AN1294

Soldering method

Doc ID 7604 Rev 3

13/20

The screen itself consists of the screen mesh, the frame that holds the screen mesh aligned

with the board, and the mask. The screen mesh is designed to hold the solder paste in place

until it is squeezed through the mask by the squeegee. The screen mesh count refers to the

number of openings per inch, which is selected according to the size of the solder particles

in the paste used. For screen printing solder paste, the mesh count may vary from 60 to 150

and, in general, the size of the mesh opening should be chosen to be at least three times

the size of the mean particle size in the solder paste. However, if the area of the openings is

too large, there is a risk of the solder paste forming short-circuit bridges. The distance

between the PCB and the screen mesh is called the snap-on. When the squeegee passes

over the screen, the mesh is stretched down to the board and then snapped back to this

distance. The snap-off has to be set correctly to avoid the print being smeared. This

parameter should be specified by the screen printer manufacturer and depends on the size

of the board. The squeegee hardness and angle of attack also affect the results of the

screen printing. The screen and the squeegee should be restored frequently to obtain a

good solder print on the board.

6.4

Placement of parts and drying

The surface mount components should be placed immediately after the solder paste is

applied to the PCB. Some misalignment is permitted because the surface tension of the

molten solder will align the PowerSO-10RF package with the pad layout of the board. The

drying step follows after placement of the components is completed. The entire application

should be baked in an oven for 45 minutes at 50-80° C to evaporate the moisture content of

the solder paste and to minimize flux and solvent bubbling during the reflow solder process.

This reduces the risk of voids, pinholes and poor wetting.

6.5

Avoiding stresses

There are two main stresses to the package during soldering. The first is due to high

pressure caused by trapped moisture prior to soldering. The second is caused by different

thermal expansion coefficients of the materials used in the package.

Usually the melting point of solder exceeds the maximum rating of the device, so if the

device is heated entirely to such temperatures it may be damaged. Therefore, the thermal

stress to which the devices are exposed must be minimized. This is generally achieved by

using the appropriate solder heating profile. However, the correct soldering heating profile

must be determined by experiment for each particular circuit.