RoHS Assembly Concerns

Solder Alloy and Flux
Unfortunately, despite a great deal of research, comprehensive and comparative data on lead-free alloys is lacking. The list of solder alloy requirements is lengthy and involved.  In general, technical requirements include being non-"hazardous", mechanically reliable, thermal fatigue resistant, good wetting, relatively low melt ing temperature and compatible with a variety of lead-bearing and lead-free surface coatings. In addition, one must consider logistical issues such as alloy cost, availability and patent issues. While most of the world has settled on the tin-silver-family of alloys, a good deal of debate still exists as to which exact composition is ideal, and of course others will choose alloys from outside of this family. As with all other technical issues, although there has been much consortia work on alloy selection, the alloy of choice will come down to the specific requirements of each unique assembly. Your choice of alloy is dependent upon your application and should be proven out to your standards.

As with alloys, what is a suitable flux (paste, liquid flux and cored wire) for one manufacturer may not be for another. Select flux chemistries suitable for lead-free processing and your particular application. One should consider a fluxs activation temperature, activity level, compatibility with chosen alloy and reliability properties such as SIR, electromigration.

Process Considerations
Once it is confirmed that the parts and materials to be used in lead-free assembly are available, suitable and reliable, it is time to get the processes optimized in order to achieve maximum throughput and reliability.  To do so, Engineering must refocus attention to paste handling, printing, reflow, wave soldering, rework & repair and cleaning.


Paste Handling
Shelf-lives with lead-free pastes may be reduced as compared to tin/lead, and storage conditions may be slightly more stringent. However, in general, the same rules as with tin/lead apply. For example, prevent/minimize paste's exposure to heat and humidity, allow paste to come to room temp erature before using and do not mix old and new paste in the same jar. If one follows proper paste handling procedures now and has good results from these, there should be very few issues when transitioning to lead-free paste use.


Printing
In general, no major changes to the printing process should be necessary.  That is, lead-free pastes should exhibit similar features on the stencil and the same equipment set points should transition well.  One can expect similar performance in terms of stencil life, aperture release, print definition, high-speed print capabilities, print repeatability, etc. However, this depends on the paste manufacturer and if they have density issues resolved.

As tin/lead solder alloys tend to have better wetting than most lead-free alloys, some stencil design modifications may be needed to maximize spread of paste and counteract inferior wetting. Engineers should run tests with lead-free alloys on their current stencils to confirm adequate spread and wetting. If wetting is not sufficient and cannot be rectified by other means, stencil design modifications may be in order.


Reflow
This is the SMT process area that will be most affected by a switch to lead-free processing. Most lead-free alloys require higher reflow temperatures than the 210-220 C peak temperature of tin/lead; anywhere from 235-260 C is common. This higher reflow temperature dictates that one should minimize T and maximize wetting through the reflow profile (including cooling), and could possibly mandate reflow equipment changes.

Profile - Depending upon the oven utilized and the density of the assembly being processed, the Ramp -to- Spike process is generally recommended for lead-free assembly.


Wave Soldering
Depending upon the alloy selected, wave soldering will require a pot temperature of 260-275 C. This increase of temperature and the change in solder alloy will require some additional process changes.

Flux- May require a change in liquid fluxes to compensate for the poor wetting of some alloys and high thermal stresses of the wave process. If changing fluxes, particular attention should be paid to both to operating window it offers and the material's reliability characteristics.

Equipment- Most modern wave solder machines can provide the necessary heat (preheat and wave) for lead-free soldering.


Rework and Repair
Materials - Operators must be re-trained for lead-free rework, as the lead-free solders do not flow as well as tin/lead. This could also require stronger cored wire fluxes to be used. As with any change of flux chemistry, if changing wire solders, particular attention should be paid to both to operating window it
offers and the materials reliability characteristics. Some wires often assumed to be safe to leave uncleaned are actually classified as rosin fully-activated and could cause field failures.

All rework should use the same lead-free solder alloy as originally used on the solder joint; different leadfree solder formulations should not be mixed on the same joint. If more than one alloy is in use in the production process (i.e., Sn/Ag/Cu for SMT and Sn/Cu for wave soldering), operators should be trained to use the correct wire for each part. For this reason alone, it is advisable to use a single solder alloy for all assembly operations.

Equipment- It is necessary to ensure that the desoldering and soldering stations are suitable for lead-free processing, i.e. can reach the necessary temperatures for lead-free soldering. It should be noted that leadfree soldering can wear out tips at a much higher rate than tin/lead.


Maintenance
The main challenge for maintenance is the additional wear and tear that lead-free assembly puts on assembly equipment. This is especially true of reflow ovens and wave solder machines. This is the result of the higher melting temperatures at which the lead-free solders require the equipment to work and the tendency of the lead-free materials to wear out the materials they come in direct contact with.