QFN devices require x-ray inspection
Steve Scheiber, Contributing Technical Editor -- Test & Measurement World, 4/1/2007 2:00:00 AM
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Jeremy Jessen Technical Marketing Engineer Agilent Technologies Courtesy of Agilent Technologies |
Quad-flat-pack no-lead devices (QFNs) attach to boards in different ways from conventional quad flat-packs (QFPs). Manufacturers have to inspect boards with QFNs differently as well. I asked Jeremy Jessen, technical marketing engineer at Agilent Technologies in Loveland, CO, to discuss the impact of no-lead devices. During our discussion, Jessen explained that QFNs not only offer space savings, but they also demonstrate superior electrical and thermal characteristics.
Q: How do QFNs make contact with boards?
A: By soldering an exposed metallic pad beneath the package body to the printed-circuit board instead of using leads around the device perimeter.
Q: How do QFN failure mechanisms compare with those of QFPs?
A: QFPs are leaded devices, where the leads are soldered to the land pattern on the board. Most QFP failures occur at the leads—such as insufficient solder or lifted leads. Sometimes the solder wicks up to the top of the lead during reflow, causing a short.
Because QFNs have no leads, there are no leads to fail, although a pad might still contain insufficient solder. Excess solder on the pad might cause the device to tilt. In that case, one side would show very strong joint formations, while the other side shows nothing but opens.
Q: How do people inspect the boards in each case?
A: Boards with typical leaded devices can use either optical or x-ray inspection. Non-leaded devices like QFNs have limited to no line of sight, so they require x-ray inspection.
Inspection of leaded devices is fairly simple. Leaded-device solder fillets are fairly uniform and include a heel, a center, and a toe. The heel sits relatively high on the board. The solder drops down at the center, then rises again (although not as much) for the toe. The inspection step compares these relative heights. A higher-than-normal center could denote a lifted lead. A joint’s wicking characteristics determine whether it is wide enough, so measuring length alone will verify sufficient solder volume.
QFN solder joints don’t all look the same. Depending on the type of pad on the QFN, a joint might form a large toe outside the perimeter of the package, while other types form a joint beneath the package. The heel, center, and toe may be the same height, forming a solder plateau.
Inspection systems measure the length and height of each component of the joint, but they also measure the width of each section by looking across the joint at gray levels per pixel in the x-ray image. Too narrow a center might mean an open. Com-paring the measurement results to an “average” ideal (with appropriate tolerances) decides pass or fail.
Q: How has the switch to lead-free solder affected failure characteristics?
A: In the examples we have seen, wicking behavior generally didn’t change much. Open joints are not as flat as with leaded solder, showing more of a peak. We do experience a significant increase in the number and volume of voids in the solder. If the voids represent no more than about 10% of the total solder volume, we don’t call it a fault, but we would fail a joint with 20% voids. Some x-ray systems can misinterpret the information from the voids, calculating solder heights too low and inappropriately failing some good joints as open. We compensate for the voids to reduce that effect.
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