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TechTips Toolbox
December 31, 1969 |Estimated reading time: 4 minutes
BY CHARLES DENNEHY
When we consider the transition to lead-free soldering, with all of its metallurgical complexities, it’s easy to understand how people might forget the basics. This is especially true for the “back-end” of the manufacturing process: rework. For example, nowhere is control of chemistry more difficult than in the rework area. That is true now and will forever be the case. That control will be especially challenging during the “transition” period from tin-lead solders to lead-free.
For those who might not be associated with rework, control in this domain is particularly troublesome because the rework and repair element of a company (including the field service portion), represents a composite picture of each company’s solder and flux history. Once product has passed through assembly, it has a tendency to keep showing up for rework or repair at the original manufacture or design facility throughout its life cycle. It may be nearly impossible, but it is crucial to keep rework chemistry consistent with the original manufacture, or chemical conflicts and unwanted residues might cause a variety of failures on the assembly as it ages.
Accomplishing this control requires foresight, patience, and an occasional time machine. It is important to organize and manage solders, flux, and wick so that rework technicians have access to these items when required, but do not have the opportunity to mix and match. Choosing a central location for distribution of these items is an acceptable solution for many. Whatever you do, don’t ignore potential chemistry mix-ups or you will send latent failures into service unintentionally.
There are many choices for lead-free solder and accompanying fluxes. The leading candidates are: tin, silver, copper (Sn, Ag, Cu, or SAC), but there are many others. If you haven’t picked up on the peculiar lingo of the new solders, there are plenty of consultants and Websites available to get you up to speed. In the rework area, once choices are made about which lead-free chemistry to use, it is useful for rework personnel to know whether or not the board or component requiring rework is lead-free. This seems like an obvious question that should have a clear answer, but many contractors are receiving more “surprises” as lead-free technology creeps quietly into our world. Believe it or not, we have received many lead-free ball grid array (BGA) components to place on boards originally manufactured with eutectic solder. Is it possible to solder this component on the board when it may require a temperature increase of 30°C to successfully place the part? It depends, but it would be nice to know the answer before you start to solder a component onto a board.
The best way to get the scoop on the metallurgy of a component is to ask the vendor or customer who supplied it. Sometimes they don’t know, unfortunately, because there is a labyrinthine stream of supply flowing into the end product. The good news is that there are a variety of spectrometric and chemical tests available to determine if a component contains lead. It is a good idea to have access to those. In the rework and repair world, “unleaded” components aren’t always a problem. But in the BGA area, the added temperature of reflow on some heat-sensitive assemblies may be disastrous. We have our own quick chemical test to perform when we suspect an unleaded part has slipped in unannounced. It has saved us a few headaches.
Rework and repair typically are at the end of the assembly line and are the last things considered from a design and manufacture standpoint. That isn’t going to change any time soon, but if the lead-free juggernaut is coming to a facility, those involved are well advised to acclimate the rework and repair technicians to the look and feel of lead-free solders. Technicians make a living with their hands and eyes. Touch and feel are everything to them. Lead-free chemistry changes much of what they have become used to in the past 20 years. Once lead-free solders are an integral element of a product, there are clear differences in board finishes, joint appearance, overall heating, solder flow, flux stability, tip maintenance, and BGA voiding, to mention a few things. The flow, spread, and cosmetic finish of lead-free solders are different than leaded solders, so rework technicians and inspectors will require time and training to adjust. It will pay down the road to train all involved in the “new” metallurgy and chemistries.
The basics of organization and management will be tested as we adjust to the lead-free environment. Knowing that and preparing the rework and repair crews for the change will bring you a long way to a successful adjustment to the lead-free world. Good luck.
CHARLES DENNEHY, president, Circuit Technology Center, Inc., may be contacted at (978) 374-5000; e-mail: CDennehy@circuittechctr.com.