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In the reflow soldering process of Surface Mount Technology (SMT) in electronic assembly, the occurrence of “Graping” solder defect refers to the situation where the solder paste does not completely melt and joint together during reflow. Instead, individual solder powders gather and stack together, resembling a cluster of grapes.
So, how does this “Graping” solder defect happen? Solder paste is essentially a mixture of tiny solder powders combined with flux. When the flux in the solder paste evaporates, it leaves behind individual solder powders.
Several possible causes for the occurrence of SMT Graping solder defects:
1. Moisture Oxidation of Solder Paste:
The primary cause of graping solder defect is the oxidation of solder paste. Various factors contribute to solder paste oxidation, such as improper handling, expired or improperly stored solder paste, and inadequate reflow or stirring, leading to moisture absorption and oxidation of the solder paste.
One possible cause is the insufficient drying of some stencils after solvent cleaning before going into production, leading to solvent contamination of the solder paste.
I’ve encountered situations on SMT assembly lines where air guns had water spraying issues. Additionally, Workingbear have witnessed operators using their mouths to expedite the drying of stencils after cleaning, inadvertently causing saliva to adhere to the stencil. These instances can contribute to solder paste absorbing moisture and undergoing oxidation.
2. Premature Evaporation of Flux in Solder Paste:
The flux in solder paste is another critical factor that can affect the solder melting process. The purpose of flux extends beyond removing metal surface oxides and reducing the surface tension of soldering metals. It serves an additional function of enveloping the solder powders to protect them from direct exposure to air before the soldering process. If the flux evaporates prematurely, it fails to eliminate metal surface oxides effectively and doesn’t shield the solder powders from oxidation. Therefore, it’s essential to use solder paste promptly after opening to prevent premature flux evaporation, ensuring the solder remains viable and oxidation-free.
Moreover, an excessively long or high-temperature preheat zone during reflow soldering can lead to premature and excessive flux evaporation, potentially contributing to the graping defect.
Once the reflow temperature (actually, it should be heat) is insufficient to provide conditions for complete solder paste melting, there’s a chance of encountering the graping solder defect. In other words, the solder paste may not melt entirely.
The mentioned item 1 and 2 among the three potential causes lead to a trend: the less solder paste printed on the PCB, the faster the oxidation of solder paste and the evaporation of flux. This is because with less printed solder paste, the ratio of solder paste exposed to air is higher, making it more prone to the graping solder defect. This explains why small chip with a 0201 size are more likely to experience graping than those with a 0603 size.
Another factor to consider is the type of solder powder used. Smaller-diameter solder powder is more susceptible to oxidation due to its larger surface area exposed to air under same volume. To mitigate this, nitrogen gas (N2) can be used to reduce the oxidation rate and achieve better soldering results, especially when using solder powder with a number higher than 5.
Strategies to address and improve the SMT graping solder defect:
Avoid solder paste oxidation.
Select the solder paste with better activity or stronger anti-oxidation capabilities.
(Note: Stronger anti-oxidation ability solder paste may result in more quickly erosion risk)
Appropriately increase the amount of solder paste. This can be achieved by widening or thickening stencil openings to enhance the volume of flux and solder paste, thereby improving anti-oxidation capabilities.
Optimize the reflow profile by reducing preheat time, increasing the heating rate, and lowering flux evaporation.
Introduce nitrogen gas (N2) to reduce the oxidation rate of solder paste.
By the way, if the solder paste has already oxidized before entering the reflow oven, then there’s nothing that can be done during the reflow heating process.
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