In Unit 5 we are focusing on tin-silver-copper and tin-copper alloys, the main solders that emerged during Unit 4 as the front runners for evaluation of lead-free solders for hand, wave and reflow soldering. This is deliberately a simplification, but the points we are making about the mechanical and physical properties of the alloys can easily be extended to a wider variety of materials. We shall be exploring the way in which these solders wet surfaces, at the microstructure of the resulting joints, and in aspects of joint geometry and surface finish that will have an affect on design, manufacturing and inspection tasks.
Inevitably, when you look for information on the joint, you will come across both photographs and comment about joints that have failed – bear this in mind for Unit 8, which will pull together a number of reliability aspects.
Eutectic tin-lead solder is highly fluid, especially when operated, as is usual, at least 20°C above its liquidus temperature, and wets both well and fast, even when using fluxes of relatively low activity. So how do lead-free equivalents compare? Do they have different wetting properties?
How do the wetting properties of typical lead-free alloys compare with eutectic tin-lead? And what impact will this have on the soldering processes?
To view the video a Real media player is required.
Search for wetting + "lead-free solder". Though this gives too many hits for exhaustive research, there are usually some gems on the early pages.
Make your search more specific by substituting "wetting characteristics", "wetting angle" or "wetting time"
If you haven't already done so, read the paper A comparison of tin-silver-copper lead-free solder alloys by Karl Seelig and David Suraski
|Look at slides we have selected from the Soltec resources, covering wetting times and contact angle.|
We have established that the wetting characteristics of lead-free solders are different from the lead-containing materials for which our processes and designs have previously been optimized, and this has implications for the materials we use, the process settings, and the shape of the joint.
But how will a reduction in wetting capability translate into assembly defects? Or to a reduction in reliability? Review the resources you discovered during your research on wetting and draw conclusions as to the kind of issues you would expect as a result of the differences in wetting.
Even some years ago, Harrison found that process defect levels for the best lead-free process were only slightly higher than for a tin-lead equivalent. Since then, similar experiences have been reported by others, and the conclusion seems to be that attention to detail in design, materials choice, process optimisation and process control will give a product at high yield, in the same way that it has in the past for lead-containing solders.
However, the optimal conditions will be different; in the same way that
lead-free solders do not wet as far as tin lead, neither do they
wet as fast, with the result that increased soldering times are needed. Be
aware that there is some difficulty in determining when wetting has actually
occurred, which would normally be taken as the zero-crossing point on the
output from a wetting balance. If you don’t know what this means, then
read the section on solderability at this
When tin-lead solder solidifies, because of the limits of solid solution of tin in lead and vice versa, we end up with a structure that contains both lead-rich and tin-rich areas, their grain size and structure depending on the cooling condition, and this has an impact on the strength and rigidity of the joint. We also know that there is a tendency for solid state diffusion to take place throughout life, with the result that the grain size ‘coarsens’, especially at elevated temperatures, so that the mechanical performance is affected. So does something similar happen with the high-tin alloys we are considering for use as lead-free alternatives?
What happens with binary alloys such as the tin-silver system is similar to eutectic tin-lead, except that the materials into which the molten solder differentiates are different. The major phase consists of essentially pure tin, with less than 0.04% silver in solution, which grows in large dendrites. The silver that cannot dissolve in the tin, instead of containing some tin itself as one might expect by analogy with the tin-lead situation, actually precipitates as an intermetallic compound Ag3Sn. The structure of the solid eutectic alloy therefore consists of rod-like crystals of the intermetallic phase in a nearly pure tin matrix. As with tin-lead, the size and shape of the intermetallic depends on the cooling rate.
Eutectic tin-copper solder has a similar microstructure that consists of large dendrites of tin with a fine dispersion of Cu6Sn5 intermetallics, which may form a network structure, depending on the cooling conditions.
The situation with tin-silver-copper systems is much more complex. This is because, while binary silver-copper systems are relatively well-behaved, a number of different crystal structures are possible in the copper-tin and silver-tin systems. As with tin-silver and tin-copper, the resultant solid alloy is mostly tin, with intermetallics, because the limits of solid solution of copper and silver (effectively impurities) are low.
The system contains both a ternary eutectic (three materials) and pairs of potential binary systems, so solidification actually takes place in stages over a small range of temperatures, which is easiest to see from this slide. The near-ternary eutectic tin-silver-copper solidifies into three phases, ß-tin (with a very small amount of silver and copper dissolved), Ag3Sn and Cu6Sn5. Ag3Sn nucleates first, as its platelets require only a minimal degree of undercooling to initiate solidification. However, because the ß-tin phase requires a significant amount of undercooling, this provides time for the Ag3Sn platelets to become quite large, and this can have adverse affects on the fatigue life of the joints. The cooling rate therefore has an effect on the structure, and rates above 1.5°C/second are reported as minimizing the effect. However, very rapid cooling can create a non-uniform microstructure, so should be avoided.
Given that most of these lead-free solders are almost pure tin, typical lead-free structures look more homogenous than tin-lead and have fewer visible grains. However, there will usually be areas containing intermetallics or similar concentrations of the impurity metal. To give you an idea of typical structures, we suggest these links.
As with all tin-containing solders, when the liquid solder is in contact with copper pads and leads, the surface of the copper dissolves to form copper-tin and copper-nickel intermetallics. The situation can get quite complex, as seen in this slide. These intermetallics play an important part in the final strength of the joint, and grow and coarsen with time.
The microstructure that you end up with depends on a number of factors which include the initial composition of the solder, the materials being joined, and the rate at which these dissolve in the solder, the time during which the solder is held in the molten state, the cooling rate, and the length of time since the solder joint was made.
If the section is being examined with an optical microscope, then the look of the section will also depend on the way that the sample has been prepared, particularly on any etching stages that followed polishing. When a scanning electron microscope is used, then the results will depend on the electron energy, the size of the beam and the kind of sensor in use. Fortunately, when using this method, it is often possible to conduct a degree of surface analysis, at least to the point where the likely composition of structural features can be established.
Detailed discussion of this aspect is beyond the scope of this module, but you should be aware that the results obtained depend very much on the process, and that interfaces are very important, particularly where they contain an elevated percentage of whatever materials formed the surface of board or component.
So far in our discussion we have concentrated on the solder as added to the joint, and considered only the dissolution of copper or nickel from the pads into the solder. However, pads and components will normally have protective surface finishes. What happens to these? Clearly a very thin gold flash will have no significant effect, whilst tin and silver finishes will dissolve more or less completely in the main mass of solder. But what happens if the surface contains lead?
To get a view of the impact on lead contamination, browse briefly on "lead-free solder" +"lead contamination". We’ll review this in Unit 8, as a potential source of failure.
And how about the look of the joint? Can we expect the same appearance? Will inspectors reject what are actually good joints? Will we need to ‘re-train’ the automated optical inspection equipment in which we invested so much?
Use the search terms below to draw conclusions as to the shape of lead-free joints and how they compare with eutectic tin-lead solder:
"fillet shape" "lead-free solder"
finish "lead-free solder" "joint surface"
Whether there is an issue about the different look of joints depends on the materials chosen, the process adopted, and whether the joint contains any lead. It also depends on your perspective – a well-known manufacturer of inspection equipment has made the comment “In comparison to conventional Sn37Pb solder, the joints and connections formed by the new lead-free techniques are visually quite different, in terms of the surface finish, reflectivity and fillet shape.”
Of course they are right in flagging that there may be a difference, and assemblers certainly need to be aware of the potential problem and to take appropriate action. There is also an undisputed point to be made that lead-free processes have a narrower process window, and are thus more susceptible to temperature variations during manufacture, so that visual inspection plays an important part in ensuring adequate reflow. IPC technical committees have been slightly slow to recognise this, but it was reported in the July 2004 issue of Circuits Assembly that lead-free assembly acceptance criteria are expected to be included in the revision of IPC-A-610 due for publication in late-2004.
The surface finish of a joint is comparatively unimportant when it comes to structural strength, but plays an important part in convincing an observer that the joint is in fact of high quality. Partly this is because tin-lead solder joints that have been overheated, or disturbed during the cooling process, have a granular appearance that reflects their internally inadequate integrity – as for example the so-called ‘cold’ joint.
The surface finish will also have an impact on the ability of certain kinds of AOI equipment to assess bond quality. Depending on the type of equipment, more or less re-setting and ‘re-training’ of the equipment may be needed. But of course one always gets the best results with AOI equipment when care is taken to optimise the lighting quality. An interesting paper on this aspect is Lead-Free Soldering and Advances in Automatic Inspection by Jim Fishburn of Omron.
As a lead-in to the discussion on mechanical properties, we would like you to watch the made by Bill Plumbridge of the Open University. Whilst this was made some time ago, the findings are still pretty accurate, and there is more to be found on the SMART Group website report on his contribution to the 25 March 2004 seminar.
|Interview with Bill Plumbridge (2001)|
|video||To view the video a Real media player is required.
Click here to download a free version of Real Player 10.
Metallurgists typically work on bars of material of substantial size, as this is the only way they can get consistent reliable results. Much work has indeed been done in this area with lead-free materials, comparing them with Sn37Pb alloy, as a result of which there are web resources that will give the mechanical properties of the bulk materials.
However, one has to be very careful when making a direct comparison between the database figures for different materials:
Real joints have their properties modified by the fact that they are relatively small compared with the parts they are joining; not until a joint is significantly thick (of the order of 0.25 mm) will its properties approach those of the bulk solder. There is more discussion on this issue at this link.
Because of the difference in melting points. You will recall (or be able to find a reminder at this link) the idea of homologous temperature, and the way in which the properties of a material like solder, which is operating near its melting point, change with temperature. Typically a material reduces in tensile strength and is more liable to permanent extension under stress as the temperature increases. For this reason, we should not be unduly surprised that, for a given temperature, SAC and similar lead-free solders appear to be comparatively rigid.
Obviously the amount of solder and the shape of the fillet have an effect on the mechanical properties of the joint. The paper A Mechanical Evaluation of Lead Free Solder Alloys by Mark McMeen and Jason Gjesvold of Soldering Technology International (2003) gives confirmation of this, by relating the wetting results to the tensile strength of the terminations. The results suggest that the surface coverage area, which is larger when the solder wicks further up the lead, is the basis for the mechanical strength of the soldered interconnect. A copy of the full paper can be obtained by email, but is also on the SMTA web site for SMART Group members who have registered for (free) membership of SMTA. Encouragingly, whilst the results show that good wetting equals a good joint (but didn’t we know that anyway?!), the differences in tensile strength are relatively slight, given that a margin of over-design is always built into joints.
Whether lead-containing or lead-free, typical solder joints are adequate for purpose, at least as far as through-hole components, chip components and leaded components are concerned. The only concerns relate to ball grid arrays, where the relative excursion of the two surfaces is large, and the joint has significant thickness.
Another reason for concern is if the final joint is not homogeneous, but has a number of voids. The reasons for void formation are many, relating principally to the solvents in the paste, to trapped volatiles in board or components, and to the pre-heating conditions. In this respect lead-free materials are not too dissimilar from lead-containing – voids of all sorts cause concern. However, as has been pointed out elsewhere, the impact on reliability of voids depends on their number and location, and a significant amount of voiding is allowed by current specifications. This is a topic we shall be returning to in Unit 8.