Article 1. Introduction to the Wetting Balance

The intent of this article is to demonstrate the superiority of the Wetting Balance over the traditional methods of solderability testing for Quality control and Process development.

Background: All of you are familiar with the traditional methods of solderability testing such as edge dip, solder float, wave solder testing, solder spread tests etc and for most of you these tests appear to be adequate for you requirements. After all the parts appear to solder and if not, that’s what rework and return policies are for, despite the additional costs involved. It is this author’s belief that the current methodology of testing has not kept pace with Technology, particularly the implementation of Lead-free assembly. It has been the authors experience over 20 years in this industry to witness a myriad of failures of product that “passed” solderability based on these old standards and techniques.

Reasons for non-compliance are many: the skill, training and interest of the Technician performing the test, tests with immersion rates to fast or too slow, dwell times starting before the specimen is in the solder, inserting the samples to deep into the pot, the list goes on and on. When a test method is created one important aspect that needs to be demonstrated is its Gauge Repeatability and Reproducibility (GR&R). If the outcome of a test is influenced positively or negatively by the interaction of the tester then the test method is no good. If the outcome of the test is likewise not repeatable then the test method is no good. When you run a GR&R on a Test, or piece of test equipment you are looking to have excellent repeatability and reproducibility. For solderability testing using any method except the wetting balance the GR&R values are unacceptable. The physics of immersing the solder into the pot can be mechanized to reduce the operator influence – immersion speed/depth/dwell time – but the evaluation of the test is still being performed by a visual method

The wetting balance is the only solderability tester that has an acceptable GR&R value, takes into account the input parameters such as solder density, flux surface tension, physical dimensions of the part, is easy to use and most important produces an output based solely on the laws of Physics.

The wetting balance consists of a very accurate/sensitive sensor head to which the specimen holder is attached. The specimen is immersed at a controlled rate into the solder pot – dross automatically removed prior to testing – held for a pre-determined period of time in the solder pot and withdrawn at a controlled rate. Options for preheating and testing in an inert atmosphere are also an option on a modern wetting balance. Fig one shows a block diagram of a typical wetting balance set up.

Fig One: BLOCK DIAGRAM OF THE WETTING BALANCE:

As components age over time as a function of their storage conditions, oxide layers on the surface plating tend to increase in thickness as well as Intermetallic compounds migrating through the bulk of the deposit. Both these normally occurring conditions are responsible for the degradation of the solderability performance of the component / pwb etc. Just how much oxide is there on the surface or how much IMC is in the bulk or worse case on the surface is something that needs to be known in order to make a decision on how “old” parts may impact the assembly lines performance. As an example let us look at the results of solderability testing of a PWB with Electroless Nick Immersion Gold (ENIG) as the surface finish that has been in storage for fourteen months. This surface finish is know to have excellent shelf life but what effect if any has the storage of the parts had on its performance and will it effect the assembly reliability?

If a straight forward dip and look test is run the following is the outcome:

1)       original test – dip samples in flux – blot excess – dip in solder pot at 235°C for ten seconds – remove flux residue – look at sample – repeat five times -result is a PASS

2)       Fourteen months later - dip samples in flux – blot excess – dip in solder pot at 235°C for ten seconds – remove flux residue – look at sample – repeat five times - result is a PASS

Test indicates that everything should be okay.

With a wetting balance test the results are as follows:

1)       original test – dip sample in flux – blot excess – place on wetting balance – run test – repeat five times-out put is below

results indicate that:

1)                          all five part are very consistent in their wetting times, in fact it would be considered to be almost instantaneous wetting.

2)                          The consistent slow rise of the graph shows the gold dissolution by the solder in the bath

3)                          The “flat line of the maximum wetting shows nothing wrong with the basis Nickel solderability.

2)       Fourteen months later :dip sample in flux – blot excess – place on wetting balance – run test – repeat five times-out put is below

Results indicate that :

1)       the time to produce positive wetting (speed of wetting) has increased but only slightly -0.375 seconds

2)       four out of the five samples are very consistent regarding time to reach maximum wetting force

3)       four of the five samples have seen a an approximate drop in wetting force of 20% compared to the original testing – indicative of the passivation of the nickel layer and very acceptable given 14 months in storage. However one sample has seen a 30% reduction in wetting force – might warrant another solderability test.

visually all samples look the same however all five samples are not the same

Another example of the power of the wetting balance is to detect issues with the basis metal of the component. In most cases the surface finish plating is applied to protect the solderability of the underlying basis metal- it is to this that the solder joint is ultimately made. It has been the authors experience that problems, unless catastrophic – are missed with traditional dip and look testing in detecting problems with the basis metal. Problems with the basis metal may result in incorrect IMC formation, which will ultimately reduce long-term reliability.

The results from the testing of the above 0402 resistors by dip and look showed acceptable results and would have deemed the lot acceptable. However by wetting balance testing the results show:

1)       Instantaneous wetting,

2)       Very rapid rise to maximum wetting force

3)       Dewetting of the sample – a reduction of approx 22% of the maximum wetting force produced.

The data from the above test led to the discovery of a poor rinsing practice post nickel plating prior to solder plating which was leaving plating chemistry in contact with the nickel surface and effecting the wettability of the deposit.

The table below outlines very simply some of the benefits of wetting balance compared to dip and look type testing.

Summary:  Apart from QA/QC type applications the Wetting Balance is one of the most cost effective tools for process development. By using it for screening tests, the choice of flux, alloy, surface finish, suppliers of all the above can be reduced to a manageable and more importantly to a cost justifiable size. For liquid fluxes the trend is to VOC free – there are many on the market – which is the right one for a given set of components, plating finishes and alloy choice? Do the initial testing on a wetting balance and the time and cost to selecting the finalists for line evaluation is greatly reduced.

Similarly for Lead Free – how do you choose the alloy and lead free final finish? How are my choices affected by the use of Nitrogen?  By using the Wetting Balance these questions can be answered quickly, scientifically and from a piece of test equipment with a GR&R below 10% .

In our second article we will demonstrate in detail the use of the Wetting Balance to screen the selection of VOC free fluxes.

With the industry moving inexorably forward toward Lead-free assembly our third article should be of great interest to those who see the writing on the wall and are either investigating or preparing to switch to Lead- free assembly. We will show in detail how to use the Wetting Balance, fitted with a Nitrogen Atmosphere Unit, to select alloys and fluxes and develop processes. The Industry leaders are well on their way. If you are to stay competitive you cannot afford to ignore this directive