How do manufacturers inspect BGAs?
|Not long ago, manufacturers soldered components to printed circuit boards by passing the underside of a loaded board over a large "wave" of molten solder. Human inspectors checked the boards to ensure that all leads were properly soldered. Now, manufacturers carefully print solder on a PCB and position SMT components with closely spaced leads on the solder. A reflow oven replaces the solder wave.
One of the newest SMT packages, the ball-grid array (BGA), no longer relies on wire leads. Solder alone forms the electrical and mechanical link between the BGA and the underlying circuit board. Thus, BGA manufacturers must use exactly the right size solder balls, and they must position them properly. To ensure the quality of the small solder balls and their proper placement, they rely on automated inspection systems. The system may come as part of the BGA assembly equipment, or the BGA manufacturer may have added it later. Many companies supply complete inspection systems as well as individual machine-vision components.
A BGA is nothing more than a small piece of material onto which a manufacturer places an IC die (Fig. 1). A wire bonder connects the die’s contacts to contacts on the substrate. Instead of individual fragile leads such as those found on the outside edges of a quad flatpack, the BGA package provides an array of conductive pads on its underside. During manufacture, a machine "prints" a small amount of flux on each pad and then precisely places a solder ball on each fluxed pad. Next, the BGA passes through a solder-reflow process, which melts the solder balls to attach them to the pads, thus forming bumps on the BGA.
Usually BGA manufactures place inspection systems at more than one point on a production line. Inspection may take place after placement of the solder balls on the fluxed pads, after reflow, after dividing strips into individual BGAs, and just before shipping. Most inspection systems have a high throughput rate, typically from 2500 to 4000 solder balls per second, so they don’t slow production. Even when you include time to move, or "index," the strip of BGAs, the inspection system can easily keep up with ball-placement rates (Fig. 2).
The first inspection usually takes place after a placement tool positions the solder balls on the fluxed pads. This inspection looks for missing solder balls as well as extra solder balls that may have fallen onto the BGA. The inspection system also may check to ensure the solder balls are the proper size. The system may also check for oxidized solder balls and may look for ball-to-pad variations.
The inspection system checks each ball on every BGA in a strip. BGA manufacturers use the information obtained during inspection to reject defective BGAs or to route them to a rework station. They also can use the inspection information to improve their production processes.
If balls are missing, it may indicate that the tool that places the balls on the fluxed pads simply misses a ball once in a while. But when BGAs consistently lack solder balls in the same area, the culprit may be a placement tool that requires maintenance. Residue at one of the points on a placement tool may prevent proper pickup of a ball.
Cleaning the placement tool may solve the problem. Once in a while, flux will get into the tool, requiring an extensive cleaning of the ball-placement equipment and scrapping of contaminated solder balls in the tool’s hopper.
Although placement problems might be caused by a batch of solder balls that includes balls of the wrong size, these days mis-sized solder balls are not a problem. Solder manufacturers ensure high-quality same-size balls within a lot. (See "How Do They Make Small Solder Balls?" below.)
Excess Solder Indicates Problems
Extra solder balls also can indicate a problem with the placement tool. In some cases, the tool may feed two balls onto a BGA pad. One lands on the fluxed pad and the other lands close by. The tool may require maintenance so it feeds only one solder ball at a time at each position. An extra solder ball can cause problems during reflow by combining with a solder ball on a pad; this results in a larger-than-specified solder ball and can ruin the BGA. Or the extra ball may cause bridging between solder balls on adjacent pads, which also ruins the BGA.
The inspection system at the ball-placement step notes any missing and extra solder balls and instructs the manufacturing machine to move the BGA strip to a rework tray rather than to reflow soldering. During rework, an operator can remove any extra solder balls and place a new ball on an empty fluxed pad by hand. By repairing the BGA prior to reflow, the manufacturing operation reduces the number of parts it rejects. Adding a ball to an empty position after reflow can be difficult. And some customers will not accept a BGA that has gone through the reflow process twice—once during regular manufacturing and again during rework. So, spending money to catch problems early enough to correct them makes good economic sense.
In most cases, an inspection system graphically shows which BGA strips contain devices with problems. The tracking software shows the rework-station operator which BGA on a strip requires rework and at what pad or pads. Without this type of information, the operator would face an impossible task of locating a problem site from among as many as 10,000 other sites.
Inspection Systems Learn Quickly
You may wonder how the inspection system knows what to look for when it inspects a BGA. The inspection software can "learn" about a new type of BGA in several ways. The software can accept CAD files that identify solder-ball sites, learn solder-ball placement information from known-good devices, or let operators enter grid information by hand. (As new package styles appear, the software should be smart enough to take them into account.) No matter which approach the inspection-system supplier offers, operators must adjust the specifications for their particular production equipment and parameters. For example, they must establish their own tolerances for ball placement and ball sizes.
Because production processes all have variations and imperfections, production engineers will routinely refine the inspection parameters to best fit their quality and process needs. For example, a BGA manufacturer may want to place balls precisely on the center of fluxed pads. But a production engineer may allow a variation of up to 30% in ball placement on individual pads. The engineer knows that during reflow the melted solder tends to recenter itself due to the surface tension of the solder and the adhesion to the BGA pads.
After setting up the solder-ball grid and the starting set of parameters, manufacturers run known-good BGAs and defective BGAs through the system to be sure the inspection system properly distinguishes between them. Using the defective parts also helps verify that the system can accurately and reproducibly identify specific problems.
Don’t Give Up On People
If you visit a BGA production line at a plant run by Amkor (Chandler, AZ), a large IC packaging and testing company, you’ll find human inspectors examine all of the BGA strips as they come out of the aqueous-cleaning process that follows reflow soldering. According to Tony LoBianco, an end-of-line engineering manager at Amkor, an automated inspection step after ball placement catches gross defects. The human inspectors identify secondary problems such as bridged pads, empty pads, double balls, and so on, that occur during reflow.
After passing an inspection by a person, the BGA strips move to a stamper which "singulates," or cuts, the individual BGAs from the strip. This step also can benefit from an inspection for mold problems such as incomplete material fill of the BGA package, excess mold flashing, and surface pits.
This inspection step also checks the device for problems with marking: illegible printing, double printing, improper print orientation, and so on. The inspection system also may check the BGA’s substrate to locate any nicks, mouse bites, burrs on the edges, metal on the edges, and other substrate-related defects.
Inspect the Package, Too
After reflowing the solder, performing an inspection, and stamping the BGAs into individual devices, manufacturers may perform yet another automatic inspection. But this time they gather geometric information about the solder balls from which they can deduce the state of the BGA package itself. They want to be sure that when a PCB assembler user places a BGA on a circuit board, all the solder balls properly contact the solder paste at the corresponding circuit-board pads.
If the manufacturing processes warp a BGA slightly, some solder balls may not properly attach to pads (Fig. 3). Reworking boards that contain defective BGAs can be expensive, so manufacturers aim to ship only BGAs that meet a set "flatness" or coplanarity spec. Keep in mind that the coplanarity measurements are on the order of only a few mils.
The 3-D inspection checks the coplanarity of the solder balls to be sure the tops of the balls are within either 6 or 8 mils of each other, depending on the customer’s requirements. If the tops extend outside of that range, the BGA may have a problem that could prevent all the balls from making proper contact with a PCB.
After the inspection system makes its measurements, it calculates either a three-point seating plane or a regression plane. The three-point seating-plane calculation locates the tops of the three highest balls and calculates the equation that represents a plane through those three points. The software then checks to be sure all the other ball tops on the BGA exist within the space between the seating plane and a parallel plane 6 mils (or 8 mils) below it (Fig. 4). Because the software identifies the three highest balls, all the other ball tops exist below the seating plane.
This technique may not work well for BGAs that include a large thermal matrix—a densely packed array of solder balls—in the center of the ball grid. A thermal matrix provides a large number of solder balls in a small space, increasing the probability that the three highest balls will be found there. When the three highest balls exist in the center of a BGA, they produce a small reference plane that can result in poor coplanarity measurements. Those measurements can cause the inspection system to reject good parts.
Regression Offers an Alternative
To overcome the limits of the three-point seating-plane technique, inspection-equipment manufacturers may offer a regression-coplanarity technique. This technique uses regression analysis to find the best plane that will fit the ball-height data for all the solder balls, not just the three highest ones.
The software then "moves" this plane so it rests on top of the highest ball. Finally, the software checks the top of each solder ball to determine whether or not it resides between the calculated regression plane and the parallel plane 6 mils below it.
With this technique, a concentration of high solder balls in the center of a BGA will not skew the plane. The regression technique provides repeatable results that are generally the same for each BGA in a production lot—unless the BGAs have serious solder-ball problems, which the technique will detect. BGA suppliers must make sure their customers understand the choices of coplanarity measurements at their disposal and the possible effect of each technique on production yield.
According to Jeff Woolstenhulme, industry marketing manager at Cognex (Natick, MA), a company that manufactures 2-D inspection systems, coplanarity is an issue for some leaded SMT IC packages, because the fragile leads are so susceptible to damage. Today, he says, some BGA manufacturers are finding they don’t need to determine coplanarity. As they improve their processes, the problems of warpage during reflow recede. And, thus, the need for full 3-D inspection lessens, too.
Not everyone agrees that the need for 3-D measurements will ebb. Arye Malek, vice president of marketing at PPT Vision (Eden Prairie, MN) takes a different view. He feels that a fast 3-D inspection system that scans 100% of the BGAs coming out of production can provide manufacturers with additional information. His company offers a proprietary scanner that employs scanning moiré interferometry (SMI).
Laser-based systems acquire data a point at a time, but the SMI pro-cess obtains x, y, and z coordinate information over an entire area at once (Fig. 5).
Even though inspection systems play important roles in the proper placement of solder balls on BGAs, production-equipment manufacturers and BGA manufacturers debate about where to place inspection stations to best increase production yields. The BGA manufacturers could place an inspection system at every step, including punching, printing, solder-ball placement, reflow, and so on. But they have to weigh the costs of the added inspection systems, including software and maintenance, against the potential for increasing the yield of BGAs and the prospect of improving their production processes.