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The supporting role that X-ray inspection plays for DFM is critical to the SMT process.
By Jack Yettaw and Steve Zweig
As printed circuit board (PCB) assemblies have evolved into highly complex entities, the design for manufacturability (DFM) process has assumed a critical role in determining the success of the overall production process. A number of technology and marketplace factors have contributed to this situation.
With the multiplicity of advanced substrates, packages and interconnect technologies now on the market, and with new ones emerging on what seems to be a daily basis, the options available to design engineers have expanded dramatically. Multilayer boards can be populated on both sides with a variety of through-hole, traditional SMT components, and advanced packages such as ball grid arrays (BGA) and chip scale packages (CSP) with each type of component having its own process requirement. At the same time, board real estate continues to shrink while performance and speed requirements rise steadily, increasing the density of the assembly.
To complicate the equation, both time-to-market windows and product life cycles have become significantly shorter. Lengthy pre-production evaluation procedures are no longer economically feasible, especially for new products that may have a life cycle as short as six months before a newer technology comes along.
For the company that designs a new product and the contract manufacturer (CM) that assembles the product, the road to productivity and profitability must be straight and short. Neither company can afford to have production detoured by incorrectly aligned multilayer boards, inappropriately sized vias or masks, BGAs that are placed too close together or a host of other issues.
Given these constraints, DFM is required to ensure that all issues related to manufacturability are resolved before an assembly reaches the production stage. Not only must the process be comprehensive, it must also be quick (often with a turnaround as short as 16 to 24 hours).
One of the most valuable tools supporting such a timely and thorough process is real-time X-ray inspection, because it is the only way to obtain immediate feedback on the numerous hidden factors than can affect an assembly. A review of the DFM process offered by one CM illustrates the value of the process and the critical supporting role of real-time X-ray inspection.
The DFM ProcessDFM starts when the customer, usually a design engineering team at a new product development company, provides components and documentation for a prototype build, often consisting of only three to five assemblies. The CM may supply the boards and some of the more standard components, with the customer supplying the more advanced parts. In these low quantities, it is usually more economical to hand-load the boards, using a mini-stencil to apply solder paste for BGA components.
Figure 1. Valuable data for the DFM process: real-time X-ray inspection reveals misregistration of hole-to-pad alignment on a multilayer PCB.
Following reflow, 100 percent of the components on the prototype boards, as well as the boards themselves, are inspected using a real-time X-ray system. The purpose is to seek out and identify flaws of all types in the boards, the components and the design, and to answer the design engineers' questions about the assembly's overall circuitry, functionality and manufacturability. A report is issued, accompanied by prints of the X-ray images to provide documentation on any items that pose potential problems (Figures 1, 2, 3 and 4).
Figure 2. Broken capacitors can be identified through real-time X-ray inspection.
Alternatively, the design team can visit the CM to review the results in person. Providing an accessible environment that is conducive to thorough DFM review by all parties involved adds value to the process. It enables the two companies to work together to resolve design issues because it is not unusual for this report to raise more questions than at any other stage of the development process. It is also common to go through two or three prototype builds and DFM reviews in the course of bringing a new product to the point where it is ready for full-scale production.
Figure 3. A short on a component is easily identified.
DFM questions may be generated by the boards themselves and resolved by X-ray inspection. These questions often relate to whether the sizes of the vias are appropriate for a particular application. Via placement can be another concern if the vias are too close to a solder pad, bridging can occur. Inner layer misalignment in multilayer boards, which can lead to shorts, needs to be considered, as well. The mask must also be inspected to ensure that solder is not deposited in areas where it is not required.
Figure 4. When a part is viewed at a 45° angle, the real-time X-ray image reveals hidden problems, such as solder filling the via (shown at the arrow).
For SMT components, the board's pad sizes need to be inspected and compared with the component size, to ensure compatibility. A pad that is too large or too small for a specific package is sure to cause problems in production. Fine-pitch SMT parts need to be inspected to verify the bond between the leads and the board surface.
Standards relating to the particular industries in which the assemblies will be used also need to be considered at this early stage. Whether an end product will be used in a personal computer, workstation, Internet technology, communications device, data storage or medical product, determines whether it needs to meet a specific requirement such as a telecommunications standard or FDA compliance.
During DFM review, many manufacturability questions are raised by the numerous leadless BGA-type components available on the market. Plastic BGAs (PBGA) are now used in a wide variety of assembly applications. There is also an increase in the use of the more challenging SuperBGAs, which, with their dense metal upper surfaces, tend to readily disperse heat. Because SuperBGAs are frequently used in products that must function in high-temperature environments, verifying their properties is critical.
As an example, one current data storage product consists of a large copper board with 17 BGAs (both PBGAs and SuperBGAs). In this case, the engineers were interested in how closely to each other the BGAs could be placed. To determine this, it was necessary to consider issues of clearance to allow for future rework and electrical properties such as impedance.
To inspect BGAs, real-time X-ray is the only method that resolves the "design engineer's nightmare," allowing the component's internal structure to be reviewed. Even though most BGAs tend to be self-centering, there is no way to be certain of solder joint integrity without X-ray inspection. Among the problems that can be revealed are missing solder spheres, spreading of the solder spheres, solder voids, tilting of the component and the phenomenon known as "popcorning."
This last problem derives from the BGAs' hydroscopic tendency. When only a few BGAs are removed from a tray for use in a prototype build, resealing the package tightly is critical. Otherwise, when that package is reopened at a later date, the remaining BGAs may have absorbed moisture from the environment, which will not become apparent until after reflow.
X-ray InspectionIn every aspect of DFM review, real-time X-ray has proven to be a valuable tool for revealing the internal structures of an assembly, allowing design issues to be resolved and ensuring manufacturability before full-scale production begins. The same inspection system used for prototype builds can also serve a dual function in performing process control for BGA production.
When selecting an X-ray inspection system, image accuracy and lack of distortion are important criteria. A system with high resolution, even at low voltages, provides the most accurate image. In addition, the X-ray camera should minimize the tendency for defects, e.g., voids that "bloom" as voltage is increased.
The reliability of the system is another critical factor. With 16- to 24-hour turnarounds, and as many as 10 new boards under review at any one time, the X-ray system must operate around the clock. A CM offering DFM review simply cannot afford equipment downtime. Therefore, it is important to select a reliable system from a supplier that can provide rapid service on those few occasions when it is required.
The ability to consult with the X-ray system supplier when there are questions about inspection matters is also an advantage. Just as the design engineer relies heavily on the expertise of the DFM reviewer, the reviewer in turn relies on the expertise of the X-ray inspection system supplier.
JACK YETTAW, director of quality, may be contacted at United Manufacturing Assembly, 42680 Christy St., Fremont, CA 94538; (510) 490-4689; Fax: (510) 490-4380; Web site: www.umai.com. STEVE ZWEIG, sales manager, may be contacted at Glenbrook Technologies, 11 Emery Ave., Randolph, NJ 07869; (973) 361-8866; Fax: (973) 361-9286; Web site: www.glenbrooktech.com.