Void Caused by the Inherent Characteristics of Solder Paste Flux

Void is the presence of air bubble in a solder joint, which affects the mechanical strength, thermal conductivity and electrical properties of the solder joint. Under the same PCB and device conditions, some soldering materials are prone to void formation, while some solder pastes exhibit excellent properties for controlling solder joint void. Flux changes during the soldering process is a very complex process involving a variety of physical and chemical changes, the flux formula determines the soldering effect and characteristics.

 

During the welding process, the flux removes metal oxides from the weld end surface to generate water and gasification to form void; the organic acids in the flux esterify to generate water and gasification to form void; and the organic matter in the flux cracks to form void.

 

Basic reaction principle

(a) welding process flux to remove the weld end face metal oxide layer, generating metal salts and water.

2RCOOH+SnO=(RCOO)2Sn+H2O^

2HBr+CuO=CuBr2+H2O^

 

(b) Flux organic acid lipidation during soldering to form water vapour escapes.

RCOOH +R'OH = RCOOR' + H2O^

 

(c) Insufficient flux activity during the soldering process, unable to effectively reduce the surface tension of the liquid solder, the flux in the organic cracking of water vapour produced by water vapour is wrapped up in the solder joint to form void.

 

 

Relationship between flux characteristics and void rate

Void rate in the solder joint is closely related to the activity of the solder paste, Figure 1 solder joint bubbles and flux activity relationship shows that the flux activity is inversely proportional to the bubbles in the solder joint - the higher the content of the active agent, the lower the rate of void in the solder joint.

 

 

Figure 1: Solder paste activity and solder joint void rate relationship

 

Solder paste flux viscosity and solder joint void rate is also inversely proportional to the relationship, Figure 2: as the flux viscosity increases, the amount of air bubbles in the solder joint decreased.

 

 

Figure 2: Relationship between solder paste viscosity and solder joint void rate

 

Solder paste flux boiling point and the bubble content in the solder joint is also inversely proportional to the relationship in Figure 3: the higher the boiling point of the flux, the smaller the hole in the solder joint.

 

 

Figure 3: Relationship between solder paste flux boiling point and solder joint void rate

 

 

The high content of Flux active agent in the solder paste is conducive to controlling the bubble content in the solder joint, but the residue of active agent will corrode the solder joint, which is unfavourable to the reliability of the product. High flux viscosity is conducive to reducing the void within the solder joint, but higher viscosity is unfavourable to the printing of solder paste and affects the printed product.

 

In addition, the boiling point of the flux also affects the bubble content within the solder joint. Selection of flux with a high boiling point can control the bubble content in the solder joint, such as resin-based flux and high boiling point organic solvents; resin decomposition into organic acids at high temperatures is easy to obtain the effect of a high boiling point, but the resin decomposition will bring the negative effect of too much residue.

 

Fitech flux

FWF-5100, produced by Shenzhen Fitech Co., Ltd, is a water-soluble flux with strong activity, good fluxing performance, high reliability, easy to clean the residue after welding, and can effectively reduce the rate of welding void.