip chips frequently provides insufficient magnification because of the long distance separating the solder joints from the X-ray source due to sample tilting. This can be overcome only if the X-ray system has a high geometrical magnification even at angled inspection or oblique view at highest magnification (OVHM).
Automated BGA solder joint evaluations are performed simply by comparing the measured data with threshold values. Thus, all X-ray images can be quickly, accurately and repeatedly determined for pass-fail conditions making even semiautomated 100 percent sorting possible.
Inspection Methodology2-D X-ray inspection techniques are done using a fixed top-down view. The target board under examination is positioned between the sources and the image intensifier. The image resolution is determined by the size of the focal spot, while the ability to resolve errors is affected by the magnification. Because of the top-down alignment, magnification is typically in the 200 to 400X range, although some ultra-high-resolution systems offer up to 1,400X.
2-D X-ray with OVHM systems uses an open-tube source (see sidebar) that provides a high irradiation angle (170°) vs. the narrow top-down typology (approximately 38°). This method maintains magnification during board tilting and rotation for microBGA and other fine-pitch devices.
3-D imaging systems build on the 2-D X-ray with OVHM; however, both the X-ray source and detector move, and the software engine captures multiple images to compile the 3-D image. This method allows slice-by-slice analysis of the component, joint or board, but takes time to accumulate the image. Additionally, 3-D system magnification typically is limited to 2 to 10X.
2-D and 2-D with OVHM offer fast image acquisition, as compared to a 3-D image representing 20 to 100 acquired images. Both 3-D and 2-D with OVHM systems allow similar image and board rotation, providing an oblique view to see voids and other misshapen solder balls.
Test CriteriaThe test criteria that can be evaluated automatically during a series inspection are listed in Table 1. Following is a discussion of each of these tests to determine the most beneficial method. All testers, whether 2-D, 2-D with OVHM or 3-D, can perform these measurements. The goal is to determine the most cost-effective and reliable method.
Short Circuits. Using a top-down, nontilted method will find 99.9 percent of all short circuits or solder bridges on single-sided assemblies. Double-sided boards will require tilting or rotation, as well as varying intensity and magnification to accurately determine faults. As a result, a 2-D machine is the most cost-effective solution for single-sided printed circuit boards (PCB). Double-sided boards will require 2-D with OVHM or 3-D systems.
Missing Solder Joints. Missing solder joints (i.e., opens) are seen easily on single-sided boards with a simple top-down 2-D system. Double-sided boards still will require an oblique angle to differentiate the side on which the open is located. As with short circuits, a 2-D machine is sufficient for the majority of these applications.
Misalignments. Again, a top-down 2-D system is the most cost-effective method to capture these errors. The exception is a tilted component that is pulled off the board. As with the easily viewed shorts and opens described above, all systems can perform these measurements, yet the top-down 2-D system still is the most cost-efficient system.
Deviating Diameter, Excessive Voiding, Gray Level Deviation and Non-circular Shape Solder Joints. These measurements are difficult to determine using a top-down 2-D system because further criteria are required for consideration. As shown in Figure 1, the X-ray image has characteristic dark rings at the pad edges that are due to the additional solder in this area. When joining pads, the areas defined by a solder mask overlapping a pad or the etched copper pad appear as dark circular areas if they are well wetted. Only an oblique view at high magnification enables a more detailed investigation of the wetting status at the pads.
Figure 2. Typical result image of an automated BGA evaluation (PBGA 352) after auto-set-up ? three solder joints deviate from a true circle by more than 13 percent, one solder bridge is present and two solder joints show a voiding percentage exceeding 5 percent. Below the solder joints, the voiding percentages are indicated.
Furthermore, detecting voids in the solder joints is important in evaluating a connection's reliability. Voids are imaged as bright circular areas inside the solder joints, as displayed in Figure 1.
The visualization of these features also requires a high-performance X-ray system because the strongly absorbing solder joints must be penetrated with sufficient intensity, yet not cause saturation effects in the low absorbing vicinity. That effect might, for example, shrink the diameter of the solder balls. In practice, this implies that the X-ray system has to be operated at high tube voltage (120 to 130 kV), at low tube current (4 to 20 µA) and with a small focus spot (<10 µm).
Quantification of Test ResultsTo go beyond the fundamental investigations of solder joint quality and process parameter dependence requires suitable control of the series production process to ensure reliability. This, in general, has to fall back on statistical methods; hence, quantitative measurements of objective and well-defined quantities for as many samples as possible (up to 100 percent) are needed.
For example, the void presence will not necessarily affect reliability, but rather amount of void per solder joint area. Usually stated as a percentage, this is decisive.1 The percentage sensitively depends on the process parameters2, and requires a measurement of the relative void size for all individual solder joints.
Figure 3. X-ray image of six microBGA solder joints in top-down view (a) OVHM X-ray image of the same six solder joints (b).
Figure 2 displays an individual result image of a BGA series inspection. The maximum void percentage accepted was 5 percent. It should be noted that the clearly visible background structures (solder at the PCB's back side) are suppressed totally during evaluation and do not influence the test result.
As previously stated, abnormal solder joints must be inspected in both the top-down image and the OVHM mode to get to the bottom of the defect mechanism. Figure 3a, for example, displays six solder joints of a microBGA component (slight misalignment and irregular solder joint diameters are recognized immediately). At the marked solder joints, wetting defects have been identified according to the above-mentioned criteria (no dark ring, missing plateau). In the OVHM image (Figure 4) these solder joints turned out to be open the solder is attached only to the component pads.
Figure 4a-b. The 3-D image shown above provides a great visualization and shows two opens while the 2-D with OVHM image shown at the bottom illustrates that there actually are three opens.
The solder joint at the lower left shows the central plateau in top-down view. While this appears to be a good ball in the OVHM image (Figure 3b), it is identified easily as an open because of the missing solder meniscus. While a 3-D image also would have found some errors, the faster 2-D with OVHM system is the more cost-effective solution for these errors and also will identify opens more reliably (Figures 4a and 4b).
Cutting Inspection TimeIt is obvious that a fully automated or semiautomated tester will be faster than a manual manipulation system. However, image analysis software packages are an important addition to accurately and repeatedly analyze a solder joint.
For standard BGA, FBGA and CSP solder joints, the user can set up the software in a self-teach mode the system recognizing the BGA layout and the right solder joint diameter and gray level directly during the evaluation process (i.e., without set-up time). The evaluation yielded all above mentioned test results (Table 1) according to the pre-adjusted threshold values. Typically, an auto-set-up program also makes it unnecessary to observe exact positioning and constant magnification during inspection, a time saving advantage. Moreover, the evaluation result is independent of the adjusted X-ray parameters within a certain range. With the auto-set-up, if at least 80 percent of the solder joints in the field of view are acceptable, the software automatically analyzes all solder joints in the field of view. For the automated evaluation of a standard PBGA 255 including four to six views, test time is a maximum of 1 to 2 minutes.
ConclusionWhen choosing an X-ray inspection system, be aware that 2-D systems are very effective in finding short circuits (solder bridges), missing solder joints (opens) and linear misalignments. 2-D systems, even those with rotation, fall short for double-sided boards, very fine-pitch devices and the determination of vertical misalignments because of lack of magnification.
Until recently, only 3-D machines were seen as reliable tools for these measurements. Now, new 2-D with OVHM systems offer an alternative to the more costly 3-D systems. While 3-D still is needed for in-depth bit slice analysis, fully automated double-sided board inspection and Z-height measurement, 2-D with OVHM offers a lower cost alternative, which enables solder joint analysis with a finer pitch (CSP, flip chip) at much higher magnification in oblique views, and as seen from Figures 3 and 4 can identify more failures.
2-D with OVHM technology, with a variety of suppliers, is available in manual, semiautomated and fully automated systems. These tools are a very cost-efficient solution for 99 percent of today's testing needs.
REFERENCES
- S. Liu and Y.H. Mei, "Effects of Voids and Their Interactions on SMT Solder Joint Reliability," Soldering and Surface Mount Technology 18, 1994, p. 21-32.
- W.B. O'Hara and N.C. Lee, "How Voids Develop in BGA Solder Joints," SMT Magazine, January 1996, p. 44-47.
HOLGER ROTH, Ph.D., applications engineer, may be contacted at phoenix|x-ray Systems + Services GmbH, Zweigstelle Stuttgart, Motorstra
_______________________________
Open Tube and Sealed Tube Microfocus X-rayThe majority of today's microfocus X-ray system manufacturers offer tools based on either open tube or sealed tube technology. It is not surprising, then, that the majority of suppliers argue that one technology is better than the other. In actual fact, however, both technologies have advantages and disadvantages, depending upon the application.
Before looking into the pros and cons, let us summarize what sealed tube and open tube mean. A "sealed tube" X-ray source generally is a glass tube analogous to a light bulb. Like a light bulb, the contents of the tube are in a vacuum, which is created at the time of manufacture.
An "open tube" source, as its name implies, may be opened. These tubes typically are constructed from steel and the vacuum is created by a two-stage vacuum pumping process that, in turn, creates the vacuum every time the system is switched on from cold.
There are some fundamentals that help define the performance of real-time microfocus X-ray systems that we can use as a yardstick for identifying the advantages and disadvantages of each technology. These fundamentals are focal spot size, tube voltage, geometric magnification, tube lifetime, cost of ownership and system price.
Focal Spot Size. Focal spot size is perhaps the most fundamental of microfocus X-ray parameters and the most difficult to quantify. There are many techniques for the calculation of this parameter in existence today. The most relevant determine the lateral resolution, or the smallest features that can be seen by the system in a top-down view, and then back-calculate the spot size. In essence, the smaller the focal spot size, the less the geometric distinction and, thus, the sharper the image. Sealed tubes have a larger spot size compared to open tube and do not offer such a sharp image.
Figure 1. FOD = focus to object distance; FID = focus to intensifier distance.
Tube Voltage. A tube with a high voltage can penetrate a denser material than one with a lower voltage. Unfortunately, the higher the tube voltage, the more likely a tube "discharge" will occur. Frequent discharges in a sealed tube will make the tube "gassy," resulting in a shorter tube lifetime. Discharges in an open tube, however, are less critical because gasses will be removed by recreating the tube vacuum. Consequently, sealed tubes have a lower kV rating than open tubes. Typical kVs for sealed tubes include 50, 80, 120 and 130 kV, and typical open tube kVs include 100, 160, 200 and 225 kV.
Geometric Magnification. The larger the geometric magnification, the smaller the feature that can be observed. Geometric magnification may be defined as:
Geometrical Magnification Mgeo = FID/FOD (Figure 1).
Sealed tubes have a larger minimum FOD, resulting in a smaller achievable geometric magnification. Thus, open tubes are better for applications demanding high magnification.
Tube Lifetime. The lifetime of a tube depends on the following:
- Target degradation
- Filament degradation
- Gas build-up in tube vacuum
- Vacuum loss.
The target and filament within a sealed tube are not replaceable because the tube is held under a permanent vacuum. Thus, if one of these items fails, the entire tube must be replaced. Typical sealed tube lifetime lasts between 5 to 10,000 hours.
Open tubes (as previously mentioned) may be opened, enabling target and filament replacement. Open tubes often are referred to as having "unlimited" lifetime because all major components may be exchanged.
Cost of Ownership. Cost of ownership has always been an area of contention between sealed and open tube manufacturers. Sealed tube suppliers and manufacturers claim high cost of repair for open tubes, while open tube companies claim high replacement costs for sealed tubes. Both technologies have their associated costs, but the actual costs over a five year period are, in fact, very similar.
System Price. Open tubes are more expensive because of the addition of vacuum pumps and more complex X-ray control systems. Open tube systems may be up to 50 percent more expensive than sealed tube systems (Table 1).
Which Technology Is Best?The choice between open tube and sealed tube depends on both the application and budget. If your components or PCBs can be inspected with a geometric magnification of less than 60X under 130 kV and with a low tube current (less than 10 W), a sealed tube system probably is best. If your application requires higher geometric magnifications, higher voltage or tube current and your budget permits an open tube would be the more suitable product.