AN1235

Soldering assembly recommendations

Doc ID 7272 Rev 8

9/14

5.2

PCB assembly guidelines

For Flip-Chip mounting on the PCB, STMicroelectronics recommends the use of a solder

stencil aperture of 330 x 330 µm maximum and a typical stencil thickness of 125 µm.

Flip Chips are fully compatible with the use of near eutectic 95.8% Sn, 3.5% Ag, 0.7% Cu

solder paste with no-clean flux. ST's recommendations for Flip-Chip board mounting are

illustrated on the soldering reflow profile shown in Figure 9.

Figure 9.

mounting on PCB (definitions)

Dwell time in the soldering zone (with temperature higher than 220 °C) has to be kept as

short as possible to prevent component and substrate damage. Peak temperature must not

reflow, especially above 150 °C.

Flip Chips are able to withstand three times the previous recommended reflow profile to be

compatible with a double reflow when SMDs are mounted on both sides of the PCB plus one

additional repair.

A maximum of three soldering reflows are allowed for these lead-free packages (with repair

step included).

The use of a no-clean paste is highly recommended to avoid any cleaning operation. To

prevent any bump cracks, ultrasonic cleaning methods are not recommended.

Table 4.

mounting on PCB (value)

Profile

Value

Typical

Max.

Temp. gradient in preheat (T = 70 – 180 °C)

0.9 °C/s

3 °C/s

Temp. gradient (T = 200 – 225 °C)

2 °C/s

3 °C/s

Peak temp. in reflow

240 - 245 °C

260 °C

Time above 220 °C

60 s

90 s

Temp. gradient in cooling

-2 to - 3 °C/s

-6 °C/s

Time from 50 to 220 °C

160 to 220 s

250

0

50

100

150

200

240

210

180

150

120

90

60

30

300

270

-6°C/s

240-245 °C

2 - 3 °C/s

Temperature (°C)

-2 °C/s

-3 °C/s

Time (s)

0.9 °C/s

60 sec

(90 max)