Analysis of QFN package bridging phenomenon and suggestions for improvement

1. Overview of the bridging phenomenon

Bridging in QFN packages is particularly common between the inner rows of solder joints in double-row QFNs, and relatively rare in single-row QFNs.Bridging occurs when solder is pressed against a non-wetted surface, which usually results in an electrical short that can seriously affect the performance and reliability of a circuit.As shown in the figure below.

2.  Causes

Solder Extrusion: The bottom of the QFN package has a large heat-sink pad that determines the height of the solder seam.Instead of printing the solder paste on the heat sink pad in a monolithic pattern, it is usually windowed or striped to vent and control voids in the solder seam.However, this design reduces the area covered by the solder paste and reduces the total amount of solder paste.When QFN reflow collapses, the pin solder squeezes outward, easily triggering bridging.

QFN Deformation: Deformation of QFN package is also one of the causes of bridge connection.When there is a large difference in the height of the solder joints between the inner and outer rows of the QFN (sometimes more than ten microns), the solder is more susceptible to extrusion and flow during the reflow process, thus increasing the risk of bridging.

Correspondence between voiding rate and bridging: Usually, there is a correspondence between bridging and the voiding rate of the signal pads.When the bridging rate is high, there tends to be more voids.This further proves that solder extrusion and poor venting are the main causes of bridging.

3. Suggestions for improvement

In view of the causes of the QFN package bridge phenomenon, we can improve the following aspects:

Reduce the amount of inner ring solder paste printing: theoretically, the same amount of leakage area should be designed according to the pad coverage of heat sink pads.Considering that the QFN itself has a relatively small pad and is difficult to print, the amount of solder paste can usually be reduced by scaling down the size of the inner ring pad opening.This reduces the degree of solder extrusion during reflow and reduces the risk of bridging.A suggested design for a dual-row QFN to prevent bridging is shown in the figure below.

Provide Solder Tolerance: Provide more solder tolerance for the solder through inter-pad de-soldering designs, wide pads and narrow openings process designs, and hot-sink pad solder resistance definitions.This helps reduce the risk of bridging by minimising solder crush and flow during reflow.However, it is important to note that for small pads, excessive increase in solder space may lead to stress concentration problems, so careful design is required.

In summary, the causes of QFN package bridging phenomenon mainly include solder extrusion, QFN deformation, and the correspondence between void ratio and bridging.To address these issues, we can reduce the amount of solder paste printed on the inner ring and provide tin space to improve the two aspects.Through reasonable process design and optimisation, we can effectively reduce the risk of QFN package bridging and improve the performance and reliability of the circuit.