If you have ever carried out any soldering, or been involved in fault-finding, you will appreciate that problems associated with soldering may derive from the quality of the surfaces to be soldered, from the soldering technique and, in the case of hand soldering, from the ability and training of the operator!
For the assembly industry as a whole, however, many of the common problems associated with soldering can be traced back to poor solderability. The trend towards materials with mild fluxing activity has accentuated the problem.
Imagine that you are an assembly house experiencing problems in assembling a circuit. You get poor yield, or a lot of rework, or find that you have to adjust the process to get good results. You suspect that the problem is to do with solderability, either of the board or the components. How do you set about finding where the problem lies, with the supplies you buy in, with your materials, or with your process?
Before we look at how solderability might be assessed, we are going to have a quick review of the requirements for soldering.
Even with the most solderable metals, some cleaning of the surfaces to be soldered is needed. This is because most metals show a strong tendency to form compounds with oxygen from the atmosphere and are rapidly covered with an oxide film when exposed to the air, even at room temperature. On tin and copper, an oxide layer about 2nm thick is formed almost instantaneously. These oxides are generally removed by fluxes, which are capable of reacting chemically and physically with the oxides, and of dissolving or dispersing the reaction products.
Fluxes are applied before or during soldering and cover both the base metal and the solder. This retards oxidation by oxygen from the atmosphere, which would otherwise occur even more strongly at soldering temperature. In areas where the oxide film has been removed, a direct metallic contact is established between solder and base metal, so that one single interface with low energy exists instead of the original two interfaces. From this point of contact the solder will spread.
Note that, if one works at too low a temperature, an oxide layer may still be present between the base metal and the solder. [Joints of this type are often called “cold joints”, although the term is sometimes criticised – in fact defective joints can be produced by diffusion processes caused by excessive exposure to temperature, although the interface may then be an intermetallic rather than an oxide layer.]
For all such defective joints, the initial resistivity will be higher than for a good joint, but not necessarily unacceptable. However, the joint resistance is likely to increase the life, and the joint may become electrically noisy or even intermittently open-circuit. Adhesion is also poor, and the solder can easily loosen when subjected to a mechanical load.
In order for a joint to be made, the solder needs to wet the conductive surfaces on the board and the component within the process time available. There are two measures of surface wettability which impact on the ability of component and board metallisation to make a satisfactory joint:
Related to these, and often tested at the same time, is the capacity of the metallisation to withstand exposure to molten solder without dewetting. Such tests are related to the ‘process window’ shown in Figure 1. This indicates diagrammatically the way in which a surface is first wetted by solder, and then at a later stage dewetted.
Note that different components show different characteristics; some wet quickly, others take a long time to wet. Similarly, some components are resistant to dewetting (take a long time to do this) whereas others dewet quickly.
Typically those components which wet quickly take a long time to dewet, whereas those that wet poorly dewet first. The process window is between the time when a component must wet (given the soldering conditions obtaining) and the first occasion on which components may dewet.
How would you expect the process window shown in Figure 1 to change if you were using a flux with insufficient activity?
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The idea of the process window helps us to explain our observations, particularly in terms of dewetting, but the assessment of solderability goes back to the basic ideas of “how far and how fast”. However, it is just these fundamental measures which are difficult to carry out in production, except one might take a copper coupon with a bead of solder, and watch it spread.
In assembly shops the wettability of individual parts is most likely to be estimated using production equipment and observation. Whilst this helps answer the question “Will the parts work?”, such tests are difficult to replicate and are not suitable as standardised assessments. Standardised tests need to have as few variable parameters as possible, and specify tight control of these.
Of the standardised tests available, the wetting balance test method has for long been regarded as the most versatile, and has been used for assessing the efficacy of fluxes as well as the solderability of components. Examination of the results gives information about both the speed and extent of wetting and the behaviour of the surface upon prolonged exposure.
The specimen is fluxed, then suspended from a sensitive balance and held over a solder bath which preheats it by convection. It is then immersed in the bath at a controlled rate and to a set depth. The specimen experiences vertical forces of buoyancy upwards and the surface tension downwards, which are detected by a transducer (Figure 2). The transducer output varies with time and is continuously recorded on a high-speed chart recorder or data acquisition computer: the forces change very rapidly and a fast response time is essential to avoid distorting the test results.
In Figure 3, the six stages of the testing of a readily wettable specimen are shown:
Some representative curves are also shown in Figure 4. In each case the full horizontal line represents the force at the start of the cycle and the dotted horizontal line the buoyancy level at which the wetting force is zero (a calculation based on the weight of solder displaced).
In the globule balance normally used for SM components, the solder bath is replaced by a small globule of molten solder (Figure 5).
There are several benefits:
Not everyone accepts that the wetting balance test is suited for determining the solderability of plating materials. The ceramic plate test (CPT), developed at Compaq in 1988, is claimed to evaluate devices under conditions more closely simulating actual surface mount processing. The method consists of printing solder paste onto a ceramic substrate, followed by package placement and reflow. The reflowed solder on the leads is then inspected visually to determine the wetting characteristics, using a rating system for solder appearance (0 = best; 10 = worst).
Although this test is not universally accepted, the most important point to make is that the test results obtained with the ceramic plate test are not mirrored by the wetting balance test results, tending to be less favourable.
Solderability testing should ideally reflect the solderability of the surface after typical life. Unfortunately, board fabricators need to have immediate assessments, and other test methods have been proposed to try and give a more real-time test. One of these is Selective Electrochemical Reduction Analysis (SERA), which can be applied to assess the chemical constitution of the through-holes as well as the surface features.
Two test approaches are required to get a complete picture of the finish. The first is a non-destructive reduction test which assesses the presence and quantity of surface oxides of tin. The second uses a destructive oxidation test where the electrolyte strips selectively through the layers of tin and the underlying intermetallics. Measuring the amount of free surface tin gives an indication of solderability. Whilst this test is under evaluation, and an interesting laboratory tool, the concern is that it may not be a good predictor of long-term solderability because the growth rates of intermetallic vary with the different types of tin finish.
The production manager has asked you for advice on how to determine whether or not a batch of boards is useable for reflow assembly. What are the options available, and how do these compare?
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Component solderability is extremely variable, depending on the type of material and the conditions in which it has been manufactured and stored. Damp and heat combined will do much to reduce the wettability of even the more robust surface. The solderability of components and boards can be assessed by a number of methods, the most reproducible of which is the wetting balance.
The deterioration with time of the solderability of a tin-lead coated surface is shown schematically in Figure 6.
During Stage I an oxide coating develops and solderability deteriorates: during Stage II, the wetting time stays constant, although the oxide continues to grow. The explanation proposed is that unsolderable oxide coatings are broken up by the flux action (rather than reduced to the metals) and then float away from the soldering site. The surface becomes unsolderable (Stage III) once the oxide has become thick enough to prevent this happening.
Figure 7 relates the thickness of solderable coatings to their deterioration with time, which demonstrates the benefits of a thick coating.
As you would expect, given the fact that steam ageing accelerates effects that are found naturally in life, this technique has been used to demonstrate the ability of coatings to withstand atmospheric conditions. One would hope for good correlation between the performance of samples aged in steam and those that have been ‘shelf-aged’. However, always take care when placing too much confidence in any technique:
The wetting balance test deliberately uses a minimum amount of flux, in fact the mildest flux available. This is not necessarily representative of the performance of real products, where a more active flux may be available. As a designer, you need to know something of the materials used by your assembler, because there may be incompatibilities between certain combinations of fluxes and coatings. For example, Electrochemicals report that not all no-clean fluxes may be compatible with OSP coatings.
How might you assess whether or not a component with tin-lead plated leads might be suitable for your Defence application, where a batch of components is to be purchased now for gradual use over a three year period for the assembly and rework of small quantities of assemblies.
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