RoHS - Information
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Hand Soldering
lead Free Hand soldering of through hole component terminations is still very common for small volume printed board assembly. It is also necessary after component removal to re-solder new replacement components. Even in large companies hand soldering is common in second stage assembly where through hole parts cannot go through reflow or wave soldering processes. With the move to lead-free soldering processes care needs to be taken when using new alloys and disciplines in place on care of iron tips. Also staff should be aware of difference in the solder flow, joint appearance and temperature requirements. |
First the component terminations are inserted into the through hole; they may or may not have their leads clinched slightly depending on the quality standard being used. It is often the case that large or multi leaded parts have a couple of corner leads clinched at approximately 45 deg to hold the part in place during board inversion and soldering. This guarantees the part to be correctly seated on the surface of the board. For lead-free the components will look the same but the plating on the leads may be different, traditionally most components are coated with 60/40 tin/lead or 90/10 tin lead plating.
Next the correct solder tip temperature is confirmed; this is normally 600- 800degF (315-426degC) for work on printed circuit boards. The higher the temperature the faster the oxide formation on the tip and the dewetting of the tip face; wherever possible the lowest temperature should be used. The tip face size should be a similar size to the terminations being soldered. The cored solder wire used for soldering is available in different gauges commonly available between 0.6 - 1mm. Cored wire is available in tin/silver/copper and tin/copper, which are the most common lead-free alternatives. First the tip is wiped clean on a moist sponge and the tip tinned with cored solder wire. Care should be taken not to use a wet sponge, this tends to spit, increase tip and iron corrosion, drop the tip temperature and is very bad practice.
When starting soldering it is good practice to re-clean the tip again and re-tin just to confirm that the tip is clean and is correctly wetted with solder. The tinning stage is very important if the tip cannot be wet efficient soldering will be more difficult. Also make sure that the solder alloy wire you are using is the correct type by checking the label. If you have changed to lead-free, tin/lead wire should not be on the shop floor.
Lead Free Component Requirements
With the impending move to lead-free production there are really two main component issues that engineers must address as quickly as possible.These are the temperature requirements of processing lead-free solder alloys and the termination finishes used as alternatives to tin/lead. New design are being conceived every week and design engineers must consider component temperature requirements for all future design, now. They need to ask suppliers about process limitations at the same time as questioning the components electrical parameters. If not they need to pass on the specific requirements in a specification to purchasing staff as often the purchasing department are the main interface with component producers or distributors.
The most commonly adopted alternative solder alloy is Tin/Silver/Copper, Sn/Au/Cu which has a higher liquidus temperature of around 217 deg C as opposed to tin-lead at 183degC. This will probably equate to a reflow temperature on most convection processes of between 240-245 degC and 230 or 240 degC with vapour phase reflow. There are lower temperature alternatives available like tin/silver/bismuth, tin/zinc and tin/bismuth, some of which were initially adopted in Japan due to component peak temperature limitations. Tin/silver/copper will be the choice for the vast majority of users, with moves to this alloy in Japan when component compatibility issues are overcome.
There are temperature guidelines available in the industry for components from IPC, IEC and JEDEC, but very few people consider these on existing or future designs. Most suppliers have well documented component specifications which should be reviewed. Many companies use alternative suppliers for parts, some of which do not meet current temperature limits and are unlikely to meet future lead-free requirements. The move to lead free will hopefully focus attention on what is the component’s maximum rated temperature and hopefully, where alternatives are used both design
and purchasing staff will ask the right questions of the supplier. Generally this question is not asked and design and development staff very rarely specify it to purchasing departments.
Don’t just think electronic components are an issue there are also the interconnections and mechanical parts that need to be considered. Wire insulation on cables and IDC’s that need to be soldered in position what are the temperature specifications of connectors and connector housings on the topside of the board. In some cases the plastic used to form the parts does meet specification but still the pin and sockets can move in the plastic leading to misalignment.
Lead Free Information PDF's
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PIHR Production lines
(236k) |
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PIHR Checklist (45k) |
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PCB Solder Finishes (304k) A paper about the types of solder finishes currently available |
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Simple Solderabiltiy Lead-free (164k) Testing component solderability |
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All lead-free information has been provided by courtesy of Bob Willis, our partner in Lead-Free manufacturing. Bob can be contacted at www.leadfreesoldering.com
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