低温焊锡研究

1、The damage from popcorning is immediate and usually detectable, but there are other thermally induced damages that cancause long-term problems, such as warping of printed circuit boards or damage to ICs, which would also be reduced withlower peak temperatures.Step Soldering. The availability of sold

2、ers with lower melting points will make multiple reflow processes on a single boardpossible. For example, all of the normal components that can tolerate higher reflow temperatures could be soldered to aboard using the standard process, and then the lower-temperature components could be added in anot

3、her reflow process.Since step soldering is a bulk reflow process, it takes less time and is more uniform than hand soldering, and doesnt takeany different equipment or special training.Possible Pb Elimination. Many low-temperature solders contain no lead.Selection of Low Melting AlloysWe call a sold

4、er alloy low melting if it melts at temperatures below 183°C and above 50°C. Most of the alloys that meet thisrequirement are made of four elements: Sn (tin), Pb (lead), Bi (bismuth), and In (indium). The Cd (cadmium) bearing alloysare not considered because of their extreme toxicity. Vari

5、ous compositions of these elements produce alloys that melt atany given temperature between 50°C and 183°C. Commercially available low-melting alloys are listed in Table I. Thenumbers associated with each alloy in Table I are the percentages by weight of the components that make up the all

6、oy.To better understand the correlation between the alloy compositions and their melting temperatures, we can use the ternarydiagram of melting temperature. A ternary diagram uses a triangle to represent chemical compositions of a three-elementalloy system. A physical property, such as melting tempe

7、rature, is plotted over the triangle. Figs. 1a to 1d show the meltingpoints of ternary systems of all possible combinations of the elements BiPbSn, BiInSn, InPbSn, and BiInPb.These diagrams show what are called the liquidus temperatures, as opposed to the solidus temperatures. A typical alloymelts n

8、ot at a single temperature but over a temperature range. The solidus temperature is the highest temperature at whichan alloy remains solid, while the liquidus temperature is the lowest temperature at which an alloy remains liquid. At thetemperatures between the solidus and liquidus temperatures, an

9、alloy is a mixture of solid and liquid. The solidustemperatures of these alloy systems are not shown in Fig. 1. However, for a few specific compositions labeled “e” or “E” inFig. 1, the so-called eutectic alloys, the solidus and liquidus temperatures are equal. Alloys with eutectic compositions orsm

10、all differences between their liquidus and solidus temperatures are often favored for soldering applications because theymelt and solidify rapidly instead of over a range of temperatures.Not all the compositions found on the ternary phase diagram are suitable for soldering applications. To determine

11、 which aremost appropriate, see Table 1.Wettability. A metal is said to have wetted with a surface if it forms a sound metallurgical bonding with the surface.Wetting is essential in the soldering process because it ensures that the joint created wont come apart at the inter-face. Any new alloy must

12、be able to wet to the common pad surface finishes: Cu, PbSn, and Ni coated with Pd orAu.Reliability. Lower-temperature alloys should still be reliable, so we measure the following properties to estimatehow reliable solder joints made of an alloy will be: shear strength, creep resistance, isothermal

13、fatigue resistance,and thermal fatigue resistance.Long-term stability. Microstructural evolution, grain growth, and recrystallization contribute to changes in the sol-der joint mechanical properties over time, so we want to make sure that the changes are slow and stable and wontreduce the mechanical

14、 properties of the solder joints to unacceptable levels over the life of the joint.Practicality. Alloys used for mass production should be cheap and widely available. It should be possible to makethem into solder pastes so that they can be used in standard assembly processes, and suitable fluxes sho

15、uld beavailable. The alloys shouldnt be more toxic than whats currently used.To begin our alloy selection and evaluation, we found references in the available literature to low-temperature alloys thatmight fit these requirements. Three alloys were selected for further evaluation:43Sn43Pb14Bi. The so

16、lidus temperature of this alloy is 144°C and the liquidus temperature is 163°C, 20°C lower than63Sn37Pb, but with similar mechanical properties.58Bi42Sn. This composition is a eutectic alloy that melts at 139°C. It is lead-free and strong, but brittle. Also, itsfatigue resistance

17、 is questionable.1,240Sn40In20Pb. The solidus temperature of this alloy is 121°C and the liquidus temperature is 130°C. It is soft andductile. It doesnt have the problem of embrittlement when soldering to thick gold surfaces, like PbSn, becauseof the high In content. Unfortunately, the hig

18、h In content drives the price of this alloy up because In is extremelyexpensive right now.Article 10August 1996 Hewlett-Packard Journal 2ConclusionThe application of low-temperature solders in surface mount assembly processes for products that do not experience harshtemperature environments is techn

19、ically feasible. Low-temperature assembly appears promising as an addition to the surfacemount landscape as a way of increasing process flexibility and component reliability. However, one single alloy wont be auniversal solution. Specific component and assembly requirements will have to be considere

20、d in choosing or tailoring thebest solder alloy for each application.AcknowledgmentsThe authors would like to thank Jerry Gleason for providing direction and guidance for this project in its early, criticalstages. We would also like to thank Judy Glazer, Fay Hua, Jim Baker, Charlie Martin, and Meng

21、Chow for their help andsupport.References1. J. Seyyedi, “Thermal fatigue of low-temperature solder alloys in insertion mount assembly,” Journal of Electronic Packaging,Vol. 115, 1993, pp. 305-311.2. J. Seyyedi, “Thermal fatigue behavior of low melting point solder joints,” Journal of Electronic Pack

22、aging, Vol. 115, 1993, pp.305-311 (sidebar).3. Z. Mei, H. Vander Plas, J. Gleason, and J. Baker, Proceedings of the Electronic Materials and Processing Symposium, 1994, LosAngeles, California, pp. 485-495.4. H.A. Vander Plas, R.B. Cinque, Z. Mei, and J. Baker, “The Assessment of Low-Temperature Flux

23、es,” HP EAMC Conference Pro-ceedings, 1995.5. H. Vander Plas, J. Gleason, Z. Mei, and G. Carter, Results of building BLD Ponderosa formatter boards with 58Bi-42Sn solderpaste, HP internal report, August 1994.6. Z. Mei, A failure mechanism of 58Bi-42Sn solder joints, HP internal report, September 199

24、4.Bibliography1. G. Humpston and D.M. Jacobson, Principles of Soldering and Brazing, ASM International, 1993, p. 63.2. Choongun Kim and J.W. Morris, Jr., University of California at Berkeley, unpublished work.3. R. Strauss and S. Smernos, “Low Temperature Soldering,” Circuit World, Vol. 10, no. 3, Spring 1984, pp. 23-25.4. A. Prince, “A Note on the Bi-In-Pb Ternary Phase Diagram,” Materials Research Bulletin, Vol. 11