Selecting the Right Solder Paste for Your PCB Assembly

Choosing the soldering paste is essential and we may follow some guidelines when going for it. Variations in flux residue, alloy strength, alloy flexibility, void control, wetting properties, and other performance metrics can all have a big impact. In PCB assembly, solder paste is a sticky conductive medium which makes the joint between two metals, in case of electronics we may say two wires or the lead of two components. A seamless, error-free production run or expensive rework or product problems might be caused by the choice of solder paste. Simple solder paste needs to cover:

• Quality: Pick a solder paste that complements your design, materials, and heating method the best.
• Throughput: Make use of solder that facilitates quicker, more effective heating and printing.
• Cost: To lower total manufacturing costs, strike a balance between material.

Everything you need to know about selecting the best solder paste for your PCB assembly will be covered in this extensive guide. It includes types of paste, compositions of flux types, particle sizes and important industry standards.

What is Solder Paste?

A combination of flux and powdered solder alloy, solder paste is a viscous substance that can be used for PCB screen or stencil printing. The solder particles melt when heated (reflowed), creating robust, conductive connections between PCB pads and component leads.

Basic Composition of Solder Paste:

Metal powder alloy (92–96% by weight) and flux (6–10% by weight).

A detailed list of alloy and flux medium types is provided below. We have written a comprehensive article on the benefits and drawbacks of lead-free and leaded solders because some alloys contain lead.

Further read: How to Use Solder Paste: Stencil, Syringe, and Soldering Iron Methods

Types of Solder Paste by Alloy Composition

Your first step is to select the appropriate alloy type. Which is based on performance requirements and regulatory compliance. Lead content, melting temperature and alloy powder particle size are the three main factors, when selecting a solder alloy. The compositions in different types of alloys are as follows:

Flux Types in Solder Paste:

The flux component has a major impact on the quality of the solder junction and the need for cleaning after assembly. Here is a proper guide to flux selection that we have created.

1.  No-Clean: Used in consumer goods where cleaning is unfeasible, this type of product does not require post-soldering cleaning.

2. RMA (Rosin Mildly Activated): It is utilized in general PCB assembly and is good for moderate activities with less residue

3. Water-Soluble: Applied to high-activity applications where soldering is more necessary. It is dependable and easily cleaned with a water dip.

Alloy Defects in Soldering:

Since lead-free solder is still a relatively new technology, various issues are still being resolved. Typical soldering flaws include:

Composition Issues: Composition problems include tin whiskers (in tin-rich alloys), changes in alloy composition due to prolonged heating, and problems with melting temperature or particle size.

Usage Errors: It includes poor wetting on pads at high temperatures, solder beading from inadequate paste deposition, tombstoning from uneven paste, and cracks from thermal or mechanical stress.

Other Causes: They include surface tension issues in lead-free soldering, cracks from elevated soldering temperatures, voids from mismatched flux or materials, and PCB warping.

Other Parameters for a Right Solder Paste:

1. Mesh Size (Particle Size):

Particle size is a determining factor in solder paste's appropriateness for various PCB designs. Type 5 solder paste is advised for ultra-fine pitch applications of 0.3 mm or smaller. Fine-pitch components of about 0.4 mm are best suited for Type 4 paste. Which has particles that range in size from 20 to 40 μm. Type 3 solder paste is frequently used for conventional SMT applications with a pitch of 0.5 mm. Here is the basic guide:

• Type 3: Suitable for standard SMT (≥0.5 mm pitch)

• Type 4: Ideal for fine-pitch (0.4 mm pitch)

• Type 5: Best for ultra-fine pitch (0.3 mm or smaller)

Finer powder is indicated by a higher mesh number, whereas larger particles are indicated by a lower mesh number. For best results, the particle size should be 1/4th of the stencil aperture.

2. Viscosity:

This factor influences the paste's behavior while handling and printing. Lower viscosity ones can print more quickly, also they need lower cleaning and are easy to dispense. For fine-pitch printing, higher viscosity is preferable.

• 200–600 Pa·S: Best for automated dispensing.

• 600–1200 Pa·S: Best for stencil printing.

Viscosity rises when at rest and falls when disturbed. Additionally, it is sensitive to temperature; greater temperatures cause viscosity to decrease.

Conclusion

A PCB assembly requires careful consideration of alloy composition, flux type and particle. Lead-free, no-clean solder paste with Type 3/4 particle sizes will satisfy performance for electronics. And it also comes under regulatory requirements for the majority of contemporary devices. Choose a solder paste that comes with fewer faults. The solder paste must also be compatible with the assembly process and industry standards processes. The information compiled for the best-case scenario is as follows:

• Flux Type: Choose No-clean for standard electronics and WS for aerospace.

• Particle Size: Choose Type 4/5 for fine-pitch and Type 3 for general use ones.

• Melting Point: Lead-free (215–220°C) and Leaded has (185°C).