JULY 2019 I SMT007 MAGAZINE 83
lic formed with the aluminum. Figure 7 shows
some cross-sections.
As can be seen, while the solder wet and
bonded well to the aluminum in most areas,
voids were within the acceptable limit and are
seen under the capacitor and battery holder's
pad. These were further examined by doing
an energy dispersive X-ray spectroscopy
(EDS). Figure 8 shows an EDS of one such
resistor pad.
As can be seen in the EDS, there is evidence
that the surface treatment results in the forma-
tion of a silver-aluminum (Ag-Al) intermetallic
with some presence of bismuth.
Conclusions
The surface treatment was shown to enable
soldering to aluminum. Since solders vary in
metal composition and flux, they each will
have specific reflow profiles. Additional work
may be needed to fine-tune reflow parameters
for specific applications to get consistent wet-
ting and good solder joints. Most new soldering
applications would require such process devel-
opment. Overall, this surface treatment has the
potential to open the path for large scale use of
Al-PET substrates. This could revolutionize the
manufacturing of smart tags, RFID tags, and
single-layer aluminum circuits.
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References
1. Weast, R.C. Handbook of Chemistry and Physics: 66
th
Edition,
CRC Press, 1985.
2. Anixter Inc. "Copper vs. Aluminum Conductors."
3. Based on price listing on Market Insider in September
2018. Actual prices will vary on a daily basis.
4. MacDermid. "Metex 6811 Cyanide Free Liquid Zincate
Datasheet."
5. Averatek. "Mina: Facile Soldering to Aluminum for
RFIDs."
6. "Dielectric Constants at 20°C."
7. Omnexus. "Glass Transition Temperature Values of
Several Plastics."
This paper was first presented at the IPC APEX EXPO
2019 Technical Conference and published in the 2019
Technical Conference Proceedings.
Divyakant Kadiwala is director
of manufacturing at Averatek
Corporation.
Figure 8: EDS spectra from the pad side after pulling resistor from pad showing Ag-Al intermetallic.
