Many assemblers are doing both leaded and lead-free soldering in the same location, what needs to be done to avoid cross-contamination?
This question comes up more often during an audit where an assembler has just transitioned. Lead can create havoc in a lead-free wave operation. Lead-free bar comes in at lead levels < 0.05% while the RoHS limit is 0.1%. The unintentional addition of two 1 lb bars of Sn63Pb37 can elevate the Lead level beyond the RoHS limit. There is no practical way to remove lead in a wave pot so pot dumping or extensive dilution will be required.

Small amounts of lead under 2% do not seem to impact joint robustness however when soldering leaded parts within lead-free solders this is not exactly known, secondly if segregation issues are encountered due to insufficient temperatures during soldering than higher levels of lead may be present within certain parts of the joint. This could lead to reduced reliability. This applies to all soldering situations such as reflow, wave, hand-soldering and rework.

The first item in avoiding cross-contamination is a complete training of all operators.  This includes those doing lead-free and leaded assembly. Identifying lead-free work zones by the use of large signage, green work station mats, identifying equipment with lead-free signs and insuring lead-free or RoHS parts and components are identified clearly, here some use green RoHS Compliant labels.

To summarize, the following could act as a good starting point to avoiding confusion.
· Identify the equipment with Lead-free and Lead-free symbols and signs
· Use LF bar solder that is triangular in shape instead of rectangular, usually Sn63Pb37
· Use soldering products with different packaging
· Different colors of dross bins for leaded and lead-free dross
· Train all procurement and process personnel about leaded and lead-free identification systems used by suppliers, example IPC-1066 or JES-D97
· Train all process personnel about the differentiating processes in place for lead-free and leaded, pre and post soldered assemblies
· Identify parts that are lead-free but not necessarily RoHS compliant or that are not lead-free process capable
· Use solder pot covers with triangular openings to avoid unintentional addition of leaded bars to a pot, where feasible
· Analyze solder pots regularly for main constituents but also lead and iron
· Use green “RoHS Compliant” labels on components and board packages
· Use green mats to identify lead-free work areas
· Identify soldering stations with a green stripe to differentiate them from leaded
· Identify stencils solely used for lead-free, especially if hand cleaned.
· Identify or segregate lead-free and leaded assemblies requiring further work or hand-soldering
· Separate, identify leaded and lead-free finished products

If I am cleaning soldered assemblies can leaded and lead-free assemblies be cleaned in the same cleaning machine?
The first thing to verify is if the present cleaning process and chemistries can in fact clean-off all lead-free residues. Lead-free flux residues will have seen higher temperatures. In reflow soldering, higher peak temperatures in the range of 240-255ºC are typical.  In wave soldering applications, longer contact times and higher pot temperatures are common.  During hand soldering, higher contact temperatures may reach 800º F. This can result in some charring of residues or polymerization of the flux resins resulting in possible cleaning difficulties.

Asking the manufacturer if the flux residues can be easily cleaned after higher temperature processing is a good start.  As an example, Kester works closely with the manufacturers of cleaning solutions to verify cross-compatibility. Materials that have been designed for lead-free operations will have been verified as lead-free process capable from a soldering and cleaning perspective.

If the flux is removable in your present process cleaning both leaded and lead-free soldered assemblies will be acceptable. Lead is very insoluble in water and most cleaning solvents, so it will not contaminate the lead-free products.

Checking ionic cleanliness is always a good idea, especially with the higher activity water washable fluxes. In lead-free soldering, higher temperatures can increase the risk of ionics remaining after the cleaning process. Checking ionic contamination levels after a flux chemistry switch is always a good practice.

How about cleaning stencils contaminated with lead-free pastes or leaded?
The same stencil cleaning solution will clean-off both leaded and lead-free solder paste residues. The resins and activators used in both lead-free and leaded pastes are usually from the same chemical family and the cleaning chemistries will very often remove the residues without change to cycle time and temperature.

Contamination of lead should not be an issue with stencil cleaning machines.

If stencils are cleaned by hand this can a demanding process and insuring all particles of leaded pastes are removed is critical. Examination of the apertures is important. As the powder diameter decreases, cleaning becomes more difficult. It helps that often stencils destined for lead-free processes are not used in a leaded process.

In reference to parts and components used in lead-free assembly, is RoHS compliancy sufficient information to receive from suppliers?
In a recent lead-free wave build a connector exhibited minor plastic cracking after soldering. After examination, it was found that the connector was lead-free, but was not designed for the higher temperatures of lead-free wave soldering.

A part may have a lead-free finish on its terminations but not necessarily be able to take the added heat during lead-free reflow, hand soldering or wave soldering especially if long contact times are used.

Moisture sensitivity levels are also critical for hermetically sealed SMDs. The floor life of some components destined for lead-free reflow may be reduced. To avoid delamiantion, popcorning and die stresses during the soldering process, a bake-out procedure may be necessary. Understanding the MSL rating is important.

Other components may be tinned but not able to withstand higher temperatures without charring, discoloration or, in extreme cases, melting.

The other questions to ask besides lead-free and RoHS compliant are:
· Maximum processing temperature
· Type of termination finish and composition
· MSL rating for lead-free assembly
· The use of nickel under the tin to reduce tin whiskers

What should I choose for a lead-free wave solder?
Two popular choices exist, SAC305 and SnCu based solders. Another option is the low silver SAC also known as SACX. Presently 60% of assemblers have decided on SAC based solders and about 20% have selected SnCu based solders. The remainder is still undecided.

SAC305 has the advantage of having more data available about its reliability and processing. SnCu based solders have less data available at this time. However, both have their place in the lead-free wave solder world.

SAC305 contains 3% Silver and therefore costs more, elevating operating costs much higher than SnCu based solders. The wetting speeds during wetting balance tests indicated that SnCu-based solders were slower than SAC-based solders. This could translate into slower conveyor speeds and extended contact times in the use of these SAC alternates. This could be a bigger issue however with thicker boards in excess of 0.093” or difficult to solder boards such as OSP having seen a previous heating cycle.

Some characteristics of SAC305 in wave applications are summarized below:
More information available
High operational costs
Patent-free
Faster wetting than SnCu-based
Grainy cosmetics
Higher incidence of hot-tears on joint surfaces
Higher oxidation than SnCu based solders with dross-reducers
Higher dissolution of copper
Higher dissolution of iron based solder pots

Some of the characteristics of SnCu based solders (such as K100LD) include lower costs, less dross, less surface shrinkage effects, bright solder joints, less dissolution of copper, less leaching of iron in solder pots and reduced maintenance costs.

It is important to note that the above information compares SAC305 to Kester’s K100LD solder bar. This patent-pending alloy out-performs all other lead-free solders in reference to the rate of Copper dissolution. It is a SnCu based solder with special additives to enable excellent wetting, reduced dross and extremely low copper dissolution. This reduces the cost of the overall operation as well.
                                                                                        
What should I choose as a lead-free tinning solder?
Solder tinning can also accommodate both SAC305 and SnCu based solders. The key to is to give a good solderable surface and both SAC305 and SnCu based solders are lead-free and easily solderable surfaces.

SnCu based is the lowest cost option and can produce a bright finish if the alloy contains a nickel dopant.

The slightly higher dip tinning temperatures used in this process do not impact wetting speeds. However, choosing SnCu based solders with certain additives can reduce dissolution of base metals reducing termination leaching and solder pot maintenance.

In the tinning of particularly thin diameter terminations the lower dissolution of K100LD can be a potential advantage and increase the reliability of the tinning.

During lead-free through-hole rework operations dissolution of copper is noticed at the elbow of the barrels is this a problem and how it can be reduced?
Dissolution of copper in the rework fountain process can occur with lead-free solders because of the higher leaching potential of the solder, higher temperatures and more importantly extended contact times.

A good paper on this issue is the technical paper delivered by Celestica’s Craig Hamilton at the 2006 CMAP International Lead-free Conference. His work indicated a dramatic lose of copper at the elbow of bottom-side barrels which could lead to a reduction in reliability especially if the board sees thermal cycling events. His experiments have indicated at times a 00% dissolution of copper.

In a wave soldering process contact time with lead-free solders can be from 1-6 seconds however in fountain rework processes the times are longer 15-60 seconds. The flow rates of solder are also faster in fountain rework.

Longer contacts and faster flow rates in these types of processes accelerates the dissolution of copper but also other metals found on boards and terminations.

A higher degree of solder contamination occurs which also increases analysis frequency
and pot maintenance.

SAC305 can be particularly a problem since dissolution rates are higher with this material. Ways to get around the issue are reducing contact times and using the lowest temperatures possible. Another option is to use lead-free solders with lower potentials for dissolution such as SnCu based solders with additives such as nickel and other elements. These alloys will have less ability to dissolve base metals. It is wise to ask for dissolution data of the various lead-free solders that fall in this category.

In some cases micro-voids are noticed in BGA assembly with SAC305 how can this be prevented?
Large voids present in BGAs need not necessarily be an issue if they meet IPC criteria however with lead-free in recent years micro-voids at the substrate interface have been noticed. Especially noticeable with BGAs and occurring with all types of finishes these smaller voids in large numbers have caused concern for the long term reliability of these types of joints.

The reason for their occurrence is complex and the following are some of the variables associated with their occurrence.

Solder paste flux chemistry
Thermal profile used
Board out-gassing
Board pre-plating cleanliness
Plating chemistries
Plating thickness
Organic inclusions in the plating

It is important to verify BGA lead-free joint integrity after soldering and X-Ray but also cross-sectional testing are needed to exclude this micro-voiding phenomena.

Not all board plated finishes are the same in reference to micro-voiding potential. For example a silver immersion chemistry may more likely promote this than an other. It is important to ask for voiding potential data from the plating chemistry

About the author:
Peter Biocca is Senior Market Development Engineer with Kester in Des Plaines, IL. He is a chemist with 24 years experience in soldering technologies. He has presented around the world in matters relating to process optimization and assembly. He has been working with lead-free for more than eight years, involved in numerous national and global consortia within this time; he has assisted many companies implement lead-free successfully. He is an active member of IPC, SMTA, IMAPS and ASM. He is the author of over a 100 technical papers delivered globally. He is also a Certified SMT Process Engineer.
   

For further information please contact Peter Biocca at Kester, 972.390.1197; E-mail: pbiocca@kester.com.