'Lead-free' is finally upon us. Microtronix is starting to get enquiries on a daily basis as to our current lead-free capabilities so the time has clearly come for us to stop talking about it and actually get down to the nuts and bolts of producing a completely lead-free PCB.
Like most people in the industry we have sent our technical engineers, our process engineers and our production managers to all 99 different seminars that have been held in and around the country by the various equipment and consumer suppliers, where on average we have learnt mostly about the pitfalls and complications that can be expected during the process. From the outset I can verify that the process of lead-free will probably do far more harm to our planet due to the large number of trees we will have to cut down to produce paper to manage the process and traceability. I can also confirm that the manpower and resources needed to manage the process will place a huge burden on human resources in smaller companies.
The first challenge we faced was to confirm that all the parts selected for the initial production run were, in fact, lead-free. The bill of material that we started with had a component count of around 35 items. Our end customer is based in Japan and therefore insisted on each and every item passing a burn test. This is a test where each component is incinerated and then tested for banned substances. All the datasheets from each component had to be looked at and the verification process completed. In quite a few cases, even though the suppliers assured us of the lead-free status of the components and all our orders for the parts stated lead-free, the actual parts arrived: 'non' lead-free. These then had to be returned and re-ordered. It became apparent that when specifying the components to be used a great deal of detail has to be taken to include every last dash and letter, if not, it can lead to confusion. In total it took almost two weeks to collect all the datasheets and verify between the suppliers and the customer and ourselves, that all the parts to be used were, in fact lead-free. Ten complete sets of parts were then sent off to a lab in the UK to do the burn test. In a few cases, especially for the resistors and capacitors of size 0805, we had to resend a few hundred of each part as it was impossible to verify the results due to the small sample quantity.
The actual process proved to be fairly easy once we had established all the correct parameters - talk about a narrow process window! The screening of the PCBs was pretty standard. The stencil was a laser cut stencil supplied by Laser Stencil Technologies with approximately 5% reduction all round. The placement was pretty standard but special care was taken to mount a QFP 144 very accurately to ensure good joints all round. Now the fun started; to get a good profile on the oven for the consistent soldering of all the parts.
The oven used is a 6 metre Soltec Vitronics. Anyone who has done oven profiling knows how time consuming an accurate profile can take and this was no exception. The problem with the lead-free process is that although the joints looked good on our first few attempts the PCBs did not all function correctly the first time. Once we increased the profile temperature the joints looked no different but the pass rate increased. When the profile became too hot the fall out increased again due to actual component failure.
The profiler was placed on four different parts of the PCB; the critical part being an Altera QFP 144. Our first profile showed an average temperature across the PCB of 232°C, the joints looked good but we were experiencing a 30% failure rate due to unsoldered joints. These were easily fixed by resoldering the QPF (using lead-free solder of course). With an average temperature of 242°C we had a 5-10% fall out where we suspected that the QFP had been damaged due to excessive heat exposure. Once the QFPs were replaced the PCBs worked fine. The ideal profile was one which reached a maximum of 237°C for around 75 seconds, which has given us a good yield of around 98% first time pass rate.
Hand assembly presented its own set of problems, the main one being the ability to heat the joints up to the required temperature. Our entire factory uses Pace irons and we had to use the extended temperature module to get the tip temperature up to allow the connector joints to flow neatly. Each iron had to get a new lead-free tip so as not to contaminate any of the joints at the start. We have set up an entire area marked 'Lead-free' with a green sticker on the irons, the solder wire and the flux pencils, this has worked well with the staff being aware of the importance of keeping the lead-free tools apart from the rest of the factory.
The importance of a good oven is beyond doubt and the process window is very tight but not impossible. In summary I feel that the paperwork is by far the biggest hurdle to overcome initially and must not be underestimated - that is, providing you intend supplying a lead-free product not a lead-reduced product.
We have just completed our third batch of 750 PCBs which is ready for export to Japan. The first 1500 are installed and commissioned and have been accepted by our customer. We are currently working on a further three customer products due for production in August.
I would also suggest tackling a more simple PCB to start with and work up from there. Anyone with any queries is most welcome to contact the author for any information that can practically help one with problems or queries regarding the 'leap to lead-free'.
For more information contact Michael Goodyer, Microtronix, +27 (0)11 792 5322.