8 Parameter Measurement Information

8.1 Solder Heat Shift

The materials used in the manufacture of the REF35 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 TBD mm and the area is TBD mm × TBD mm.

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C01

Figure 8-1. Reflow Profile

The reference output voltage is measured before and after the reflow process; the typical shift is displayed in

Figure 8-2. 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 last pass to minimize its exposure to thermal

stress.

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REF35

SNAS809 – DECEMBER 2021

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