X-ray fluorescence (XRF) systems can be used in production for RoHS compliance. Handheld systems can be used as a first level assessment tool with laboratory-based systems as second level verification of results, with a final wet analysis technique for arbitration.
Controls are specifically necessary in factories that are running a combination of lead-free and tin/lead, which is quite common particularly in contract assembly. Using XRF as a process control tool is an option that some companies have found beneficial after wading through all the misunderstanding and inappropriate use of the equipment in the early days. Random testing is also a way of showing customers and inspectors that engineers have thoroughly thought through all aspects of demonstrating compliance.
To review a simple online video guide to RoHS Compliance go to www.ASKbobwillis.com/bobwillisRoHS.wmv
For practical RoHS advice we will be running a 'Process Advice and Lead-Free Soldering Defect Clinic' at Productronica, Germany sponsored by Balver Zinn, DAGE Industries, KIC and Thermo Scientific, providing a testing service as well as answering questions on general process issues. A special colour 'Lead Free Process Support Guide' is being produced for the exhibition and will be available free of charge to visitors to the show.
Let us now look at some of the potential areas where XRF testing could be used.
Printed circuit boards
It is fairly common for printed boards to have an alternative surface finish for lead-free manufacture. With experience, production staff can visually identify gold, tin, silver and copper OSP finishes. However, with the advancements in solder levelling over the last few years it is very difficult for lead-free and tin/lead solder levelled boards to be identified by a simple visual check. It is, however, possible to detect lead on the surface of pads through the surface or the transparent wrapping, allowing audits at goods receipt and on the line prior to production. Normally all batches of boards are marked with some form of lead-free marking on the packaging; at board level this may or may not happen.
Components
Components can be tested as a bulk sample in a small container or cup which will give a percentage reading of the bulk material content checking for all materials covered by RoHS. This is not very common in production and not very practical, as the majority of parts are supplied and used in different forms of packaging to allow automatic assembly. Individual testing of terminations is often what is required and in that case the size of the area measured is the crucial factor. The XRF system used must have a small spot size and the operator must be able to pinpoint the test location during testing. They will also have to be aware of the impact of X-ray penetration depth and the impact on the materials seen. Experience has shown that testing through packaging where the surfaces cannot be easily targeted will be difficult, leading to mistrust of the readings obtained.
Solder paste
Solder paste is well documented and traceable when supplied by the manufacturer on to the production line. However, that control can be lost when it is placed on to the stencil. After a batch of boards is printed the same paste, if still printable, is either transferred to another stencil or placed in a used container fo re-use. That material will be used and then topped up for printing. Placing cling film over the paste bead - which is normally 15 to 20 mm thick - or XRF instrument head allows a measurement to be made of the alloy to confirm if the material is tin/lead or lead-free.
Solder dross
Solder dross is taken from the surface of the wave and placed in dross recovery containers to be disposed of. It is good practice to have separately identified containers for different production lines, both tin/lead and lead-free. The dross taken from a lead-free line potentially has more value than that from a tin/lead line. The dross should be weighed for each container and held for disposal with a recognised company. Companies wishing to purchase the dross will expect this and would not accept mixed containers.
When changing technology it is important to understand the cost associated with running lead-free, especially with multiple production lines. Keep a check on the amount of dross produced per line, operating parameters and volume of boards running through a machine to determine cost per board. It should also indicate differences in the operation of each line.
Solder recovery
Some companies use solder recovery units to compress dross when in a liquid state, if production staff have poor skills or have not been trained correctly in de-drossing a wave, there is a lot of expensive solder in the oxides which can easily be re-used. A dross recovery system compresses the solder oxides and produces ingots of solder with the same level of purity currently in the solder bath. There are no markings or identification on the solder recovered, measurement of the alloy is a simple way of making sure the correct alloy is being recovered. You cannot use a dross recovery unit for tin/lead and lead-free alloys.
Solder waste and spillage
It is not uncommon to have solder spillages, floods or solder pick up on solder pallets or guides during wave soldering; often it is dumped into the dross bucket. If it is not it may be taken away from the wave solder line by maintenance staff and control is lost. With no markings it could find its way back to a line resulting in contamination of the pot. Just like normal solder analysis, XRF could be used to identify the material.
Solder bath analysis
It is good practice to take samples from the wave soldering system and send them for analysis on a regular basis, either monthly or quarterly depending on different factors. These include size of bath, volume of boards, board finish, number of through-hole components and confidence in the component finish. Analysis of a sample is usually relatively inexpensive and is very accurate and repeatable. If a solder bath was out of specification any products produced during the period may become an issue, not from a reliability point of view but from product saleability. If a RoHS product is sold and it is later demonstrated to have lead in the joints, the manufacturer may be liable.
A process control procedure could take more frequent samples daily or weekly and hold them as a reference. They are only tested analytically when and if necessary, if the regular sample fails the criteria set. This provides a simple way of pinpointing and potentially identifying more precisely stock needing quarantine for test to the day or week of manufacture rather than the month. Using XRF is a simple and quick test for bulk samples of solder; with correct calibration the accuracy obtained can be very good.
Printed board assemblies
Testing board assemblies in production is an obvious consideration; to test the solder joints or component termination requires a small targetable area to be measured. If that is possible with the equipment available then which steps in the production process should be randomly monitored?
There are some potential weak links in the production chain, providing many opportunities for mixing materials in a dual alloy manufacturing operation:
* Boards for second side assembly on a non-continuous line.
* Boards taken off line for inspection and analysis.
* Reflow boards moving to wave or selective soldering.
* Boards split into two batches due to shortages on the line.
* Boards going from production to rework area.
* Boards held for missing parts or held for component recovery.
* Components held in rework areas.
* Components from placement reject trays.
Bob Willis will be running his 'Process Advice and Soldering Defect Clinic' and seminars at Productronica Exhibition, Germany in November. Bob is a process engineer providing engineering support in conventional and surface mount assembly processes. He organises special production features at exhibitions and also provides seminars and workshops worldwide. These complement his on-site company workshops.
For further information on how Bob may be able to support your staff with process problems on-line go to www.ASKbobwillis.com
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