Strategies for Solder Paste Quality Control
Solder paste quality control should be included in your overall quality control strategy towards a zero defect manufacturing process. Catching defects after printing or reflow is too late because reprinting or rework is costly and has the potential to affect assembly reliability. Robust quality control of not only solder paste, but all materials in the manufacturing process, is critical to ensuring the reliability of final assemblies.
Solder Paste Shelf Life
Like milk, solder paste is a perishable product and has a useable shelf life. Why does solder paste expire at all? It’s helpful to go back to the basics and look at solder paste’s composition and function. Solder paste is made from a suspension of solder powder and flux, where the flux’s primary role is to remove oxides from the circuit board pads, component terminations, and solder powder during reflow. Ideally, this reaction occurs during the reflow process; however, it also slowly happens during storage.
The reaction between flux and solder powder is the primary factor that limits a solder paste’s shelf life in storage. Other contributing factors to the degradation of solder paste are evaporation of solvents and absorption of moisture after opening, for example while the solder paste is on the stencil during printing.
All these effects that influence the shelf life of solder paste and cause it to change happen at a gradual process. While manufacturers are able to provide their best estimate of a paste’s usability, any significant variations in the paste that are caused by handling, environmental changes (such as humidity), formulation, etc. may decrease the solder paste’s previously predicted shelf life. For manufacturing operations with narrow process windows, a material may not be useable throughout its full shelf life before changes in the solder paste cause defects that reach unacceptable levels, due to environmental conditions. This becomes even more prominent as fine pitch printing for miniaturization continues to push the limit of the solder paste printing process.
Primary Solder Paste Degradation Mechanisms
Solder pastes are degraded by three primary effects:
Evaporation of flux solvents
Absorption of moisture into the solder paste
The reaction occurring between the flux and powder in storage has a few different impacts on the quality of a solder paste. As the reaction proceeds, the flux activator is used up and may degrade the solder paste’s ability to remove oxides, causing graping or wetting issues. This reaction also produces a metal salt that increases the solder paste’s viscosity and may cause printing issues. In extreme cases, the flux activator can entirely remove the oxide layer on the surface of the solder powder, which causes them to cold-weld together and form hard chunks that may clog stencils. As even finer powders, which have larger surface areas, are used for solder paste for fine pitch printing, this reaction between the flux and the powder becomes even more pronounced.
Solder paste must be kept in cold storage due to acceleration of the flux-powder reaction at elevated temperature. If a solder paste storage refrigerator fails to maintain the proper storage temperature or solder paste heats up during shipping, paste degradation is accelerated.
The solvents in a solder paste are vital for proper printing and reflow performance. When exposed to the air, solvents in the solder paste flux may evaporate and dry out the solder paste, leading to increased viscosity and causing skips or other printing issues. Without the solvents, the flux activator may not be able to access the oxides, resulting in wettability issues during reflow.
Solvent evaporation occurs more readily in low humidity environments or when the solder paste is exposed to ambient air during printing.
Solder flux is hydroscopic and will absorb moisture from the air over time. Depending on the flux chemistry, this added moisture may either increase or decrease the viscosity of the solder paste. The absorption of moisture is a common cause of solder balling, spattering issues, and even voiding in cases of extreme humidity.
Moisture absorption in a solder paste occurs more readily in high humidity conditions or when the solder paste is exposed to ambient air during printing. Moisture may also be introduced through condensation if the solder paste is colder than the ambient environment, like when a jar is opened before reaching room temperature.
Practical Strategies for Solder Paste Quality Control
The IPC-J-STD-005 standard specifies the requirements for what is considered an acceptable solder paste. While this standard defines a thorough set of tests that are valuable for qualification, it would be infeasible to run this battery of tests to measure each individual container of solder paste before use for quality control purposes on the production floor. Three techniques that are most applicable to solder paste quality control are the solder ball test, viscosity measurements, and electrochemical impedance spectroscopy (EIS).
Solder Ball Test
The solder ball test as specified by IPC-TM-650 2.4.43 is a quick test that provides an indication of a solder paste’s ability to coalesce during reflow. This technique is sensitive to the three main degradation mechanisms listed above. If the flux-powder reaction has consumed too much of the flux activator or the solvents have evaporated, the solder powder will not coalesce to form a single solder ball. If the solder paste has been contaminated with moisture, spattering may result.
The solder ball test is quick and inexpensive, but its success depends on operator training. Additionally, it is a qualitative test instead of quantitative and, therefore, is subject to interpretation.
Measuring solder paste viscosity according to IPC-TM-650 2.4.34 can give an indication of the solder paste’s degradation. This technique is commonly used by solder paste manufacturers when re-certifying a solder paste for use beyond its original expiration date. This technique is sensitive to the common degradation mechanisms that limit a solder paste’s shelf life. If the flux-powder reaction has progressed too far or the solvents have evaporated, the viscosity will increase. A higher viscosity is the direct cause of some printing failures such as skipping or insufficiencies.
A viscosity test is relatively quick but requires a large volume of solder paste outlined by IPC-TM-650 2.4.34. Viscometers give a quantitative indication of solder paste quality but often have significant variability in measurements. Rheometers can also be used to measure solder paste viscosity by using a small sample of solder paste; however, they are expensive and require specialized training to use reliably, making this method impractical for regular quality control for production.
Electrochemical Impedance Spectroscopy
Electrochemical impedance spectroscopy (EIS) provides a quantitative indication of solder paste quality by monitoring the underlying chemical reactions occurring within the solder paste. This technique is sensitive to the main degradation mechanisms of a solder paste such as evaporation of solvents, moisture absorption, and change in the flux-powder interaction.
EIS measurements are quick, require minimal training, and are available to be made by using a handheld measurement tool on the production floor with Insituware’s Vision MARK-1.
Upon taking an EIS measurement with the Vision MARK-1, a small electrical signal is applied to the solder paste and the current response is measured. Results are provided as the Flux Coefficient (FC), the dielectric properties of the flux, and the Powder Reactivity Coefficient (PRC), the interaction between the flux and powder.
These properties correlate to the printing and reflow performance of a solder paste and the measurement results can be immediately viewed on the screen. Measurements are automatically synced with Insituware’s online SPC web app where materials behaviors and patterns can be viewed from multiple measurements being made over time.
Establish Your Quality Control Program
Regardless of the approach, the most important step to improve your solder paste quality control is to monitor the solder paste on the production floor. It is not enough to rely solely on a solder paste’s expiration date because handling, storage, and environmental exposure can alter the solder paste behavior. Regular materials measurements on the production floor are vital to building a world-class quality control program and producing the most reliable assemblies for volume manufacturing.