LEAD-FREE
FREQUENTLY ASKED QUESTIONS

Wave Soldering with Lead-free Alloys

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

Sn96.5Ag3.0Cu0.5 and Sn99.3Cu0.7 will be the alloys recommended for wave soldering. SnAgCu has a faster wetting speed and greater solderability than SnCu.

Can I use my present wave soldering machine, and what changes are needed?
There are a couple of issues that need to be addressed with the wave soldering machine.

• Convection type preheats will be the best heating method for lead-free wave soldering.
• Due to the corrosive nature of the tin on the solder pot lining, it may need to be replaced or re-lined.

Will there be more dross generated with lead-free alloys?
There will roughly be twice as much dross generated with lead-free alloys. SnCu will dross slightly more than SnAgCu. Nitrogen blanketing of the solder pot will reduce dross dramatically.

Will my present liquid flux adequately solder with lead-free solder alloys?

Fluxes traditional designed for Sn63 soldering may not offer the good wetting and hole-fill required for reliable lead-free wave soldering. The activator package in fluxes for lead-free are more thermally stable and can sustain the slightly higher soldering temperatures. This thermal stability will enable the flux to be active throughout the contact time with solder. No-clean fluxes are particularly sensitive to excesses in solder temperature, water washable fluxes depending on the activators used and also due to the higher content of activators may be suitable with lead-free. However, liquid fluxes are being designed specifically for lead-free wave soldering, these are recommended.

Is nitrogen needed at the wave?

Nitrogen is not necessary, but may prove beneficial for the following reasons.

• Nitrogen will eliminate most of the dross being generated.
• Nitrogen will improve the wetting and spreading of the solder on the printed circuit assembly.
• Nitrogen will give shinier joints with a lower contact angle.

How are the solder alloy metals and impurities controlled in the pot?
Silver and copper tend to not dross out of the solder pot. Depending on the metallization on the board silver (Immersion Silver) or copper (Bare Copper or OSP over Copper) may actually leach into the solder pot. Adding pure tin to the pot will control the alloy. Impurities such as copper, palladium and silver will need to be monitered because they raise the melting point of the alloy. For instance the melting point of a lead free alloy will increase 25°C for every 1% of copper in the solder alloy.

Will the solder joints made with lead-free solder look like those made with tin-lead?
Lead-free solder joints will look different than leaded solder joints. They tend to look grainier with a larger contact angle. The surface of the joint may look crazed or even have micro-fractures. This is usually a surface condition caused by the cooling of the joint.

What is fillet lifting, and how is it prevented?
Fillet lifting is caused by a combination of lead from either a HASL board or leaded components plated with tin-lead and a bismuth containing solder alloy. A tin-lead-bismuth rich layer forms just above the intermetallic layer. This layer has a low melting point of 96°C. The rest of the joint cools and contracts but the layer next to the board is still liquid. When the tin-lead-bismuth finally cools and contracts there is not enough solder to fill the gap caused by the contraction.

This phenomena can be reduced or eliminated by reducing the amount of preheat added to the board and a higher pot temperature. Thermal shock to the components will need to be considered with this method. A second solution is to quickly cool the board as soon as it comes out of the wave. The obvious prevention is to not use lead with lead-free.

Will the same soldering fluxes used for tin-lead work for lead-free soldering?
Some tin-lead fluxes do work with lead-free soldering. A better choice may be to evaluate and find a flux designed for lead-free since the lead-free fluxes almost always work for tin-lead soldering. Lead-free fluxes will be primarily water-based (VOC-free) because of their ability to handle the higher temperatures associated with lead-free soldering.

What are the main qualification tests for using liquid flux?

The main qualification tests that will be required will be to determine adequate hole-fill in relation to board and component finishes. No-clean fluxes will be less active and require particular attention, when compared to water washable organic acid fluxes, which will exhibit the best hole-fill. Having optimized preheat temperatures, solder pot temperatures, and flux volume, the conveyor speed may have to be reduced to accommodate the solder wetting when using lead-free solder. The flux will play an important role in maintaining production yields. Lower solids fluxes may require faster conveyor speeds to avoid complete flux burn-off, higher solids fluxes will be more forgiving and conveyor speeds may not have to be reduced.

How do I control the Pb, Cu and Fe concentration in my Lead-free soldering pot? Which steps do I need to make to stay within the limitations for above elements?

In Lead-free wave soldering the Cu, Fe and Pb concentrations need to be followed very closely.

Increase of Cu concentration will generate an increase in melting point of the alloy and increase of viscosity of the solder. This will result in slower wetting and reduced hole fill. If i.e. SAC305 is used the volume compensation of the solder pot should be done with SAC300 (without copper) in order to maintain a constant Copper concentration.

The Pb concentrations need to be kept below 0.1% wt. If not all components are lead-free the Pb concentration will increase rapidly. As the initial Pb value of the solder bars will be between 0.05% and 0.1% the limit of 0.1% can be achieved rapidly. Only partial removal of the contaminated solder pot will get the Pb concentration back below 0.1% wt.

If the Fe concentration increases in the solder pot this indicates that the solder pot is slowly dissolving because of the Pb-free alloy. High Sn concentration (all Pb-free alloys) is capable of dissolving mild steel. If this happens the Fe concentration will increase rapidly and Fe-crystals can be seen on the surface of the solder pot. In this case the solder pot has to be changed or coated and the solder bar has to be replaced.

Analysis of the solder pot is done by means of Atomic Absorption Spectrophotometric (AAS) or Inductive Couple Plasma (ICP). Other methods such as X-ray analysis are equally possible. Kester has laboratories capable of doing such analysis including XRF.

For lead-free wave soldering, what is the recommended control limits for impurities such as lead, copper and silver for good control?

The recommended control limits for lead is max 1000 ppm, which is in accordance with the RoHs regulation. There is no industry control limits for Copper and Lead in SnAgCu solder pot. However, it is suggested that for silver impurity, it should be controlled at a tolerance of +/- 0.2%. For copper impurity.it is recommended to control below 1%. It is also recommended that customer monitor the impurity level with the respective yield as the impurities may vary based on the process, board finish and components and then derive a suitable limit for process control.

What is a good method for solder bar addition for lead-free solder bar?

If SnAgCu solder bar is the intended alloy for lead-free, then the recommended lead-free bar for addition to the solder pot should be either SnAgCu or SnAg3 bar depending on the type of board finish used, and process conditions.

How does one minimize solder tearing in lead free joints for SAC solder after wave soldering and does this affect reliability?

Typically solder tears can be observed when using lead-free solder SAC alloy, particularly in wave soldering and with lead contamination present. This has not been considered as a defect as per IPC-610D specifications. Typically this is a cosmetic effect and some studies have been conducted by the industry that solder tearing does not lead to reported reliability failures. The mechanism for this phenomenon is caused by differences in solder solidification, which is affected by the lead from either the PCB or component lead plating. The differences in solder solidification could result in the solder shrinkage that could result in solder tears visibly noticed at the surface. In certain cases, when the board cooled, the contraction may even result in pads lifting from the surface of the laminate. This was reported in certain studies that lead or bismuth contamination of the alloy close to the joint interface can lead to a difference in solidification rates in the joint.

Hence to minimize solder tearing, this may done by super-cooling the board, eg implementing an additional cooling zone when the board immediately exits from the wave. Hence this requires the combination of efforts of the machine vendor.

Is it possible to decrease dross (oxide) generation when using Sn96.5Ag3Cu0.5 solder?

By adding Ge to Sn96.5Ag3Cu0.5 solder, a decrease of dross generation is possible. Ge content (from 0.01% to 0.05 %) is usually effective in reducing oxide formation.

Other causes of increased oxides are high pot temperatures, turbulent wave dynamics and the increase of certain metal contaminants such as zinc, iron, and copper.

Is there a lead-free solder that prevents dissolution of Cu?

It is possible to prevent Cu dissolution by making Cu content 2-6% of the lead-free solder. When Cu content of solder is increased, liquidus temperature also rises unfortunately.

Higher soldering temperatures could lead to reliability issues for components and boards.

Also certain additions of nickel to lead-free solders particularly with SnCu solders can help in reducing solder pot dissolution.