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www.us-tech.com
September, 2017
Developing an Ultra-Low-Voiding Solder Alloy
By Ralph Lauwaert, Technical Engineer, Interflux® Electronics N.V. V
oid formation under compo- nents with large soldering planes and low stand-off
heights, such as QFN components, is a well-known problem. The use of such components is on the rise, mak- ing it increasingly difficult for design- ers, SMT line operators and quality control staff to meet IPC criteria. Recently, Interflux undertook a
comparative study of common, com- mercially-available solder alloys and discovered some striking issues. The company then used these results to develop a special solder alloy with low-voiding characteristics. The new alloy is also suitable across a wide range of soldering processes.
Due to an obstinate focus on solder paste flux, investigations into the void formation of various
soldering alloys have often flown under the radar.
The formation of voids in a sol-
der joint has been the subject of study for many years. A number of void types and forming mechanisms
have been identified. Most notable are macro-voids, where the main contributor to the void formation ap- pears to be the soldering chemistry of the solder paste, which is detailed
to have been the solder paste flux. Designing a solder paste flux that ac- tively works to reduce voids seems to be the right way to go. About 50 per- cent of the flux paste will evaporate
In the study performed by In-
terflux, a baseline void percentage was established with standard avail- able alloys, including SnAgCu0.5 (SAC305), SnAg0.3Cu0.7 (low SAC - 0307) and Sn42Bi57Ag1. The same solder paste chemistry was used for each test. To level out differences between
PCB finishes, tests were performed on three commonly used finishes in the field: OSPCu, ENIG (NiAu) and ISn. To have sufficient void genera- tion, a stencil of 120 µm was used without any pad reduction. For each paste, 60 tin-finished QFN compo- nents were reflowed. They were re- flowed with a standard ramp-up pro- file that was suitable for each specif- ic soldering alloy. Each component was X-rayed
Solder paste voiding shown by X-ray inspection.
here. Micro-voids, shrinkage voids and Kirkendall voids are other well- known and well-documented types.
An Unusual Suspect The main contributor to void
formation is and has always thought
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during the reflow process, actively generating voids. Due to an obstinate focus on solder paste flux, results of investigations into the void forma- tion of various soldering alloys them- selves have often flown under the radar.
REFLOW
and the void level of the ground plane was used to determine the void percentage. Void percentage is calcu- lated by the area of voids compared to the area of the ground plane sur- face. Individual void size was not taken into consideration. The tests show rather poor results for SAC305 and low SAC0307. Better results were obtained with Sn42Bi57Ag1.
Continued on next page See us at
SOLDERING SYSTEMS
IMAPS 2017 Booth 222
FALCON ULTRA PROFILE 1200 Small Footprint • Efficient Power Use
Consistent High Yields • Precise Temperature Gradient
EMAIL:
sales@bofaamericas.com WEB:
bofaamericas.com
BOFA AMERICAS, INC
CALL: (618) 205 5007 FAX: (866) 707 2632
See at SMTAI, Booth 100
SIKAMA INTERNATIONAL, INC.
www.sikama.com
See at IMAPS, Booth 222
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