Thursday, May 26, 2016
VOLUME -24 NUMBER 7
Publication Date: 07/1/2009
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ARCHIVE >  July 2009 Issue >  Special Features: Assembly and Packaging > 

Lead-Free HASL Growing in Popularity
SnAgCu (SAC) alloy resulted in an uneven grainy finish.

The wisdom traditionally expressed in the lead-tin solder days by advocates of the hot air solder leveled (HASL) board surface finish was "nothing solders like solder". Since this dictum applies equally in the lead-free era, the question that has to be asked is why there was ever any doubt that the HASL finish would continue to have a place as a major printed circuit board finish. Until the electronics industry was forced away from the tin-lead eutectic solder, the Sn-37Pb, applied by the HASL process was the most widely used printed circuit board finish in North America and Europe in terms of the area of panels processed. HASL was also widely used in Asia but the market share was smaller because of the dominance in those markets of consumer electronics. For the consumer electronics industry the short interval between manufacture and soldering that resulted from the effective application of "Just in Time" inventory management systems meant that the solderability could in most cases be satisfactorily ensured with a cheap OSP finish.

In the period leading up to RoHS, the view that was most widely expressed was that the HASL finish would disappear as the industry moved to lead-free technologies. One concern that prompted that view was that the board would be damaged by the temperatures to which it would be subjected because of the higher melting point of the lead-free solders. Another concern was that the tightening in the requirement for coplanarity which was occurring simultaneously with the change to lead-free would mean that the use of the finish would be precluded by the unevenness of the coating thickness for which the tin-lead HASL process was notorious. The apprehension about the lead-free HASL process seemed to have been justified by the poor results from some early trials. Even with process temperatures as high as 300°C, the finish was dull, grainy and very uneven.

Among the challenges the industry has faced in implementing lead-free soldering technology has been the identification of solderable finishes that reduce rather than increase the difficulty of achieving the wetting that is a non-negotiable requirement for a reliable solder joint. In retrospect, therefore, it is difficult to understand why the industry chose to compound the challenge of lead-free implementation by the decision to abandon the finish that had served it best during the era of tin-lead soldering and switch to finishes which were already known to have limitations.

Metallurgical Continuity
A fundamental requirement of a solder joint is metallurgical continuity between the copper pad of the printed circuit board and the termination of the component. The pad is normally electrodeposited copper so the short form justification for a HASL finish could be stated more fully as "nothing solders like copper that has been prewetted with solder". That statement is more technically correct because what this finish delivers is not so much the solder that is visible but the wetted interface that lies beneath the surface of that solder at its interface with the copper. Because when a tin-containing solder wets copper, Cu6Sn5, forms almost instantaneously; the evidence that wetting has been achieved is provided by the presence of that interfacial intermetallic compound. It is reasonable to assume that the fact that once the board has been fully wetted in the HASL process, the amount of work that the flux has to do so has been reduced; therefore more activity is available for wetting the component termination and facilitating hole fill. Thus the advantages of the HASL finish could be summarized by saying that "half the solder joint has already been made".

The term "solderability" describes the ease with which solder, with the help of an appropriate flux, can wet a substrate. With wetting already established, the HASL finish delivers solderability unequaled by any of the alternative printed circuit board finishes that do not contain tin — and this in turn widens the process window. An additional benefit: when properly applied, the HASL finish can preserve solderability for a very long period of time without the need for additional protection from the environment or special storage conditions. There are many anecdotal reports of printed circuit boards with a HASL finish, being soldered easily after many years of uncontrolled storage.

One of the reasons for the poor results obtained in early attempts to apply a lead-free HASL finish was that the alloys recommended by industry consortia — Sn-3.8Ag-0.7Cu and Sn-0.7Cu — were not suitable for the process. One of the reasons for these alloys being recommended was that they are both nominally eutectic and as such, were expected to behave in a manner similar to the tin-lead eutectic they were replacing. However, because of their different metallurgy, with second and third phases being intermetallic compounds rather than the solid solutions that make up the microstructure of tin-lead, these alloys do not behave like eutectics. A feature of an alloy with true eutectic behavior is high fluidity close to its melting point and single stage solidification to a fine grained microstructure. Neither the SnAgCu (SAC) alloy nor the SnCu alloy exhibited that behavior and the consequence was the reported uneven grainy finish.

Nickel Modified Alloy
The breakthrough came with the use of a nickel-modified Sn-0.7Cu alloy marketed under the name SN100C® that behaves as a true eutectic. The similarity of this modified SnCu alloy, with the further addition of germanium as an antioxidant, to tin-lead solder is apparent in the appearance of the smooth shiny surface and the comparable fluidity.
Alloy with true eutectic behavior has high fluidity close to its melting point.


In practice, the problems had been expected with the lead-free HASL process have not materialized and the finish has found widespread use, initially in Europe. Unless the panel is passed through the HASL process more than twice and/or a solder temperature higher than necessary is used, the expected damage to the printed circuit board has not been observed even with CEM-1, CEM-2 and FR-2 grades of laminate. The likelihood of damage is being further reduced as more heat-resistant grades of laminate are introduced to accommodate other requirements of lead-free processing. The variation in the solder coating thickness of a properly applied lead-free HASL finish is typically about half that of the tin-lead HASL finish it replaces — so that its coplanarity has been found to be good enough for the finish to be used on boards with large area array components. When properly applied, the HASL finish is the most robust of all solderable finishes offering a shelf life of more than a year even without special storage conditions and providing solderability that can survive multiple lead-free reflow profiles.

Dominant Not the Best
While immersion silver and OSP have indeed established a position as the dominant lead-free printed circuit board finishes in the North American market, a consequence of the successful promotion of immersion silver has been its exposure to a wider range of operating conditions than had been the case before lead-free implementation. One phenomenon to emerge from that experience is catastrophic "creeps corrosion". This occurs when the assembled board is exposed to humid atmospheres polluted with sulfurous gases and is driven by the electrochemical cell that is created between the silver and the small areas of copper inevitably exposed near the edge of the solder resist on solder mask defined pads. Solderability issues have been experienced with OSP in the final stage of wave soldering as it is often difficult to meet barrel fill criteria even if solderability is good enough for the first stage of reflow with enough solderability surviving to the second stage of reflow.

In Europe, where lead-free implementation is well advanced, HASL has retained much of its market share with more than 300 lines currently running the lead-free process. As volumes have increased, European printed circuit board shops and their customers have looked to Asia for supply and there are now more than 100 lines running the lead-free HASL process in China, India, and Southeast Asia. Even in North America there are at least 20 lead-free HASL lines in operation. The number of lines has grown by 45 percent since the end of 2006 and continue to grow in 2009. To have achieved that growth rate, an average of more than five new lines would have been commissioned each month, and that would not be happening if major problems were being encountered with the process.
Typical lead-free PC board that has been HASL finished.


While it was the availability of alloys that delivered better results that gave the lead-free process a second chance, it was improvements in the design of HASL machines that have secured its position. Tighter temperature control and more efficient heat transfer made it possible to operate the process at lower temperatures than were expected. Improved machine design has widened the range of alloys that can be used in the process but best results are still obtained with the tin-copper-nickel-germanium alloy SN100C.

So what has happened to HASL in the lead-free era? Despite concerted attempts to consign it to history, the HASL process is playing a major and increasing role in delivering reliably solderable lead-free printed circuit boards to all sectors of the electronics industry.

Contact: Nihon Superior Co., LLC, 1395 Hawk Island Dr., Osage Beach, MO 65065 573-280-2357 fax: 619-923-2714 E-mail: k.howell@nihonsuperior.co.jp Web:
http://www.nihonsuperior.co.jp

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