Using the Wetting Balance with Nitrogen Inerting as a process development tool for Lead (Pb) Free Assembly.
In this the third and final article in this series we explore the proposed change from Lead containing alloys to the Lead free alloys – SnCu for wave soldering and SnAgCu for reflow- and how they will impact solderability testing, assembly issues and the requirement to return to Nitrogen Inerting to widen the process window.
The ban in Europe against Lead will affect the global marketplace, forcing all manufacturers to eliminate lead as effectively as a local ban. Manufacturers in all regions, including Asia, will need to prepare for this change. Even producers of equipment not directly affected by the legislation, need to assess the effects of changes in the supply chain and customer demand.
For the companies that have already accepted this fact and embraced the challenge that goes with it, i.e. alternate plating evaluations/development, reliability testing etc, one item keeps on cropping up necessary to successfully implement the change – the use of Nitrogen as an inert soldering atmosphere. When the industry changed to no clean fluxes and alternate finishes such as OSP coatings, it became the de facto requirement to use Nitrogen, This was the norm for a number of years while the solder / flux suppliers developed products that would produce the same results in a normal atmosphere. It is hoped that the same will be true for the lead free assembly world, however in this case the increase in peak reflow temperature may seal the fate of always having to run with Nitrogen.
A number of key areas are immediately impacted by the proposed lead free change. The first is solderability testing. The current JStd-003A for PWB’s and JSTD-002B for components, references test temperatures of 235°C and 245°C respectively.
Fig one shows the response of an Immersion Silver PWB tested at 235°C for both 99.3Sn 0.7Cu and 95.5Sn 4Ag 0.5Cu alloys tested in normal atmosphere.
Fig 1: testing of Immersion Ag PWB’s @ 235°C using a no clean flux. Note that the curve demonstrates negative wetting forces.
Based on the proposed peak temperature increase from NEMI et al, the test was repeated with 10°C increment to the maximum 265°C. The effect of wetting time and forces based on temperature are outlined in figs 2 and figs 3 below.
Fig 2: effect of test temperature on wetting forces & time for Immersion Ag coupon tested with SnAgCu alloy.
Fig 3: effect of temperature on wetting forces& time for SnCu alloy
As can be seen from both sets of wetting curves, the effect of temperature is dramatic with respect to both wetting times and forces recorded. It should also be restated again that if tested to the current test temperature of 235°C for JStd-003A and 245°C for JStd-002B then these parts would be rejected due to low wetting forces produced and excessive times to cross the zero line. The parts were obviously solderable as evident by the very acceptable wetting times and forces obtained by increasing the test temperature.
By using the inert gas attachment on the wetting balance we can demonstrate the impact of Nitrogen inerting. The tests were re-run using nitrogen and the results are graphed below.
Fig 4: the effect of Nitrogen Atmosphere on Immersion Ag tested with SnCu Alloy
Fig 5: the effect of Nitrogen atmosphere on Immersion Ag tested with SnAgCu alloy
From the two graphs it can be seen that there was little to no effect on either alloy tested at 235°C which would (a) imply that the thermal delta between the Liquidus of the alloy and the test temperature is too small and (b) that the JStd-003A will need to be revised to allow for testing with a Lead Free alloy, (currently being discussed). At 245°C the impact of the Nitrogen on the SnAgCu alloy is quite dramatic, producing results that would be acceptable from both a wetting force and a wetting time perspective, quite different from the same test run in air!
For demonstration purposes we chose two Lead free alloys and a particular brand of no-clean flux.
In the real world there are a large number of alloys and fluxes, the combinations of which, go beyond the scope of this article.
Re-introducing Nitrogen inerting to the production process is an expensive proposition.
By using the wetting balance as a development tool to narrow the choice of alloy, flux and Nitrogen combinations for your particular assembly operation all this work is done off line where the operator, using objective data, can replicate any number of desired conditions.
The decision to use or not use Nitrogen can be supported with the objective data provided by the wetting balance using the inert gas attachment.
Ultimately the final decision will be made under operating conditions however the user can be confident that the top choices will meet most requirements.
Furthermore, for those involved in QC/QA, as more companies switch to Lead free assembly, requests for component solderability data using Nitrogen inerting will increase.
Finally, if possible, be proactive on Lead free assembly. Don’t wait until your product is blocked at Customs or returned as unserviceable inventory to start developing your Lead free process.