8 Parameter Measurement Information

8.1 Solder Heat Shift

The materials used in the manufacture of the REF70 have differing coefficients of thermal expansion, resulting in

stress on the device die when the part is heated. Mechanical and thermal stress on the device die can cause the

output voltages to shift, degrading the initial accuracy specifications of the product. Reflow soldering is a

common cause of this error. In order to illustrate this effect, a total of 32 devices were soldered on two printed

circuit boards [16 devices on each printed circuit board (PCB)] using lead-free solder paste and the paste

manufacturer suggested reflow profile. The reflow profile is as shown in Figure 8-1. The printed circuit board is

comprised of FR4 material. The board thickness is 1.65 mm and the area is 114 mm × 152 mm.

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Figure 8-1. Reflow Profile

The reference output voltage is measured before and after the reflow process. Although all tested units exhibit

very low shifts (< TBD %), higher shifts are also possible depending on the size, thickness, and material of the

printed circuit board. An important note is that the histograms display the typical shift for exposure to a single

reflow profile. Exposure to multiple reflows, as is common on PCBs with surface-mount components on both

sides, causes additional shifts in the output bias voltage. If the PCB is exposed to multiple reflows, the device

must be soldered in the second pass to minimize its exposure to thermal stress.

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REF70

SNAS781 – OCTOBER 2020

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