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Immersion Silver PCB Finish
Summary
The standard PCB Train finish is Immersion Silver. Consequently PCB Train pcbs satisfactory comply with legislation which restricts the use of lead in electronic assemblies. This finish is an improvement on HASL and will also satisfactorily replace Immersion Gold over Electroless Nickel (Au/Ni) for most applications. The use of Immersion Silver will have no effect on customer's assembly processes or on PCB Train prices.
Legislation
The legislative requirements of EU Directive 2002/95/EC Restriction of hazardous substances (RoHS) requires compliance by 1st July 2006. It contains the text:
"a maximum concentration of 0.1% by weight in homogenous materials for lead, mercury, hexavalent chromium, polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE) and of 0.01% by weight in homogenous materials for cadmium, shall be tolerated."
Therefore from 1st July 2006, our standard Sn/Pb HASL process will be none compliant with the directive and we must replace it with a lead free finish.
HASL
Lead free HASL is possible but it is performed at higher temperatures. We shall be modifying of our HASL machine to meet the required increase in operating temperature from 240 deg C to 270 deg C in due course. In the mean time we have installed an Immersion Silver finishing process which offers significant improvements over HASL as well as being intrinsically lead free.
Immersion Silver Metallic Solderability Preservative
We have selected immersion silver as our standard finish because it meets legislative requirements, it is already very widely accepted as a good substitute for HASL, and it offers excellent SMD pad flatness. The process is labour efficient being run on conveyorised machinery with automatic loaders and unloaders. It is approved, amongst many others, by the following major companies: 3M, AVO International, Boeing, Bosch, Cisco, Ericsson, Hewlett Packard, IBM, Intel, Motorola, Nokia, Philips, Texas instruments, Toshiba, Western Digital.
About Immersion Silver
- Immersion Silver Metallic Solderability Preservative is a 0.1 to 0.2 micron layer of metallic silver coated with an organic layer of solderability preservative.
- It is a "drop in" HASL replacement, requiring no change to assembly processes.
- It has excellent wettability for soldering.
- It is environmentally friendly.
- The solder joint strength is superior to that of AU/Ni pcb finishes.
- The resistance to environmental ageing is superior to that of organic solderability preservative (OSP).
- Flatness, electrical properties and solder joint strength are superior to HASL.
- For touchpad contacts, it is better than carbon ink and as good as Au/Ni finish.
- It has similar performance Au/Ni for ultrasonic aluminium wire bonding.
- It can withstand multiple reflow.
- The process is supplied and supported by Macdermid PLC
Assembly guidelines for Immersion Silver
Download pdf of a detailed assembly guide relating to the soldering of pcbs coated with Immersion Silver.
Handling and Storage of Immersion Silver finished pcbs
- Avoid finger contact with the Immersion Silver finish. It is recommended that gloves be used for handling panels/boards. Salts/acids from fingerprints may tarnish the finish, and impair solderability.
- The generally accepted shelf storage life for Immersion Silver is 6 to 12 months in dry conditions, the same as that for HASL.
- The bright appearance of Immersion Silver finished pcbs should not suffer from any yellow or black discolouration which will indicate contamination. If contamination is suspected, please contact us for advice.
- If pcbs have been in storage for some time, it is wise to perform a solderability test prior to assembly. For SMD applications this might consist of solder paste and reflow of a bare pcb. 100% solder wetting would indicate a satisfactory pass.
- When board cleaning or washing, surfactants and/or acid cleaners should not be used for bare boards with Immersion Silver finish. Only water washes should be used, with a pH > 6.0.
- Immersion Silver finished boards should be packaged to prevent exposure to air circulation. Storage could be in sealed bags or similar containers to exclude direct contact with air. Storage conditions should be < 30?C and < 50 % RH. Use sulphur free paper to interleaf parts.
- If Immersion Silver finished boards must be stress relieved for warp & twist problems, the boards should be tightly wrapped in aluminum foil to prevent oxidation of the silver which will affect solderability. The bake conditions are: Temperature 150deg C for 2 hours. If it becomes necessary to perform a bake to remove moisture, the time and temperature should be reduced to a minimum. Do not exceed 2 hours at 122deg C.
Known compatible solder pastes and fluxes
Download pdf list of solder pastes and fluxes commonly used with Immersion Silver when used in accordance with the manufacturers instructions.
If you have any concerns about the use of Immersion Silver finish, please contact us at PCB Train.
Source: www.macdermid.co.uk
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