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The Landscape of PCB Technology is Changing Rapidly--How Will AOI Testing Keep Up? Part II
June 1, 2010 |Estimated reading time: 20 minutes
Editor's Note: Find Part I of this paper here.
4.4 Transition to lead free solder: The transition to lead-free solder was driven by legislation, not technology. Early chemistries such as Sn/Bi (tin-bismuth) oxidized rapidly and had a mottled appearance and rough surfaces. This made it quite difficult for AOI to inspect solder joints, since most AOI relies on homogenous, consistent appearances. However, current lead-free pastes such as Sn/Ag/Cu, Tin, Silver and Copper have a more consistent but less reflective appearance.
While the increase in homogeneity decreases noise, the less reflective appearance can be a problem. Most AOI solder inspection systems are built upon the principle that they can project different light patterns on the solder joint and the solder joint will reflect unique signatures back to the camera to indicate "good," "excess," and "insufficient" diagnoses. It is paramount that the solder joint reflect back the signature with clarity and intensity. As a result, most AOI systems which inspect lead-free solder have had to adjust their light intensity (or exposure duration) and algorithm thresholds to account for the change in reflectivity.
One issue that customers are reporting is that lead-free materials are more difficult to rework. They require higher processing temperatures that may stress or cause significant damage to heat-sensitive PCB materials and components. If rework becomes undesirable, then the inspection of boards must occur before reflow, at paste deposition and placement. Additionally the use of process control, to try to catch defects early and to prevent defects before they actually occur at paste and place, is gaining popularity.
4.5 Increased use of array packages: Array package types are also becoming more commonplace. One subset of Array packages that is commonly used is the ball grid array (BGA). With a relatively small size and thermal footprint, they can be designed to give more complex functionality than traditional parts. (Figure 11 shows a recently designed board where a significant portion of the real estate is devoted to array packages, indicating their popularity.) However, the majority of the leads and pads are hidden with this package type. In addition, these package types are very delicate, meaning they are not very amenable to rework, and expensive to replace.
Thus, there are only two realistic methodologies, which are not mutually exclusive, to inspect this type of part. The first is to use X-ray inspection. X-ray inspection allows one to see cross-sections of the hidden joints. Thus, one can inspect for cracks, voids, and other "non-visible" solder related issues. Because X-ray is slow and relatively expensive, many companies employ X-ray machines solely for array package inspection and often just inspect on a sampling basis to validate the process. However, X-ray inspection is naturally used after the part has been reflowed. Once the defects are found, it may not be feasible to fix them. Thus, X-ray can only be used as an indicator that something is wrong in the process.
The second method for inspecting array packages is to use pre-reflow AOI. If the paste is deposited correctly, if there are no dropped parts after the chip-shooter and before fine pitch placement, if the array package is placed correctly, and if the oven profile is correct, there is a high probability that the package will not have a defect after reflow. Additionally, if there is a problem at paste deposition or placement, these defects can be corrected.Figure 11: The use of array packages has increased significantly. This recent board devotes approximately 40% of its real-estate to array packages.
Determining if an array package (or any part) has been placed correctly requires fine measurement accuracy and repeatability. This measurement function is often called metrology. Most AOI systems who offer metrology have modified both their Image Capture modules and Image Processing modules to deal with the significant issues associated with metrology.
4.6 The increased use of shields: Mobile phones and other devices that use an RF component require shields before reflow. This means that many of the parts cannot be viewed at post-reflow inspection since they are covered. Shields additionally block lighting and camera views. Also, rework of shielded components is quite difficult. Thus, AOI must be deployed prior to shield attachment. Figure 12 shows an example of a board with one shield placed and pasted pads/land patterns ready to receive other shields.Figure 12: Example of the use of shields - The board in this figure has one shield placed in the lower left corner. However, there are land patterns that show that shields will eventually cover 70% of the board, making it difficult to do post-reflow inspection.
4.7 Decrease in board lot size: Ten years ago, high volume manufacturing dominated the PCB landscape. It was commonplace for a company to set up a PCB line for one particular board and to run that particular board for an extended period of time. In the past 10 years things have changed dramatically. Board mix has increased and board volumes have decreased. New revisions of the board are constantly introduced. Manufacturing customers are asking for more custom boards.
Older AOI technology was prone to significant false failures and false accepts and constant tweaking of the machines was required to keep them at a stable inspection level. It was common for one to "tune" an AOI program to inspect a board for weeks or months. While this process was labor intensive, it was still feasible to use the program once the variations were all managed in a high volume environment. Such a process is no longer tolerated or feasible.
Thus, designers of AOI machines have performed significant work on the program generation piece of the system. Most AOI systems compiled a library of the best settings for the Image Capture module and the Image Processing module based on their experience with customer boards. This library gives a "head start" because it is a known good place to start the debug process. More sophisticated machines employ a learning process that automatically adjusts to each board type, whether it is a new board or revision. This allows the system to provide a good inspection plan for the exact board undergoing test. It also allows the program to adjust over time as normal variations in the PCB creation process occur. In our experience, we have found that this can significantly reduce programming time and create a more stable program without significant debug work. (Figure 13 contrasts the status quo of programming a board ten years ago with the new programming and debug requirements based on a number of factors including smaller lot sizes.)
Decreased board lot size also means there is significant pressure to find defects before too many boards have been built. It would be undesirable to build a lot of 20 and to find at reflow that all had defects. Thus, high-mix, low-volume manufacturers are also seeing the benefits of using AOI pre-reflow to find defects at the point at which they occur. Additionally, by using measurement (metrology) data from their pre-reflow machines, we found they can decrease their setup time by 50% and monitor the process to reduce future defects.z
Figure 13: Contrast between the programming/debug practices from 10 years ago and the programming/debug practices that are required today. The top schematic depicts the behavior of older AOI machines whose programming methodology required significant examples of boards in the pre-production stage and constant "tweaking" or modifications in the production stage to meet and maintain acceptable false accept/false reject ppm levels. Smaller lot sizes and the desire to reduce labor costs have put significant pressure on AOI designers to develop methodologies to reduce programming time and to develop more stable programs requiring significantly less "tweaking" over time.
4.8 Summary: We touched on 7 major changes in PCB board technology that have occurred over the last two decades. Table 1 shows a summary of how the components in an AOI machine that have changed to adapt to these technologies, how customers are changing their use of AOI machines, and how customers are changing the technology they use to inspect PCB boards.
Table 1: Summary of changes in AOI technology and AOI usage over the past ten years. 5. What are the changes in PCB technology that AOI systems will have to face in the future?
We spent the prior section talking about how AOI has adapted to PCB technological changes that have already occurred. Now let's look to the future. We see 4 major classes of changes in PCB technology that are coming in the next several years. These are:
- Changes in component packaging;
- Changes in PCB materials;
- Changes in attachment materials and methods;
- Changes in the use of output data from AOI machines; and
- Increased sharing across platforms.
The sections below discuss these major changes and the implications on AOI technology and how AOI systems are used. This section is not intended to be exhaustive. The main idea is to illustrate some of the significant changes we see on the horizon.
5.1 Component Packaging: PCB component packages are evolving at an exponential rate. So far, the newly developed packages that we discussed above are amenable to the type of image capture and image analysis that is possible with AOI machines. As we see new packages, both the AOI machines and how customers use AOI will need to continually adapt or change. Below we discuss three types of new package classes.
One good example of a new component type is stacked configurations commonly called PoP or package on package. PoP was developed based on the need to reduce space and thickness, especially for handheld devices. One example of PoP consists of cost logic processors as a base with stacked memory directly on top. The manufacturing challenges of utilizing PoP include accuracy of paste deposit, flux dipping, placement accuracy, reflow profile and rework. The bottom package is placed on standard paste deposit, the top part is dipped in flux and then the combination is reflowed at the same time. The AOI challenges are three fold. The first is that pre-reflow AOI must be able to inspect the paste deposition and placement of the bottom layer. It then must be able to inspect the flux deposit and placement of the top layer, relative to the bottom layer. Post-reflow AOI must be able to inspect any visible joints from the bottom layer. Inspecting one layer relative to another layer will require a significant change in how AOI machines work.Currently, all placement inspection is performed and evaluated relative to the board or panel fiducials. In addition, most AOI machines expect to evaluate a part's position relative to its transition to the board or pads. Now AOI machines will need to inspect part layers relative to other layers underneath. This will require a change in the machine's software infrastructure and in the image processing algorithms. Customers will need to redouble their efforts pre-reflow because this part needs to be inspected prior to reflow in stages. Additionally, because the cost of these stacked parts is double or triple the cost of the standard package, verifying the paste/flux and paste pre-reflow are critical to avoid having to scrap this part later in the process.
Another example of a new package type is wafer level chip scale package (WL-CSP) or a wafer level package (WLP). This package type has pads that are etched directly onto the silicon. Conductive adhesive is used to connect the part to the PCB board. The part itself is generally very small, approximately 1.5 mm x 1 mm. (Figure 14 shows two examples of this package type.) The major issue with this type of part is that once it is adhered to the board, the part cannot be reworked. Thus, the burden of inspecting this type of part falls on the post-adhesive inspection and the post-place inspection. Probably the most significant change to AOI systems would be in the post-adhesive inspection. This type of conductive adhesive looks different from paste and normal adhesive. 3-D measurements are difficult and not-required. An optical system that can distinguish the adhesive from the board is required.
Figure 14: Two examples of wafer level chip scale packages (WL-CSP). In each case, the color of the wafer level chip scale package is blue. The images on the left show the packages in context on the boards. The images on the right show magnified views of the packages and some surrounding parts. The packages shown are very small, similar in size to a 0402.
A third example of a new package type is embedded passive designs. These are components that are not surface mounted, but embedded in the board itself. They will become common in order to reduce circuit size and increase circuit density. Embedded passives can reduce circuit size by more than 50%. The inspection requirements of these embedded passives are to determine if the printing of the passive is the right size and in the right location. This may require a change to the image processing algorithms used by AOI machines. The problem of inspecting these passives is more similar to bare board inspection than surface mount inspection. Algorithms from the former can be adapted to this new type of part.
5.2 PCB material changes: Just as PCB components are changing, the underlying PCB substrate is also changing. Most AOI machines are used to standard FR4 PCB boards. However, we are seeing a rise in two types of new board technologies.
The first is high-density interconnect (HDI). HDI boards are boards that have three-dimensional wafer-scale packaging of integrated circuits. Patterns of interconnect layouts and micro-via holes are written or drilled out by lasers. Chips can be connected to each other using standard semiconductor fabrication methods. The wafer-scale components are stacked and interconnected at their edges. As a result, HDI changes PCB boards from "large" two-dimensional surfaces into very "compact" three-dimensional spaces.
Inspection of traditional PCB components on the surface of an HDI board is difficult because of the number of distractors in the board. There are substantially more traces and vias than on normal "two-dimensional" boards. This substantially decreases the signal (part, paste, joint) to noise (distractor elements on the board) ratio. The image processing component of some AOI machines may need to be modified to ignore the distractors.
The second material change is the use of flexible film circuit technology. Components are mounted onto the flexible material using conductive adhesives, which are flexible, reduce the need for traditional connectors and cables and allow for more creative designs, switching and interface panels.
AOI systems will have to adapt in three ways as a result of this new flexible film material. Imaging techniques will be needed for post conductor printing, post placement and post reflow. As we discussed above the conductive adhesive looks different from both the paste and flux that are traditionally used and may require changes to the image capture process and the image processing portion of the AOI systems.
Flexible films themselves are a challenge for the image capture and image processing portion of AOI machines. The substrate itself is very shiny and reflections from the board may increase false calls. It is possible that different illumination techniques in addition to changes in the image processing may be required to inspect these boards. To compound the problem, flexible films are by definition flexible. In order to build loaded boards, the films are pinned to a rigid substrate. Generally, the result is that the flexible surfaces are not very flat in random portions of the boards. This creates visual non-uniformities that decrease the signal to noise ratio and cause problems for laser based systems that attempt to map the depth and distortion of the board prior to inspection.
Flexible films also have different inspection needs than traditional PCB boards. Customers are requiring bare board inspection of the lines in the conductive path to look for such defects as "rat bites". (If the "rat bites" are found they can be fixed by adding some conductive ink on the path. Adding elements of bare board inspection to surface mount AOI inspection systems may require changes to the Image Processing and Program Generation module.
Thus, PCB material changes may require changes to all of the components of an AOI system. They may be the disruptive driving force which forces the invention of a new class of AOI systems. Additionally, they will force customers to rely on pre-reflow inspection for most of their AOI tasks.
5.3 Attachment: As described above, new component types and new materials are driving the attachment methods away from traditional solder paste and flux to conductive adhesive. Traditional solder paste inspection relies on the reflection of laser light. Traditional solder joint inspection is based on reflective properties of materials. New attachment materials do not have the same shape, reflectivity or laser opaque properties of solder. Thus, the image capture and image processing modules may need to be substantially changed to inspect the adhesives.
The adhesive is more likely to be completely covered by the pins and end caps of SMT components. Additionally, once attached to the component, they cannot easily be reworked. Altogether, pre-reflow inspection seems much more relevant than post-reflow inspection for this type of attachment method.
Whether printed by piezoelectric pistons, rotary pump or time/pressure, the appearance of the resulting solder deposits are very different. Pad-shapes may also be modified to improve the reflow characteristics of the new joint. As with adhesives, this means inspection systems that rely on rectangular brick shaped deposits, will need to change. And, implied volume calculations will be less accurate.
5.4 Sharing of Data: Based on the upcoming adoption of new PCB board technology and component technology, we anticipate it will be a requirement to inspect boards and parts at multiple stages of the assembly. As distributed inspection become more common, it will be desirable to share data across all the inspection machines. Additionally, we project that correcting defects after rework will become less and less feasible. Thus, boards need to be built right the first time. Direct feedback of variable data is necessary to reduce the occurrence of assembly defects and requires the development and incorporation of standard data interface and exchange. In the past, third party SPC and proprietary SPC data analysis have been common. New standards for CAD, part descriptions and assembly data are being developed by IPC, EIA/JEDEC, IEEE and RosettaNet promoted and proposed by iNEMI. These standards will allow for a common data exchange and lead to improved manufacturing and process feedback.
5.5 Cooperation: A new awareness of the benefits of cooperation and exchange of information and ideas is underway. Driven by the economic instability of global markets, suppliers who have not always been open to communication are now aggressively pursuing relationships with competitors. Less business and smaller profits are leading brand competitors to work together. Duplications of efforts in R&D among AOI companies with nearly identical applications are prohibitive given the low margins. Also, with the need for very different types of inspection systems, usually meaning several different suppliers in a production line, AOI systems must be able to communicate with each other across platforms for input programming and distributed data analysis. As the best AOI techniques are comparable in terms of their ability to perform inspections, the competition is between the best support organizations and the best programming processes rather than the best technology.
5.6 Summary
Table 2 shows a summary of how we believe AOI systems must evolve in the future to meet the new changes in PCB technology.Table 2: Summary of theorized future changes to AOI systems and their usage to meet the coming changes in PCB technologies.
Thus, if we compare Table 1 and Table 2, we see that the previous efforts, summarized in Table 1, were more towards changing one of the modules, the Image Capture Module, Image Analysis Module or Programming Paradigm to meet evolving needs of PCB inspection over the last ten years. Table 2 suggests that we are going to see a tightly coupled evolution of the three modules and their surrounding system architecture to deal with new PCB technologies. Additionally, we are going to see significant changes in AOI usage and also sharing of data across AOI platforms. We may also see that X-ray and AOI will play an important role together in the future to inspect new PCB substrates, packages, and attachment materials.
6. What should I look for in an AOI system now that will allow me to address these new technologies?
The most important attribute of an AOI system is a flexible or object oriented system architecture that allows the AOI designers to plug in new image capture devices, image processing algorithms, and programming paradigms. The system architectures should be designed so that they can output data in a standard format to be used for analysis and process control. A properly architected system can "plug" in new modules and output new types of data without disruption of the whole system. This type of system can adapt and grow to meet new challenges and is a better investment than one that needs to be overhauled for every change.
With that said, it is impossible to evaluate the system architecture and its modules without a full engineering review. But a review of the products offered by AOI vendors in the past and the technology roadmap ought to be reviewed. Customers can review the programming interface and the quality of the results during evaluations and demos. We suggest however, that a customer focus on accuracy and repeatability of results, the ease of use, and data analysis over time rather than just looking at the hardware or, as we like to call it, "curb appeal." A supplier ought to be able to demonstrate their programming process and results rapidly. They should allow the customer to put their hands on the machine. They also ought to be able to leverage current part library data and input data formats of customers' current installed systems. Another critical point to examine is the level of knowledge required for program generation. Any programmer ought to get identical inspection coverage and the same level of diagnostic repeatability, regardless of experience.
Finally, customers need to change their mind set from using AOI as an end of line defect screening device and think more of it as a continuous process improvement tool. This change in mind set is required by the new technologies that are coming. However, it can also significantly improve yields with the existing technologies.
7. What is the role of AXI in the future?
More hidden joints and new interconnect technologies have and will continue to drive the need for AXI and other image capture techniques for examining hidden joints. The high cost of AXI as a separate inspection platform is still a deterrent to adoption. Cycle time requirements do not currently match line rates for complete inspection and many defects cannot be visualized by X-ray. System costs are high compared to AOI. Currently, AXI tends to be used as a process development and process monitoring tool. In the future, AXI is more likely to be combined with AOI in a single platform. If techniques for examination of hidden joints can be incorporated into a combined optical inspection system, and can be applied only where needed it is more likely that customers will consider them. Ideally, a well architected AOI system should be able to plug in AXI image capture methodologies and image processing techniques as well as those traditionally associated with AOI to facilitate the creation and evolution this type of combination machine.
8. Summary
For the past 20 years we have seen a number of changes in PCB technology. While these changes were dramatic, they had a relatively small impact on AOI technology. However, the adoption of new PCB technologies such as PoP, flexible circuits, and conductive adhesives may drive major changes in AOI technology. In addition, the new materials and manufacturing processes allow for little error. As a result, they may drive major changes in how customers use AOI, such as relying more on pre-reflow inspection. Finally, analysis of defect and measurement data, feedback of data to the process, and sharing of data will be required to build more boards correctly the first time.
We are also seeing a rise in the use of stencil free solder printing techniques. The advantages are obvious in terms of flexibility, cost reduction, reduced changeover and elimination of waste.