by Fred Verdi
The current movement in the worldwide electronic industry toward lead (Pb) free electronics is based on environmental and legislative, rather than technical, reasons. Pb is considered to be a toxic substance that should be eliminated from all electronics, just as the particularly harmful organic compounds of Pb have been eliminated from paint and gasoline. By 2006, Europe will require that all electronics be Pb free, and the U.S. banning of Pb in electronics is expected to be not far behind. However, the U.S. will be pulled along as other countries make the conversion. For details, see the article titled "Lead Free Soldering Processing" in this issue.


Pb is used in the existing electronic manufacturing process as a tin (Sn) Pb alloy solder to attach and electrically interconnect components such as IC (Integrated Circuit) chips and resistors to PWBs (Printed Wiring Boards). The vast majority of experience in electronics is with manufacturing processes that use SnPb solders to attach electrical components having SnPb plating on their leads to PWBs.

When Pb is banned, the electronic manufacturing industry will have to switch to solder that contains only Sn, and perhaps some small amount of silver (Ag), copper (Cu), bismuth (Bi), or antimony (Sb) to improve mechanical properties of the solder. These Pb free soldering alloys, such as SnCuAg, have much higher melting temperatures than the SnPb currently used. This higher melting temperature is the source of many manufacturing issues. One of the foremost DFM (Design for Manufacturability) issues is the selection of the base PWB material to be used in a design for Pb free electronic hardware. Figure 1 shows an example of the choices of substrate material available to the designer faced with the task of designing hardware that will use the Pb free assembly process.


FR 4 PWB substrate laminates of several varieties may be chosen for the PWB substrate to be used for Pb free manufacturing. These varieties of FR 4, with their corresponding Tg values, appear in Fig, 2.

Because of the close proximity to the Tg of the substrate PWB, Pb free soldering may cause the designer to call out a higher-Tg-substrate than PWBs used in SnPb processed assemblies. Extended time above the Tg of the PWB material in the reflow oven for SMT or in the solder wave for PTH manufacturing can cause the assemblies to warp. This makes the choice of PWB substrate materials a tougher challenge for the designer, especially since the higher Tg PWB materials tend to be more expensive.

"Popcorning" is the tendency for explosive outgassing of water vapor during the soldering temperature cycle. The moisture sensitivity rating for each component has, in most cases, been established based on SnPb soldering temperatures. Processing temperatures using the Pb free manufacturing process on the same hardware, depending on the solder alloy chosen, may be higher. It may be necessary to specify more stringent moisture sensitivity requirements for both the board and the components to compensate for these higher processing temperatures to avoid "popcorning", or delamination of the PWB or components.

Bake-out of the boards and components also becomes a more critical step with the higher temperature processes. For details on bake-out see the article titled "Lead Free Soldering Processing" in this issue. The DFM requirement is to specify a moisture sensitivity rating for each component that has been based on the higher processing temperatures for Pb free manufacturing.

The PWBs currently have surface finishes that have been optimized to allow reliable solder attachment of those components using SnPb solder, but the new Pb free process will require that PWB finishes be qualified for the new alloys, pastes, and fluxes.

The most common of these surface finishes are:

HASL (Hot Air Solder Leveled)
ENIG (Electroless Nickel, Immersion Gold)
OSP (Organic Solderability Preservative)
Immersion Ag (Immersion Silver) Immersion Sn (Immersion Tin)

At the EMPF, finishes have been tested using Pb free manufacturing methods in manual, through-hole and surfacemount modes. Pb free manufacturing has been found to be workable with relatively straightforward process accommodations except for the OSP finish.

Design and manufacturing processes are not the only complications in dealing with Pb free manufacturing. Manufacturing technology is also a concern. Component leads plated with SnPb solder will now be plated with pure Sn or other no-Pb alloys such as Ni, Pd, or Au. Unfortunately, pure Sn plating is known to grow spontaneous tin whiskers. The Sn whisker is a filamentary single crystal growth that takes place some time (typically months or years) after the plating is done. It can grow to lengths that can easily bridge between features on a PWB, or even a bracket and other electrically grounded hardware in electronic systems, causing shorts.

Sn whiskers can also break off the Sn-plated component and re-lodge themselves between features causing short circuits. In the old SnPb solder world, the Pb alloying addition in the SnPb solder was known to virtually eliminate the possibility of whisker growth. The whole subject of Sn whisker growth, for which there is currently no industry standard test, must now be studied and controlled in the Pb free electronic manufacturing environment. For more information on the tin whiskering phenomenon, please see the article titled "Tin Whiskers" in this issue.

For more information concering Lead Free processes and surrounding issues, please stop by ACI's new Lead Free Manufacturing Page to download articles contributed to ACI by some of the industry's most knowledgable individuals and organizations, as well as material generated by ACI, and documents on the legislation surrounding the Lead Free issue.