Recommendation on PCB assembly

USBULC6-2N4

8/10

Doc ID 18332 Rev 1

4.3

Placement

1.

Manual positioning is not recommended.

2.

It is recommended to use the lead recognition capabilities of the placement system, not

the outline centering.

3.

Standard tolerance of ± 0.05 mm is recommended.

4.

3.5 N placement force is recommended. Too much placement force can lead to

squeezed out solder paste and cause solder joints to short. Too low placement force

can lead to insufficient contact between package and solder paste that could cause

open solder joints or badly centered packages.

5.

To improve the package placement accuracy, a bottom side optical control should be

performed with a high resolution tool.

6.

For assembly, a perfect supporting of the PCB (all the more on flexible PCB) is

recommended during solder paste printing, pick and place and reflow soldering by

using optimized tools.

4.4

PCB design preference

1.

To control the solder paste amount, the closed via is recommended instead of open

vias.

2.

The position of tracks and open vias in the solder area should be well balanced. The

symmetrical layout is recommended, in case any tilt phenomena caused by

asymmetrical solder paste amount due to the solder flow away.

4.5

Reflow profile

Figure 15.

ST ECOPACK® recommended soldering reflow profile for PCB mounting

Note:

Minimize air convection currents in the reflow oven to avoid component movement.

0

01

2345

67

Time (min)

Temperature (°C)

2°C/s recommended

6°C/s max

220°C

125 °C

260°C max

255°C

180°C

90 sec max

10-30 sec

90 to 150 sec

3°C/s max

0

01

2345

67

Time (min)

Temperature (°C)

2°C/s recommended

6°C/s max

220°C

125 °C

260°C max

255°C

180°C

90 sec max

10-30 sec

90 to 150 sec

3°C/s max