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SMT & Inspection | March 22, 2006

Israeli company announces process for temperature sensitive components

A proprietary new packaging technique developed in Israel by Cellergy Ltd. allows temperature sensitive components like memory ICs and relays, including lead-free units, to be surface mounted by reflow soldering, despite their sensitivity to prolonged, high temperatures.
The process can also be applied to components that were marginally compatible with earlier tin-lead based reflow soldering processes with maximum reflow peak temperature of 235°C or 240°C but that are not able to withstand the 250°C peaks of the new lead-free reflow process.

The patented process was developed by Cellergy for its electrochemical double layer capacitors (EDLC), normally classified as supercapacitors. It required the development of non-corrosive thermal insulation that would prevent excessive internal temperature excursion. The technique is based on the double layer phenomena occurring between a conductive solid and a liquid interface, where capacitance results from charge separation in the interface. Electronic charge is accumulated on the solid electrode and counter charge is accumulated in the form of ionic charge in the liquid.

Cellergy achieves this using an aqueous electrolyte that will not decompose at high temperature, paired with a thin shell that creates a convective thermal resistance between the shell and the surrounding air of the reflow furnace. This approach succeeds because the liquid phase change material (PCM) undergoes phase change at or below 100°C, increasing the ability of the component to absorb heat, rather than trying to minimize heat flow into the component. The latent heat of vaporization of water is 2272 J/g, over ten times that of PCM's based on melting. Based on a 0.5
gram gap fill, this adds 1100 Joules of heat capacity, more than sufficient to keep the internal structure of the capacitor cool while its terminals undergo soldering in the reflow process. Cellergy's process incorporates a fabric in the gap that absorbs the deionized water during the rapid diffusion of the vapor when the temperature nears 100°C. This allows temperature sensitive components, including lead-free, to be reflow soldered, instead of requiring slow and expensive manual
soldering.

According to Joel Lang, Cellergy's CTO, effectiveness of this packaging method was proven in comparative endurance tests of supercapacitors during 1000 hour tests at 70 °C and rated voltage, with and without reflow soldering. Capacitance and leakage current are somewhat reduced in the reflow test, compared to their initialvalues, and ESR (Equivalent Series Resistance) is increased, but both within expected and allowable limits.

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