LEAD-FREE
FREQUENTLY ASKED QUESTIONS

Lead-free SMT

What lead-free solder alloys are recommended for reflow soldering?

The key variable to contend with in selecting an alloy for SMT assembly is the reflow temperature of the alloy. There are component thermal issues to observe in this selection, for example, can the plastic molding compounds sustain the higher temperatures associated with most lead-free solder alloys? Will higher temperatures impact component reliability? The wetting properties of lead-free alloys will differ and is dependant on the surface finishes to be soldered.

Presently tin-silver-copper is a popular choice for most SMT assembly applications. These alloys reflow within 217-221 C, and a peak temperature of 235-255 C is adequate to achieve good soldering on most lead-free surfaces such as tin, silver, gold over nickel, and bare copper OSP.

There are lower melting options such as tin-zinc and tin-silver-copper bismuth but they may require special flux systems or the leads and boards must be completely free of bismuth to achieve reliable solder joints.

What are the main qualification tests for using lead-free solder paste?

A good selection process for a lead-free paste is essential to a defect free SMT process. Firstly the board finish and component finishes must be determined and the flux chemistry chosen to adequately solder these finishes. Some pastes do well with tinned surfaces but solder poorly bare copper OSP. The solder paste manufacture will normally do extensive testing such as spread and wetting of its particular solder paste in both air and nitrogen atmospheres. This information should be requested. Other critical performance tests are:

• Hot Slump testing, done at higher temperatures such as 180-185C
• Cold Slump testing
• Solder balling test in air and nitrogen using a typical reflow thermal profile
• Wetting to different surfaces, spread testing
• Tack life
• Stencil life
• Residue character
• Pin-testability of the residue
• Cleanability of the residues, especially for water washable pastes.

Will the stencil design or printing process need to be changed for lead-free?

The stencil and print process will not change. Lead-free alloys such as tin-silver-copper do not wet-out completely to the edge of the pads, in some cases a stencil with less reduction or a 1:1 ratio will assist in reducing this effect. A well-designed lead-free solder paste will have excellent tack life and the stencil life will be similar to traditional leaded pastes. Print speeds should not be jeopardized.

In what way does a lead-free thermal profile differ from a traditional tin-lead process?

The main difference when using tin-silver-copper as the paste alloy will be the peak temperature. This alloy melts between 217-221 C; peak temperature will be between 230-255 C depending on the thermal mass of the assembly. It is recommended that the time above liquidus (TAL), be under 90 seconds to avoid charring of the residues; this will also reduce the incidence of intermetallics.

Should a nitrogen atmosphere be used in the reflow process?

The use of nitrogen will depend on the solder paste flux chemistry. New developments in flux chemistries for lead-free pastes do not require nitrogen to achieve good wetting and solder joint integrity. Nitrogen like in the tin-lead system will offer smoother solder joints and better wetting will be gained with the use of nitrogen.

Will the higher temperatures needed for melting lead-free solder create more fumes and condensate in the reflow oven?

New formulations of no-clean and water washable pastes are designed for lead-free alloys, and therefore decomposition by-products are not significantly more than tin-lead bases solder pastes. If a formulation is not designed for higher peak temperatures in the range of 230-260C as would be expected with tin-silver-copper solder paste, decomposition of flux materials may be slightly more pronounced causing a large amount of condensate to form with the reflow oven and exhaust system.

Will residues from lead-free, water-soluble paste be more difficult to remove?

If tin-silver-copper solder paste is used the peak temperature will be higher, if the flux formulation is not designed for higher temperatures removal may be more difficult. The removal of flux residues may require an assembler to re-evaluate the cleaning chemistry. In some cases increasing the pressure of the cleaning solution, reducing conveyor speeds or the temperature of the solution may promote effective cleaning. The residue's cleanability may be further degraded with double sided reflow where the residues from the first reflow are further baked onto the board. Selecting a paste with good cleaning properties is essential.

What happens to no-clean flux residues at the higher reflow temperatures?

Since time above liquidus temperatures are higher when using tin-silver-copper, flux residues may exhibit darkening when reflowed in air. A flux system designed for lead-free will not exhibit this phenomena to a great extent. The flux residue will tend to polymerize rendering it harder, increasing the pressures required to make them probable. Pin-testable flux residues remain soft after reflow and will be pin-testable, the flux is designed not to harden at the higher peak temperatures.

Is the appearance of the lead-free solder joints different from those made with tin-lead solder?

Traditional Sn63 solder offers bright solder joints after reflow, tin-silver-copper alloys give dull joints with a slightly crazed surface; this is typical of this alloy and does not suggest poor solder joints. The other difference that will be noticed is the larger contact angles and reduced wetting around the pads, tin-silver-copper doesn't wet as rapidly and completely as Sn63.

What are the main soldering defects associated with lead-free reflow soldering?

Lead-free can increase the incidence of solder defects and requires a good understanding of the properties of the lead-free alloy and flux system to prevent them. Defects such as bridging, non-wetting, de-wetting and the potential for solder balls can increase. Choosing the correct flux chemistry compatible with the metals to be soldered, and having an optimized reflow profile will prevent the increase in defects. Insuring the solderability of the boards and components by proper storage and handling methods will also enable good soldering with lead-free pastes. If the chemistry is chosen carefully and the SMT process controlled the results in yields will be the same as the Sn63 process.

What other issues can be expected with the conversion to lead-free reflow soldering?

Some of the issues to address before and after the implementation of a lead-free SMT can be summarized as follows:

• Determining process compatible lead-free board finishes
• Determining availability of lead-free components
• Determining thermal compatibility of both boards and components to new thermal profile
• Selecting solder paste chemistry to suit assembly process and the soldered assemblies reliability and operating conditions
• Process optimization and statistical process control development
• Training of operators and line managers to new lead-free process
• Material and logistical control for dual systems, if running both a leaded and a lead-free process
• Defining a proper rework process for lead-free assemblies
• Identifying the lead-free assembly for field service