Read about it in Quality Digest: http://www.qualitydigest.com/print/7638
By Steven A. Zweig
Ball grid array package handling has it own set of problems and requirements. In replies from the field, five assemblers report how the technology is meeting quality demands.
When board designers began incorporating ball grid array (BGA) packages into their PCB layouts, electronics assemblers were faced with new and unknown challenges for processing these new forms of leadless components. Manufacturing and process engineers had a variety of theories, some of them conflicting, but little practical experience to guide them in adapting their processes to accommodate BGA packages. Contract manufacturers, in particular, often found themselves with the need to produce BGA boards on short notice. One major concern was the issue of inspection: How to verify a solder bond that cannot be observed? The use of X-ray inspection was proposed and, in some cases, summarily dismissed as too expensive. However, a survey of five different manufacturers currently processing BGAs reveals that X-ray inspection systems have proven to be valuable and cost-efficient tools for establishing process parameters and auditing production.
Establishing a Process
One contract manufacturer using film-based transmission X-ray to inspect BGAs is Fine Pitch Technology (FPT) of San Jose, Calif., a wholly-owned subsidiary of Solectron, with 160 employees. The company spent about eight months developing a process and has been producing BGA boards for use in computers (both Internet and networking products) for about 18 months. Frank Vernon, process engineering manager, says that the company also does significant BGA rework, often on boards not originally assembled at its facility, on a rapid turnaround basis.
For rework projects, boards are usually X-rayed on arrival to see what problems may exist from the original processing, and again after rework. For work that originates at FPT, Vernon has found few problems. “The BGA process works well. We generally have not had to repair anything we’ve done … yields are excellent.”
|FIGURE 1: Bridging, a common defect shown here, can be identified using the X-ray system.|
Vernon notes that the X-ray system is useful for identifying bridging, shorts and collapsible opens (Figure 1). Having gained experience with both BGA and X-ray inspection over time, he adds, “We’ve begun to develop a sense of will and will not work.” He recalls one instance where a design was received with a mask that was done incorrectly. After pointing out the problem, he was told to go ahead anyway. Only five boards were run, each was X-rayed and the problem was found; the remainder of the run was re-masked. “The X-ray enabled us to spot the problem early on,” states Vernon. And, on the issue of voiding, Vernon reports that process control is the best way to prevent it. “It’s a question of getting your process right before you start.”
The images that are produced during X-ray inspection are generally filed and have often proven useful in helping to answer questions. “If a customer calls and says he thinks there’s a short under a BGA, we ask for the serial number and pull the photo,” he continues. “We can say with certainty whether a short exists. On request, we can even send the photo along for verification.”
Checking Both BGAs and ICs
Another large contract manufacturer handling BGAs is Altron, located in Anoka, Minn. (200 employees and annual sales of nearly $30 million.) The company has been processing boards bearing both plastic and ceramic BGAs in a variety of sizes up to 503 I/Os for about two years. Production supervisor Gary Smith explains that X-ray inspection is used to establish process parameters and then for a 1percent production audit at two points in the process: after placement to check for component alignment and after reflow to inspect for bridging, opens and solder voids.
As to BGAs, Smith says, “We like them. We haven’t encountered many problems or found any major defects yet. Bridging is very minimal. And we haven’t found that solder voids are a problem down the line. The few we found were very severe and were sent back to the component manufacturer.”
He reports that boards are more likely to be rejected for misaligned components, improper rotation, insufficient solder or major outgassing. Even though the transmission X-ray reveals obvious outgassing (but not minute holes), this has not proven to be a concern for Altron over the past two years. “The minute holes don’t affect functionality and there is no spec on BGA yet. If they do have any long-term effect, they haven’t come back to us yet,” Smith says.
In addition to BGA inspection, the X-ray system has been used by the test department to track internal shorts in boards and to identify problems with ICs. In one case, Smith explains, “We were experiencing 30 percent failures. The component manufacturer claimed that our process was at fault. To determine the truth, we took the parts directly from the waffle packs and X-rayed them. Sure enough, we found internal shorts.” The company was able to present X-ray images of the problem ICs to the manufacturer to verify their findings. Overall, Smith finds the X-ray system “a useful tool for verifying your process.”
A Place with Smaller Assemblers
Accu-Tronics of Raleigh, N.C., with 75 employees and under $10 million in annual sales, specializes in high technology, moderate-volume production and rapid prototyping. It has been assembling boards with both PBGAs and CBGAs for two years. President Scott Brown says the company inspects 100 percent of prototypes, then samples 10 percent of low volume production runs. It also performs a first-piece X-ray inspection for placement accuracy prior to reflow.
Overall, defect levels are extremely low since the BGA is an inherently superior package to the fine-pitch QFP. “On occasion we find bridging, misregistration and missing solder balls,” Brown reveals (Figure 2). “Our X-ray inspection system is good for these problem classifications and we’re still evaluating the acceptability of void content in the solder joints. I think everyone’s trying to figure that out.”
|FIGURE 2: Although BGA defects are relatively low, missing solder balls can be detected using the X-ray system.|
Another small manufacturer uses real-time X-ray to inspect BGAs. World Electronics of Morgantown, Pa., with 65 employees, manufactures elevator electronics and also does contract assembly. The company has been processing BGAs for its contract customers for about eight months. Quality Control manager John Menuchak explains that, because the process is new and most of the runs are small (40 to 50 boards average), 100 percent of BGA production is currently inspected. As both volume and confidence in the process increase, he anticipates moving to production audits for inspection. Among the few defects: “Only one or two with bridging, one misregistration,” Menuchak reports. “We haven’t seen any problems yet with missing bumps or voids. Of our total BGA production, defects have been a fairly low percentage.” World Electronics has installed a portable RTX Mini real-time X-ray system* in its quality control laboratory.
To Speed Prototyping
Encore Computer Corp., in Melbourne, Fla., is a manufacturer of scalable real-time data-storage, data-retrieval and data-share technologies. The company is in the process of developing boards with BGAs and uses a portable real-time RTX Mini X-ray system for 100 percent of its prototype runs. “X-ray inspection is imperative so that we know the process,” says process engineer Pat McDonough. Initially, the company inspected components both before and after reflow. “When you can’t see under the component,” says McDonough, “that’s a process engineer’s worst nightmare.” But as prototyping progressed, the first inspection has been eliminated. “We have confidence that BGAs are self-aligning and we know that we are placing them accurately,” he explains. In post-reflow inspection, McDonough finds that transmission X-ray is very good at picking up voiding and shorts. “We experienced some bridging, but once we corrected our process, it disappeared.” Overall, he finds that “BGAs are highly manufacturable parts. Defect rates are as low as 1 to 4 ppm.”
The X-ray inspection system has also been useful in identifying popcorned (cracked molding) BGAs. Some components had been used on an early prototype run but apparently the remainder were not properly sealed in their plastic packaging. When they were used again a month later, they had absorbed moisture. “Plastic BGAs are very sensitive to moisture” explains McDonough. “After X-raying the first board and seeing the popcorn effect, we baked the rest of the components and eliminated the problem. There’s no way we could have established process parameters as quickly as we have without the X-ray.” he continues. “Absent that capability, those boards with popcorned BGAs may have sat for months on a test table for debugging. We’ve come to learn a lot using X-ray, because it tells you a lot about your process.”
*RTX Mini is a registered trademark of Glenbrook Technologies.
As reprinted from SMT, August 1997.
By Gilbert Zweig
The incorporation of BGA packages into electronics assembly has made inspection issues of these leadless components a primary concern. Because solder bonds hidden underneath the package preclude the use of visual inspection as a way of confirming solder joint integrity, many assembly manufacturers are turning to analytical laboratory x-ray systems for BGA inspection. Using film, real-time x-ray systems or a combination of both, assemblers have been able to fine-tune their process parameters in advance of production. Occasional audits are then conducted during production to maintain the quality of assembly processes.
After several years of experience using BGAs, reports from the field have identified four major categories of defects common to BGAs, all of which can be identifies with x-ray inspection. The first type is missing solder balls (Figure 1). Assemblers report that it is not uncommon for these tiny spheres to become dislodged during the handling process, resulting in an incomplete solder bond.
|FIGURE 1: An x-ray view of a BGA package reveals missing solder balls — a problem related to BGA handling prior to placement on the substrate.|
The second category, misregistration (Figure 2), is the result of errors either in component placement or adhesive printing. The precise alignment of the solder balls on the underside of BGAs to the adhesive pads on the surface of the PCB is critical.
|FIGURE 2: Misregistration between solder balls and adhesive is the result of imprecise placement or imprinting.|
Solder bridging (Figure 3) between two or more solder balls is a defect that seems to be related to rework. When a board is being reworked to correct some other defect, there are indications that solder bridging may be associated with the resoldering process.
|FIGURE 3: Solder bridging is a flaw that appears to be related to rework.|
The fourth problem that can be identified with x-ray inspection is the presence of voids within the solder balls on the underside of the component (Figure 4). A related condition known as “popcorning” occurs if moisture is trapped in the solder paste. When heat is applied during reflow, the moisture expands suddenly, leaving either solder voids or irregular shapes that resemble pieces of popcorn. Both conditions have an adverse effect on solder joint integrity.
|FIGURE 4: Voids within the solder balls result from moisture that has not been thoroughly baked out.|
When assemblers first started using BGA components, they had concerns about a fifth type of defect that would not appear on an x-ray image: the lack of solder wetting of a ball with respect to the pad. Since x-ray by its nature reveals the presence or absence of material, the image appears the same whether the solder does in fact flow and merge with the adhesive, or whether the two substances remain intact and separate. Field experience, however, has shown that this defect may be observed by other means
The solder balls used on many BGAs are designed to be collapsible when reflow heat is applied. When this occurs over the entire underside of the component, the whole component will sit lower on the board. If the balls under only one portion or side of the component collapse, that component will be tilted at an angle, indicating that the noncollapsed balls have not wetted.
A second reason for a lack of solder wetting is the condition known as “potato chipping,” or warpage, of either the component or substrate. This problem is readily observable, as visual inspection enables assemblers to identify dropped, tilted or potato-chipped components that are symptomatic of a lack of solder wetting. Therefore, a combination of both x-ray and visual inspection has proven effective in identifying flaws that may occur when using BGAs.
The Root of the Problem
Identifying these defects is only the first step in achieving higher yields, as each defect is actually symptomatic of a process problem that needs to be addressed. If missing balls are a common occurrence, perhaps in-house handling procedures need to be revised. Or, going back another step, both this situation and a frequent finding of solder voids may require further attention from the BGA vendor, with inspection data being used as evidence.
If misregistration is the issue, both imprinting and placement processes need to be studied. It may prove necessary to upgrade one or the other system to achieve the required degree of accuracy. It has observed, however, that if a BGA package is slightly skewed after placement, the surface tension occurring during solder reflow can pull the component back into the proper registration. Whether this is an isolated occurrence or a quantifiable trend remains to be seen as the application of BGA expands.
The issue of non-wetting due to collapsed solder balls may indicate an uneven distribution of heat in the solder reflow process. It could also be due to moisture remaining in either the packages or the substrates — the same factor that results in popcorning and potato chipping. To inhibit these problems, all materials involved in the assembly process must be baked thoroughly to remove any remaining moisture, either as an in-house procedure or by the raw material supplier.
The role of analytical x-ray inspection, therefore, is to act as an early warning system. Consider an assembler who is choosing between having continuous in-process x-ray inspection, or using analytical x-ray inspection to establish process parameters and then make occasional production audits. The assembler is, in effect, deciding whether to have a machine “pass judgment” on the process, or to rely on the expertise of people who have developed a thorough understanding of the contribution of materials and processes to the yields of the manufacturing process. Developing diagnostic skills that identify and resolve manufacturing problems does not come without effort. Learning how to use analytical x-ray inspection to perform manufacturing research as well as establishing rigorous parameters to ensure the process is essential. Additional cost considerations may include a capital investment in x-ray equipment of approximately $15,000 to $60,000 — an amount considerably less than in-line systems.
Answering all of the questions raised by the emergence of BGA is not easy. Each company must operate within the confines of its skills, experience, talent and financial resources. This does not mean that new technologies should be avoided because there is a perception that they require massive capital investments. On the other hand, no matter how deep its pockets, no company can afford to let automated inspection take the place of talent and insight when it comes to understanding a particular manufacturing process. Such insight can be developed only by a detailed and analytical study of cause and effect, which is the essence of the manufacturing research process that lays the foundation for quality production.
Reprinted from Advanced Packaging, Nov./Dec. 1995.