Factors influencing QFN process
Figure 4. The component from Figure 3, after reflow—the first three pads are not soldered.
package. If data is not fed properly, it will lead to bridging in QFN package. Let me explain here. Normally, the machine is pro- grammed for a component’s basic param- eters: length, width and thickness. Suppose the thickness of the machine is H, and we program it for H+0.1. Te nozzle of pick and place machine will go down little extra and press the component on the pads. If there is solder paste on the pad, it will press
Solder joint embrittlement —continued from p. 59
1. Luef , C., et al., “Te In–Pd–Sn Phase Diagram (xPd
Intermetallics Vol. 13 (2005), pp.1207- 1213.
2. Wild, R., “1974 IRAD Study—Fatigue Properties
of Solder Technical Report No. Joints,” IBM M45-74-002, Contract No. IBM 4A69, January 5, 1975. 1 masking 2 masking
Werner Engelmaier will be giving some of his reliability workshops on April 4 & 5, 2011 in Tel Aviv. He is available for in-house workshops and consulting in both English and German on a world-wide basis;
for
details go to
www.engelmaier.com or email
engelmaier@aol.com. Werner Engelmaier has over 45 years
experience in electronic packaging and interconnection technology and has pub- lished over 210 papers,
columns, book Figure 5. Solder paste masking examples.
should one start production. Also be sure to use solder paste height measurement equipment to periodically check the height of the component.
PCB pad design While designing the PCB for QFN package, the following should be taken into account: 1. All of the pads should be of same size and shape. A slight change, bigger or smaller, could lead to either exces- sive deposition of the solder paste or less deposition of the solder paste, as explained above.
2. Masking on the solder paste should be properly done or else it will lead to process failure. In Figure 5, the mask- ing #1 is absolutely wrong—as you can see, the mask is far away from the pad. In this case, solder would flow towards the track, reducing the height of the solder on the pad. Tis could lead to either very thin joint or no joint at all. Masking #2 is perfect. It should be as close as possible to the pad. Please refer to PCB design pad and masking standards from the QFN manufacturer. Only aſter that send your design for PCB manufacturing..
Pick and place machine Te pick and place machine’s parameters and its calibration also play a very impor- tant role in joint formation of the QFN
www.globalsmt.net
into the solder paste and result in bridging of the joint. In another example, having the parameters set properly does little good if the the machine is not calibrated. Suppose the nozzle is 0.2 mm below the datum level. Again it will press the component onto the PCB, resulting in solder paste spreading across the pad, leading to bridging.
Stencil aperture To avoid defects due to spreading of the solder paste, try to make the pads’ opening 20% less than the actual pad size. We have done this in our process and we are 100% successful in it. As you can see, by following the basic
rules we can avoid process defects when dealing with QFN packages. Te rule of thumb is to stick the basics and try to follow the component manufacturer rec- ommendations for process and product design.
chapters and White Papers. Known as ‘Mr. Reliability’ in the industry, he is the presi- dent of Engelmaier Associates, L.C., a firm providing consulting services on reliability, manufacturing and processing aspects of electronic packaging and interconnection technology. He is the chairman of the IPC Main Committee on Product Reliability. Te TGM-Exner Medal was bestowed on him in 2009 in Vienna, Austria, he was elected into the IPC Hall of Fame 2003, and was awarded the IPC President’s Award in 1996 and the IEPS Electronic Packaging Achievement Award in 1987. He also was named a Bell Telephone Laboratories Distinguished Member of Technical Staff in 1986 and an IMAPS Fellow in 1996. More information is available at www.engelmaier. com, and he can be reached at engelmaier@
aol.com.
≤0.6): Isothermal Sections,”
Figure 6. The QFN has been pressed against the pad, causing the paste to spread.
Global SMT & Packaging – February 2011 – 61
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