One step closer to lead-free standards: IPC's technical committees continue to work on revisions for expected release in the fall.

IPC's J-STD-001 and IPC-A-610 technical committees are pushing to get lead-free assembly acceptance criteria in the next revisions. More than 40 members of the two committees met for a week in early May to continue work on these projects.

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The participants, some of whom were also members of the Electronic Manufacturer's Service Forum or the NEMI Lead-Free Rework Committee, provided a good foundation for lead-free criteria for the next revisions. Data are still incomplete, but enough information is available to apply the existing lead placement and solder fillet requirements to lead-free connections. The group recommended that solder connection acceptability requirements be unrelated to chemistry; the end requirement is a wetted connection with an uncontaminated surface finish.

Visual comparison of lead-free and traditional tin/lead connections is a different issue. Because IPC-A-610 is focused primarily on visual acceptance, the committee expended great effort in building criteria. They developed the criteria for terminal soldering and through-hole and surface-mount connections for the current draft.

Differences in surface texture and the higher wetting angles of lead-free solder connections will be the hardest thing for users to understand and will require many pictures. Dewetted and frosty-looking connections may be acceptable when using lead free. The wetting angles may be larger than with tin/lead alloys because lead-free solders do not flow the same.

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Labeling assemblies and identifying the solder alloys will be critical to the ability to make acceptance decisions.

The text proposed for the solder acceptance chapter introduction in IPC-A-610 Revision D is:

This section establishes the acceptability requirements for soldered connections of all types. Although Class 1, 2 and 3 applications and environments have been considered, the nature of the soldering process may dictate that an acceptable connection will have the same characteristics for all three classes, and an unacceptable connection would be rejected for all three classes.

Where appropriate, the type of soldering process used has been addressed specifically in the criteria description. In any case, the connection criteria apply regardless of which methods of soldering have been utilized, for example:

* Soldering irons

* Resistance soldering apparatus

* Induction wave, or drag soldering

* Reflow soldering 

* Intrusive soldering

As an exception to the above, there are specialized soldering finishes, (e.g., immersion tin, palladium, gold) that require the creation of special acceptance criteria other than as stated in this document. The criteria should be based on design, process capability and performance requirements.

Wetting cannot be judged by surface appearance. The wide range of solder alloys in use may exhibit from low or near zero degree contact angles to nearly 90[degrees] contact angles as typical.

The primary differences between the solder connections created with processes using tin-lead alloys and processes using lead-free alloys are related to the visual appearance of the solder. This standard provides visual criteria for inspection of both tin-lead and lead-free connections. Illustrations and pictures specific to lead-free connections will be identified.

Lead-free and tin-lead connections may exhibit similar appearances, but lead-free alloys are more likely to have:

* Surface roughness (grainy or dull)

* Greater wetting contact angles

Typical tin-lead connections have a shiny to a satin luster, generally smooth appearance and exhibit wetting as exemplified by a concave meniscus between the objects being soldered. High temperature solders may have a dull appearance. Touch-up (rework) of soldered connections is performed with discretion to avoid causing additional problems and to produce results that exhibit the acceptability criteria of the applicable class.

The criteria developed by the joint A-610D/J-STD-001D working group will be reformatted for J-STD-001D. This summer, the committees will review illustrations and additional comments. Recommendations from industry users are encouraged to bring the project to completion by the fall.

RELATED ARTICLE: IPC Status of Standardization

Recently Published

IPC-2581, Generic Requirements for Printed Board Assembly Products Manufacturing Description Data and Transfer Methodology

In Ballot

IPC-7095A, Design and Assembly Process Implementation for BGAs (3rd Ballot)

IPC-A-600G, Acceptability of Printed Boards ([2.sup.nd] Ballot)

IPC-6012B, Qualification and Performance Specification for Rigid Printed Boards ([2.sup.nd] Ballot)

IPC-4101, Material Declaration Handbook

IPC-7351, Generic Standard on Surface-Mount Land Pattern Design (supersedes IPC-SM-782)

IPC-9194, Implementation of Statistical Process Control (SPC) Applied to Printed Board Assembly

Jack Crawford is director of certification and assembly technology with IPC, Northbrook, IL; [phone]; email: [e-mail].