Solder Selection Guide

This guide covers the most significant steps in selecting a solder paste. There are additional details of alloy and flux performance not covered that can be very important in the selection process. It is always worth a call to your Nordson EFD solder sales specialist to review requirements to ensure you are using the best solder paste for the job.

 

 

Step 1: Select Your Alloy

When choosing a solder alloy, there are a few questions that need to be answered.

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

 

Leaded vs. lead-free

Many applications have a requirement for use of lead-free solder alloy. Sometimes this is due to the product falling under the RoHS (Restriction of Hazardous Substances) directive and sometimes it is a corporate directive. Some applications that fall under the RoHS directive are exempt from being lead-free because the reflow temperature requirements can only be met with high-lead solder alloys which are exempt under RoHS regulation.

 

 

Melting temperature

Each alloy has temperatures at which it changes from solid to liquid (Figure 1). The phase change from the solid state to the liquid state begins upon reaching the solidus and ends upon reaching the liquidus.

• Below the solidus, an alloy is 100% in a solid state.
• In between the solidus and liquidus, a region called the plastic range, some portion of the alloy is solid but the majority is liquid.
• Alloys are called eutectic when the solidus and liquidus are equal.

While wetting begins at the solidus temperature, best wetting is achieved at a peak temperature 15º C or more above the liquidus. If a solder joint needs to retain physical integrity during a later operation, such as a second reflow process, the peak temperature of the later operation needs to be below the solidus temperature of the alloy.

 

Particle size

Having selected the best alloy, particle size is next. The Powder Size chart (Figure 2) cross-references particle size to typical printing and dispensing requirements. Dimensions listed for gullwing, square/circle, and dispense dot sizes represent the smallest feature recommended for that size powder. If the feature is smaller, the application requires the next smaller powder size.

Using too large a powder will cause printing and dispensing difficulties, compromising quality. Using a smaller powder will just cost more.

 

 

Step 2: Select Your Flux

Flux categories are defined by Military Specification QQ-S-571E as well as the IPC flux rating system. There are five main categories in QQ-S-571E. Each is available with a variety of activity levels, physical qualities of their residue, and cleaning methods required.

 

Flux Comparison chart shows relative activity ranges of each flux category. Note overlap of activity levels between flux groups.

 

Rosin (R)

Rosin 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.

 

No clean (NC)

No clean 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.

 

Rosin mildly activated (RMA)

Rosin mildly activated (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 NC flux. Residue may be removed with an appropriate solvent.

 

Rosin activated (RA)

Rosin activated 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.

 

Water Soluble (WS)

Water soluble 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.

 

 

Step 3: Select Your Special Characteristics

The last area to consider when finalizing solder paste selection is any other special characteristics that might be required for a challenging application. Two flux formulas can differ greatly in performance, despite having the same QQ-S-571E and J-STD-004 classifcations. Solder pastes with specific characteristics can be used to solve technical assembly problems that other forms of solder do not. The following are a few examples of flux characteristics that modify how a solder paste performs.

 

Restricted Residue

NC 26D04 flux residue remains either on or very close to the fillet after reflow. This feature is most important with NC formulations where the joint is visible or the spread of flux to surrounding areas can cause a problem.

 

Gap filling and/or vertical surfaces

RMA 07D01 and 04D01 fluxes are designed to hold the alloy in place until liquidus is reached. These formulas are suited to bridging gaps, filling holes, and soldering joints on vertical surfaces.

 

Rapid reflow

A term used to describe the heating of solder paste in under 5 seconds. RMA 04D02 and RMA 07D02 rapid reflow solder pastes will not spatter when heated as quickly as 0.25 second. Typical reflow methods that achieve rapid reflow include laser, solder iron, hot bar, and induction.

 

Pin transfer or dipping

An application technique where the solder is applied by dipping a component or pin into the solder paste. A thin, consistent layer of NC 21T20 solder paste sticks to the component. This technique is useful in applying solder to products that do not lend themselves to printing or dispensing, such as pin arrays.

 

Low-void

IPC-7097A is the Specification for the Design and Assembly Process Implementation for BGAs. The inspection criteria for Ball Grid Array (BGA) and MicroBGA often call for voiding under 20%. A low-void solder paste is required to meet the very low-voiding limits for Class 3 assemblies.

 

UV-traceable flux

Where used by itself or mixed with alloy to form solder paste (NC 22D05 and RMA 07D05), our UV-traceable flux allows for optical confirmation of flux presence. These formulas also luminesce under a UV light source for solder paste deposit verification.

Questions? Contact our product specialists at [email protected] to identify the best solution for your application.