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.

To illustrate this effect, a total of 32 devices were soldered on one printed circuit board using lead-free solder

paste and the paste manufacturer suggested reflow profile. Figure 8-1 shows the reflow profile. The printed

circuit board is comprised of FR4 material. The board thickness is 1.66 mm and the area is

174 mm × 135 mm.

0

50

100

150

200

250

300

0

50

100

150

200

250

300

350

400

Time (seconds)

C01

Figure 8-1. Reflow Profile

The reference output voltage is measured before and after the reflow process; Figure 8-2 shows the typical

shift. Although all tested units exhibit very low shifts (< 0.03%), higher shifts are also possible depending on the

size, thickness, and material of the printed circuit board (PCB). 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.

Solder Heat Shift %

0

12

24

36

48

60

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

REF35

SNAS809A – DECEMBER 2021 – REVISED AUGUST 2022

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