SMT & Inspection | April 29, 2010

3 Steps to Successful Solder Paste Selection

Choosing a solder paste can make or break an assembly process. By choosing the right solder paste for the application, you will achieve the highest process consistency and solder joint quality.
By John Vivari, EFD, Inc., East Providence, RI

This paper covers the most significant issues in solder paste selection to meet the goals of manufacturing. The goals of any assembly operation are to maximize both quality and throughput while controlling costs. Quality is maximized by choosing a paste that has the best performance with the materials, geometry and heating processes used to manufacture a product. Throughput is maximized by picking a solder product that accommodates the optimal deposition and heating methods. Cost of production is a complex calculation that includes material, direct labor, inspection, rework, and scrap value. Quality and throughput play key roles in cost control.

Not all solder products are created equal, even if they seem the same according to their classification. Specialty solder pastes provide enhanced performance over off- the- shelf products. There are differences in wetting characteristics, void control, flux residue, alloy strength, alloy flexibility, and other performance measures that can all play significant rolls in achieving quality, throughput, and cost goals. The key is to identify the solder product that best accommodates the processes required to meet these goals.

Choosing a solder paste can make or break an assembly process. By choosing the right solder paste for the application, you will minimize problems with process consistency and solder joint quality.
Choosing the right solder paste is a three-step process:

1. Alloy Selection. The alloy requirements must be evaluated and an alloy identified as meeting all the product requirements.

2. Flux Type Selection. The flux types that are valid options must be identified. This is a process of elimination where fluxes with unacceptable criteria are removed from consideration.

3. Identifying Required Special Flux Characteristics. Issues such as difficult-to-solder surfaces, rapid reflow conditions, cleaning options, and solder joint voiding concerns should be considered before choosing a paste, not after a problem is discovered.

Alloy Selection

When choosing a solder alloy, there are four key considerations: lead content, melting temperature, alloy powder particle size, and tensile strength. Lead content, melting temperature, and strength are typically addressed at the same time. The Alloy Table (Table 1) lists melting and strength statistics along with composition for fifteen solder alloys. The green shaded alloys are lead free.

At temperatures below the solidus, the alloy will be completely solid. At temperatures above the liquidus, the alloy will be completely liquid. In-between, the alloy is in a plastic state and neither fully liquid nor fully solid and strength is near zero. For best wetting, a peak temperature 15ºC or more above liquidus is required. If physical integrity must be maintained during a later operation, such as a second reflow process, the peak temperature of the later operation needs to be below the alloy solidus.

Solder alloy tensile and shear strength values are only valid at 25ºC at a particular strain rate for a particular age of the alloy sample. Tensile strength drops as temperature increases. Near the solidus, tensile strength approaches zero.

When using strength values to make a decision, keep in mind that the listed values are a reference. Use them for comparison to determine if one alloy is likely to be better than another. Include a factor of two or more as a safety margin for joint variability and to compensate for any inaccuracy in the tensile strength value reported. Be mindful that alloys with higher solidus retain more strength at higher temperatures. Example: Sn95 Ag5 at 210ºC is weaker than Sn5 Pb95 at 210ºC, despite the large difference in tensile strength (Table 1).

Metals expand as they change state from solid to liquid. In many applications involving part encapsulation, excessive stress induced by alloy expansion can cause cracking due to strain. Molten alloy tends to follow these cracks, either breaking the part outright or creating a future field failure. When encapsulating, try to avoid using an alloy that will fully liquefy during a later heating step.

Table 1: Alloy Table

Note: green highlighted alloys are lead free

Having picked an alloy, the particle size required is the next item to identify. The Powder Size Table (Table 2) cross-references particle size to typical printing and dispensing requirements. The dimensions listed for gullwing, square/circle and dispense dot sizes represent the smallest feature recommended for that size powder. If the feature is smaller, use the next smaller powder size.

Using too large a powder size will cause printing and dispensing difficulties, compromising quality. Using too small a powder size will just cost more than necessary.

Table 2: Powder Size Table

Author: John Vivari, EFD, Inc. The article was published thanks to AMB company. The second part of the article, focused on flux choice issues, will be available soon,
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