Clearly Understanding Solder Paste Descriptions

Solder Formulation and Selection Made Easy

5,000 years ago, ancient Mesopotamians attempted to join metal surfaces using a filler metal. By 3000 BC, Sumerian swords were assembled using hand soldering¹. Unfortunately, for many of today’s solder users, comprehending what is inside their solder formulation may as well be communicated in ancient Sumerian. Seeking to counter the byzantine structure of solder formatting, Nordson EFD has crafted a clear rule set to describe solder pastes. Now, each unique Nordson formula can be easily distinguished from each other. For users, this formula description benefits you by providing a logical and unambiguous means of knowing the flux formula, application method, alloy, metal content, package type, quantity, and piston color each product contains.

A few years ago, Nordson EFD created this new formula description so that each unique formula can be easily distinguished from other chemistries. The image below shows how solder names are compiled, and how to understand what each element in the solder name means.

Solder Selection

The Nordson EFD approach to selecting a solder can be condensed into a three-step process:

• Select Your Alloy
• Select Your Flux
• Select Your Special Characteristics

Using this approach, you can easily and accurately specify the right solder for your needs. Naturally, there are additional details of alloy and flux performance that are not covered here that can be very important in the selection process. In these cases, we recommend that you contact your Nordson EFD solder sales specialist to assist you in selecting the best solder paste for the job. If you require assistance making your solder selection because you have a unique performance requirement, please contact Nordson EFD here.

Step 1: Select Your Alloy

Choosing the right solder alloy is essential for achieving the desired results from your soldering process. The alloys are either leaded or lead-free. Also, the solid and liquid temperatures of each solder type vary. Lastly, the solder formulations are developed to meet the power needs for the features in the application.

Step one is answering these three questions:

• Does your alloy need to be lead-free?
• Is there a reflow temperature requirement or limitation?
• What type or size does the power need to be for the smallest feature in your application?

Leaded vs. Lead-Free

Today’s applications typically contain a requirement for the use of leaded or lead-free solder alloy. In the case where you require a lead-free solder alloy, a common scenario is due to the product being included in the Restriction of Hazardous Substances (RoHS) directive; in other cases, using a lead-free solder can be part of a corporate mandate.

In many cases where a leaded solder is required for your application, the reason is that the application itself does not fit into the regulations for RoHS because the reflow temperature requirements can only be met with high-lead solder alloys exempt under the RoHS regulation.

Melting Temperature

Each solder alloys has a temperature range where it changes from a solid to a liquid. To define this further, the phase change from the solid state to the liquid state begins when you reach the solidus and ends upon reaching the liquidus. Below the solidus temperature, the alloy is in a 100% solid state. Between the solidus and liquidus states, there is a region called the plastic range. In the plastic range, some portion of the alloy is solid, but the majority of the solder is liquid. In addition, alloys are called eutectic when the solidus and liquidus are equal.

The wetting process is where the metal in the solder merges with the metal on your PCB or components. The solder becomes fluid and flows along component and substrate, creating the solder joint that your process requires.

Wetting begins at the solidus temperature, but best wetting occurs at a peak temperature of 15° C or more above the liquidus state. If your solder joint needs to retain physical integrity during a later operation (possibly a second reflow process), the peak temperature of the later operation needs to be below the solidus temperature of the alloy.

Particle Size

The final step in selecting an alloy is to choose the right particle size distribution. Particle sizes are presented using cross references to typical printing and dispensing requirements. Dimensions listed for gull wing, square/circle, and dispense dot sizes represent the smallest feature recommended for that size powder. If the feature is smaller, your application requires the next smaller powder size. In a situation where a manufacturer selects too large of a powder size, they will typically experience printing and dispensing issues, which results in compromises to quality. Using a smaller powder does carry a higher price point, but when necessary it ensures the best soldering results.

Step 2: Select Your Flux

There are five general categories of flux. Each is available with a range of activity levels, the qualities of their residue, and the cleaning methods required to remove residue after the soldering process.

The following descriptions offer insights about what the respective flux selections are, what they do, and how to remove the residue from each respective flux formula.

Flux Selection

Rosin (R)

R flux consists of rosin and solvent. Rosin flux has very low activity and is suitable only for easy-to-solder surfaces. IPC classification is ROL0. R residue is hard, non-corrosive, non-conductive, and may be left on. Residue may be removed with an appropriate solvent.

Rosin Mildly Activated (RMA)

RMA flux consists of rosin, solvent, and a small amount of activator. Most RMA flux is fairly low in activity and best suited to easily solderable surfaces. IPC classification is usually ROL0, ROL1, ROM0, or ROM1. RMA flux residue is clear and soft. Most are non-corrosive and non-conductive. Many RMA fluxes pass SIR testing as a No-Clean (NC) flux. Residue may be removed with an appropriate solvent.

Rosin Activated (RA)

RA flux consists of rosin, solvent, and aggressive activators. RA flux has similar and higher activity than RMA for moderately and highly oxidized surfaces. IPC classification is usually ROM0, ROM1, ROH0, or ROH1. In the absence of testing to prove otherwise, RA flux residue is assumed to be corrosive. Assemblies sensitive to corrosion or the possibility of electrical conduction through the residue should be cleaned as soon as possible after assembly. Residue may be removed with an appropriate solvent.

No Clean (NC)

NC flux consists of rosin, solvent, and a small amount of activator. NC flux typically has low-to-moderate activity and is suited to easily solderable surfaces. IPC classification is usually ROL0 or ROL1. NC residue is clear, hard, non-corrosive, non-conductive, and designed to be left on many types of assemblies. Residue may be removed with an appropriate solvent. Some, but not all, NC fluxes are more difficult to remove than RMA fluxes.

Water Soluble (WS)

WS flux consists of activators, thixotrope, and solvent. WS flux comes in a wide range of activity levels, from no activity to extremely high activity for soldering to even the most difficult surfaces, such as stainless steel. IPC classification normally starts with OR for organic. They come in L, M, H activity levels and halide content of 0 or 1. By definition, residue may be removed with water.

As explained above, each flux is available with a variety of activity levels, the qualities of their residue, and the cleaning methods required to tidy up from the soldering process. Now that you have a good understanding of the flux characteristics available, you are ready to consider special characteristics your formulation may require.

Step 3: Select Your Special Characteristics

Challenging solder applications oftentimes include specialized characteristics. Some examples of a challenging application characteristic include restricted residue, gap filling, vertical surfaces, rapid reflow, pin transfer, dipping, low-void, and UV-traceable flux.

Two flux formulas perform very differently, despite having the same QQ-S-571E and J-STD-004 classifications. Solder offering special characteristics can oftentimes solve solder challenges that formulations without these characteristics simply cannot.

Solder Paste Formulation Made Easy

Making wise solder paste selections are essential for manufacturing high quality products. This formula description benefits you by providing a logical and unambiguous means of knowing the flux formula, application method, alloy, metal content, package type, quantity, and piston color each product contains.

To learn more, please visit:

Solder Selection Guide

Solder, Flux & TIM Category Page

¹Brady, George; et al. (1996). Materials Handbook. McGraw Hill. pp. 768–70. ISBN978-0-07-007084-4.