Analysis of the Influence of Solder Mask on BGA Soldering Quality in PCBA Processing from aaronking's blog

Solder mask is the core of the dual-row QFN process design.

(1) The extension distance of the solder mask thickness on the surface of the via hole pad.

(2) The extension distance of the solder mask thickness on the surface of the wire.

(3) Solder mask thickness between pads.

Case study one:

A BGA POE PCB chip introduced by a company is an LGA package with heat sink pads. The center distance of the soldering end of this chip is 0.47mm; the soldering end is a round shape of ∅0.27mm, protruding from the package ground, and the side of the soldering end It is bare copper with poor wettability; there is a large thermal pad in the middle.

Due to the relatively small spacing between the pads, the main problem encountered in soldering during the SMT patching process is bridging. The production adopts a 0.01mm thick steel mesh and a ∅0.22mm window design, and the bridging probability is about 1%.

Reason analysis: BGA soldering with dense pins and narrow pitch is prone to bridging defects. The fundamental reason is the structure of the package itself-the protruding solder ends. Due to the unstable wetting of the side, it is easy to form a locally wet solder on the side. Sewing shape. Therefore, it is a good way to use a solder paste with relatively strong activity.

In addition, in terms of PCBA process, the low solder paste coverage of the thermal pad may be an important factor. It reduces the thickness of the solder seam and makes the surrounding solder joints more sensitive to the amount of solder paste. The manufacturer recommends a coverage of about 20%, and the main hope is to control the weld thickness of the heat sink pad within the range of 10-25m, in an attempt to force the weld to be well wetted. But this may be the main factor of the resulting bridging, which reduces the accommodating space of the molten solder. According to experience, the welding seam thickness of the heat sink pad should be increased to more than 40um.

In addition, the inverted trapezoidal solder ends further reduce the originally extremely small spacing; the thicker solder mask thickness between the pads is also the main factor that triggers bridging. This is the experience summarized by the Shenzhen SMD Rigid flex PCB processing plant over the years.