By Keith Gurnett and Tom Adams
An unavoidable consequence of the transition to lead-free solders has been the assembly of printed circuit boards containing a mixture of two types of components: those intended for use with lead-free solder, and those intended for use with traditional leaded solder. For a brief grace period after the July 1, 2006, enforcement date of RoHS (Restriction of Hazardous Substances) legislation in the U.K., for example, it was understood that such “mixed technology” or “hybrid” boards would in some instances be permitted.
This raises the question whether hybrid boards could wind up being used in military or even aerospace applications, and the best estimate seems to be that hybrid boards will, infrequently but inevitably, be used. If a hybrid board fails in these applications, the consequences could be severe. No one imagines that a hybrid board would intentionally be accepted, but it is equally hard to imagine that hybrid boards would not be acquired as COTS items.
Assembly Of hybrid boards
A good many hybrid boards have been assembled accidentally, without the assembler’s knowledge. During the transition to lead-free solder, the labeling and numbering of parts has been inconsistent. Some manufacturers, when switching a particular component from the lead version to the lead-free version, have retained the original part number and told users to refer to the date/lot code to determine which version they had. To compound an already confusing situation, an assembler might buy a component from one supplier this week and from another supplier next week. Whether the component that is being placed onto a particular board is lead or lead-free is anyone’s guess.
Hybrid boards have also been assembled purposefully. During the RoHS grace period in the U.K., some assemblers really had no choice: if one of the 20 components on a particular board was still available only in its lead version, then the board would end up being assembled with 19 lead-free components and a single lead component. Since military electronics systems, and numerous other critical systems, were generally exempted from RoHS requirements, such hybrid boards were presumably used in non-critical consumer applications.
Dr. Ronald Lasky of Dartmouth College, N.H., explains that a hybrid board must be reflowed at a temperature that is high enough to ensure that both solders melt. “You’ve got tin-lead solder that melts at 183 degrees Celsius and you’ve got lead-free solder that melts at 217 C. You need be sure to run the board through at something like 225 C, so that you get complete melting and total reflow of the lead-free solder.”
On a hybrid board, the solder plating the leads extending from the component and the solder on the pad are two different solders, with different characteristics. Generally speaking, lead solder flows more easily, wets better, and in its solid form is more plastic. Lead-free solders do not wet as well and are more rigid in their solid form. What kind of bond will form between these two solders on a bond pad is hard to predict. In some cases, though, even after reflow at adequate temperatures, the components on hybrid boards have been so loosely attached that when the board is placed vertically in a testing rack, the components fall right off.
In other cases, the bond between the two solders might be good enough to permit a normal service life. Marjory Craw-Ivanco, director of engineering services at Celestica in Toronto, says, “Where you have a mixed metal board, if the mixed metal occurs on devices that are not affected by that particular change, you could see no consequence, because not all components are affected equally, or not all component lead finishes are affected equally by this mixed-metal phenomenon. You could have it in a system and you might never find out about it. Or you could have it in your system and you might find out very quickly.”
Joe Scala, director of operations at Celestica, explains that hybrid boards exhibit one of two types of compatibility. Forward compatibility occurs when “we are talking about taking existing components and saying how will they actually fare when we start using lead-free solder. And then in other cases we’re talking about lead-free components and using them in our traditional lead-solder applications. That would be a backward-compatibility issue.” It is difficult to say which of these two types of hybrids might be most likely to occur in a COTS-procured system, but during the transition to RoHS rules in the U.K., it was not unusual to have a board with RoHS-compliant lead-free solder paste and at least a few components with lead-solder coated leads.
Lead-free solder options
Perhaps five years ago, the list of feasible lead-free solders being considered for general use was long, and included 40 or more alloys, but more recently a few alloys have predominated. Most of the lead-free solders being used globally are alloys of tin (Sn), silver (Ag), and copper (Cu), and are collectively known as SAC alloys. Two SAC alloys have captured the lion’s share of the market: Sn96.5Ag3.0Cu0.5, known as SAC 305, and Sn95.5Ag4.0Cu0.5, known as SAC 405. Lasky makes this observation: “I can tell you with pretty good confidence that in mainstream applications-in other words, companies producing cell phones or laptops for European customers, and not some crazy way-out application-the lead-free solder that is not SAC alloy is absolutely less than 10 percent and probably more like less than 2 percent.”
But despite the dominance of the SAC solders, new solders for specific applications are continually being introduced. “One of the challenges that we see is that there are constantly new alloys being introduced to build lead-free assemblies,” Marjory Craw-Ivanco says. “Every time you turn around, someone’s coming out with a new alloy with just a small amount of bismuth or nickel or some other metal that would influence the metallurgy of the solder joints.” Bismuth may seem like an odd choice, but it can be used to make alloys whose melting points are much lower than the melting points of other lead-free solders, or even of traditional lead-solder. During reflow, bismuth has the potential to form alloys whose melting points are only slightly above the normal working temperature of a board.
Part of the reason for the development of alternate, non-SAC solders, Joe Scala says, is that it permits a component to be reworked more than once. With SAC solders, you have only one shot at rework.
Experience counts
One of the difficulties in introducing a new type of lead-free solder is that the new solder can be used with confidence only after it has been fully characterized-meaning that its interaction with the rest of the component package and with the board is clearly understood. This is one of the reasons why the behavior of a hybrid board is so hard to predict-the components might fall off right after reflow, or the board might remain intact and perform flawlessly for years.
Joe Scala points out that the long experience with lead-solder lulled people into a feeling of complacency. “We had a lot of years of experience under our belts, so people stopped looking at certain things because they said, you know, I’ve been building with tin-lead solder for 50 years. Yes, packages have gotten bigger, but it’s been an evolutionary change. And now suddenly you inject all these changes all at once, and some are understood and some are not.”
As an example, Scala mentioned a BGA package that Celestica was using for one of its qualification boards-a BGA that Celestica has been using for years without a problem. But when they ordered the same BGA as a RoHS-compliant part, the package had been changed to a higher-temperature epoxy, and problems popped up. “The overmold compound on it was actually changed, so [the problems] had nothing to do with the lead restriction, but just the general other properties in terms of what makes the overmold compound,” Scala says. “That changed all the physical properties suddenly, from a thermal expansion perspective. The ability to create bridging and opens-all of those issues started rising up, and it had to do more with the material properties that were different than the previous part.”
Looking at the larger picture, what military and aerospace engineers would really prefer is to transition all at once into a fully compliant lead-free system. In this way, Scala says, they would be able to completely avoid the potential for lowered reliability that currently exists.
Until recently, Scala adds, it was widely considered that there was no difference in long-term reliability between SAC 305 and SAC 405. Using SAC 405, it was thought, would convey a cost advantage because SAC 405 contains less silver than SAC 305. But recently a team from Celestica published research showing that there were reliability differences between the two alloys when they were used in harsh environments. When the two alloys are tested in temperature ranges from -25 C to 125 C, or even from -40 C to 125 C, the difference becomes evident. “When you’re talking about a pure 405 system, it actually performs better in harsh environments than 305,” Scala says.
Board materials
In addition to changes in the physical characteristics of components and solders, assemblers also need to consider changes in board materials. An assembler making the transition to lead-free solder might still use an FR4 board, but he might specify an FR4 with a higher glass-transition temperature, Scala explains. “Typically what that means is that the boards are also getting stiffer, and that’s not a good situation because you want a little bit of flexibility in your system. So some other issues can come up.
“I’ve heard of pad cratering, as an example,” Scala notes. “You have a situation where the solder joint, because we’ve moved away from tin-lead, where the lead used to have a lot of give in it, to SAC, which is a lot more stiff, and you also have a printed wiring board that’s becoming stiffer. So everything’s becoming stiffer, but the system still needs to expand and contract in terms of thermal expansion, so that’s where you start seeing issues.”
Power cycling during service can cause eventual damage because the circuit diffused in the bulk silicon heats up first, and heats up rapidly. Plastic packaging, solder joints, and the board itself heat up at different times and with different rates of expansion.
The problem of “pulls”
Lasky points out a newer phenomenon that can result in a hybrid board, or at least a board having one or two components that are-most likely-old-style lead-solder components on what is supposed to be a lead-free board. The problem is that the one or two components may not be what the assembler thought they were.
“You ordered what you thought was lead-free, and it’s a big-name company microprocessor, and you got a good price on them,” Lasky explains. “The microprocessor is real, and it really works, but it was made in 2004, and the supplier told you that it was made in 2006. And when it was made in 2004, it had lead in the leads. It functions and everything, but it’s not lead-free.
“That kind of stuff is happening because the market for electronic components is a multiple hundred billion-dollar market...your laptop now is being recycled instead of thrown away,” says Lasky. “What people will do is, they don’t just grind it up, they take it apart, and they will actually pull components off it, and they’ll be reused again. So they may take some of these what are called ‘pulls’ that were assembled with leaded solder, pull them off, doctor them up, make them look brand new, and sell them to some unsuspecting person as a component that has lead-free leads. And it doesn’t, it has leaded leads. This kind of stuff is happening, but to say it’s common is too strong a word. It’s not rare.”
Since there are multiple ways in which a hybrid board could be procured as a COTS item and enter military use, military procurers may want to consider techniques for screening out such boards. Careful qualification of suppliers will surely help, but suppliers of boards cannot always be entirely certain of the pedigree of components that they purchase.
Ultimately, all or nearly all consumer electronics will have made the transition to lead-free solders, and the industry will have acquired enough field data to make reasonable reliability predictions for the various lead-free solders, components, and board materials being used. But that day is still a long way off, and until then, it may be advisable for incoming boards to be tested, perhaps using X-ray fluorescence guns, which appear to able to identify solders quickly and easily, and which are already being used for screening in Europe. Simply hoping that hybrid boards will have no mission-critical failures is not realistic.
