Common QFN board soldering problems in PCBA factory

- Jan 14, 2019-

1.QFN

QFN, the Quad Flat No Package, can be translated as a “square flat leadless package”.

QFN belongs to the BTC package category and is the most widely used type of bottom board solder end package. It is characterized in that the printed circuit board solder end is embedded in the package except the soldering surface.


2. Process characteristics

  1) "face one side" weld, easy to bridge

The soldering end of the printed circuit board manufacturer QFN is a flat surface, which is basically flush with the bottom surface of the QFN package (0-0.05mm), and it forms a "face-to-face" connection with the corresponding pad on the PCB. This feature determines the amount of solder paste in the PCBA manufacturer is proportional to the area of the weld. The more the amount of solder paste, the larger the area of the weld (the area of the weld shown on the X-ray image), and the more likely it is to bridge. . Figure 1 shows an X-ray pattern of a weld expansion phenomenon.

 

pcb

2) The amount of solder paste on the heat sink pad determines the height of the weld

  The structure of the QFN has one thing in common, that is, there is a relatively large heat sink pad at the bottom of the package, and its area is larger than the sum of the areas of all signal soldering ends. Because of this, the height of the heat sink pad weld determines the weld height of the QFN weld end, and the height of the heat sink weld can be controlled by adjusting the solder paste coverage printed on the heat sink pad.

This is very important, we must ensure that the heat sink pad weld height is sufficient to avoid too much slumping of the solder paste due to the heat sink pad, causing excessive expansion of the signal weld around the QFN and bridging.

3) Heat sink pads are prone to large holes

   The size of the heat sink pad is relatively large, coupled with the "face" structure of the QFN weld. When the flux is a large amount of solvent in the solder paste, it is difficult to volatilize, and it is easy to wrap in the molten solder to form a cavity, as shown in Figure 2. Show.

 

board

3.QFN and process characteristics

  The poor soldering of QFN is related to its package, mainly including bridging, soldering and voiding.

  The most bad thing about QFN welding is bridging. Taking the double-row QFN (also called double-circle QFN) as an example, the process requirements of QFN are explained. These requirements are based on the traditional multi-layer board technology II. If HDI technology is adopted, there is no big problem in the process.


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