Can existing tools survive Pb-free soldering? Most machines pass the test, but some key changes are ahead.

Ed.: For the full article, please see circuitsassembly.com/cms/content/view/1674/

In the transition to lead-free soldering, an area of concern in assembly operations is what equipment changes, if any, will be needed for lead-free electronics assembly processes. SnAgCu (or SAC), the most common lead-free solder alloy, has a higher melting point than SnPb solder, which increases the thermal demands on certain processing equipment. There are also concerns about mixing SnPb and Pb-free soldering operations and whether dedicated equipment is necessary to avoid contamination.

Screen printers. Equipment currently used for paste printing can also be used for Pb-free printing. Older Pb-free solder paste formulations often required the use of slower printer speeds with higher print pressures because of the non-optimized flow characteristics of the pastes. However, improvements in Pb-free solders have optimized the metal content and paste viscosity, so printer settings for Pb-free pastes are now very similar to those for SnPb. Stencil cleaning may need to be monitored, at least initially, and there should be dedicated and clearly labeled squeegee blades, stencils, support pins and blocks to avoid any contamination with SnPb paste.

Placement. The concern with placement machines has been whether Pb-free components (Sn-based passives, QFP, BGA, etc.) may require different component recognition and lighting adjustment than standard SnPb-coated parts. In general, such differences have not been found when existing placement equipment is used for Pb-free components. Previous iNEMI lead-free projects have not reported placement issues for SnPb or Pb-free components.

Reflow ovens. The SAC alloy melting point is 34[degrees]C higher than SnPb (217[degrees]C compared to 183[degrees]C). The higher temperatures affect the thermal performance of boards and components. Perhaps the most effective way to protect boards and components is to reduce the [[DELTA].sub.T] between solder joint and component body.

For Pb-free soldering, 10-zone convection ovens are often preferred over six-, seven- and eight-zone ovens, especially for large server-type boards. The additional zones permit sufficient time in reflow to achieve the desired preheat and soldering temperatures while reducing the [[DELTA].sub.T] between solder joint and component top and across components on the board. It also permits sufficient soak time and temperature prior to reflow without rapid temperature changes required in adjacent zones.

Note that the largest, thickest boards do not always have the greatest [[DELTA].sub.T] between solder joint and component top. In a 2002 paper (1), the greatest [[DELTA].sub.T] across the board for Pb-free soldering for a wide array of board types (disk drive, laptop, server) was found on smaller, thinner, double-sided paste-in-hole laptop boards that used a closed reflow fixture.

When deciding whether to invest in 10-zone convection ovens, manufacturers need to consider the types of boards they are building. For thinner boards, such as those used in cellphones, eight-zone reflow ovens should still be sufficient for Pb-free soldering. For laptop boards with a thickness around 0.063", a 10-zone reflow oven should be considered. A 10-zone reflow oven is recommended for thicker (>0.090" thick) boards.

Debate continues over the use of nitrogen versus air in reflow assembly. The iNEMI Pb-free Assembly and Rework Project found that nitrogen is better in terms of aesthetic appearance of Pb-free solder joints, but not necessary to ensure joint integrity. Again, manufacturers need to consider the types of boards they are building when deciding whether to use nitrogen.

Nitrogen use may be more appropriate to preserve solderability of certain board surface finishes such as OSP (organic solderability protectant). This is especially true for boards passing through multiple thermal excursions, such as when using both surface-mount and wave soldering as is typical in boards when board thickness is 0.063" or more. Nitrogen may also help to improve ICT pin probeability of no-clean solder paste flux residues, which may be harder during higher temperature Pb-free processing in air atmosphere.

Wave soldering. Dedicated wave machines for Pb-free soldering are recommended. The typical 1600 lb. solder pot makes changeover from SnPb to Pb-free wave soldering difficult and time-consuming, and lead contamination remains a risk. The high-tin content of SAC solders increases erosion on certain stainless steel parts, especially those in contact with the wave, and they must be replaced more often than when using Sn63Pb37 solder. Some wave-solder equipment suppliers once provided nitride-coated parts, but titanium parts appear to be better. Titanium parts are scratch-resistant (during maintenance) and should have longer manufacturing service life.

Jasbir Bath is an advisory process engineer for Solectron Corp. (solectron.com); [e-mail]. He has led various iNEMI lead-free programs. He is currently organizing a new iNEMI initiative in Pb-free rework optimization.