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Harwin and the Elimination of Lead

Lead has been used as an additive in soldering processes for over 50 years, to lower the melting temperature of Tin, to improve solderability and solder joint integrity. Prior to twenty years ago, the standard plating finish for solder terminations was 60/40 tin/lead, to match the typical mix found in solders. Since then the ratio had been changed to 90/10, reducing the amount of lead in the terminations.

However, studies within the last 25 years have shown an increasing amount of Lead contaminating our water sources from landfill sites. A report from the Environmental Protection Agency shows the hazardous nature of lead as a pollutant. The following reports show some studies in the contributing factors of electronic waste to overall percentage of lead in waste:

• EPA report on 30 years of Lead and Cadmium in Municipal waste - www.p2pays.org/ref/03/02062.pdf
• Electronic Industries Alliance report on the elimination of Lead - www.eiae.org/chemicals/files/PosPaperlead%2701.pdf
• EPA summary report on computers and electronics in waste - http://www.epa.gov/reg3wcmd/pdf/pcrecycling601.pdf

Therefore, the reduction of lead from artificial sources has been targeted, initially through the elimination of lead in Electronics (particularly Consumer Electronics). Although the total impact of waste electronics as a significant contributing factor in lead pollution is still under debate, technological advances have made the elimination of lead in electronics a viable proposition.

From the announcement of the impending legislations, Harwin had been further reducing its lead consumption. Given the wide diversity of alternative solders that the industry had been considering, 100% pure Tin was chosen as the best plating finish alternative to 90/10 Tin/Lead, to match all conditions. The alternative Gold finish has already been available on many products for a number of years.

Some Harwin products have now been lead-free for over 10 years. Many others were converted to a lead-free format in time for the RoHS legislation. For an overview of the current finish options, please view our RoHS compliance diagram.

Lead Exemptions

As the legislation stands, some industry sectors (such as medical or military, network servers and other specific applications) are still exempt from the current version of the Directive. This is partly due to the original emphasis of the legislation being aimed at consumer electronics, and also due to the safety-critical and mission-critical status of medical and military equipment. However, these other market sectors are due for consideration at future reviews, and may be included at some point, pending further research. Harwin are continuing to offer leaded connectors for these exempt sectors wherever possible.

These sectors may use the term "RoHS-5", which is a short-hand term for products that meet all the requirements of RoHS, except for lead in the solder finishes. All our non-compliant products effectively fall into this category, and RoHS-5 statements can be supplied upon request (contact leadfree@harwin.co.uk).

The other major Lead exemption that is used by certain Harwin product is listed in the Annex of the RoHS legislation under paragraph 6, namely:

• Lead as an alloying element in steel containing up to 0.35% lead by weight, aluminium containing up to 0.4% lead by weight and as a copper alloy containing up to 4% lead by weight.

This exemption covers all free-machining Brass alloys used in Harwin products, which typically contain 2.5-3.5% lead by weight.

Impact of Soldering Temperatures

As mentioned above, one of the primary reasons for the use of lead in solder was to reduce the temperature required to satisfactorily melt the solder mix. Although alternative chemicals have been added to the lead-free solders, none are as effective as the Lead. As a consequence, soldering temperatures have risen by typically 10° to 20°C, and potentially as high as 270°C. This now has an impact on the plastic elements of Harwin connectors.

One of the favoured materials for the plastic moulding in connectors had been PBT (Polybutylene Terephthalate), which gives good mechanical strength and ease of moulding. However, it typically has softening temperatures of 210° to 230°C – which is not suitable in a lead-free soldering process. Generally, maximum soldering temperatures for non-RoHS products will be recommended at 235°C for 5 seconds.

For all our solder-terminating products, Harwin have committed to offering a RoHS compliant alternative with a plastic designed to withstand the lead-free soldering process. This has involved the moulding material being changed to a High-temperature mould material, with softening temperatures higher than those of PBT. Typically, the following mould materials are regarded as high-temperature, and will withstand a lead-free soldering process:

• Nylon (Polyamide) 46
• Nylon (Polyamide) 6T
• Nylon (Polyamide) 9T
• LCP (Liquid Crystal Polymer)
• PPS (Polyphenylene Sulphide)

Products carrying a RoHS compliant Finish code (the last two numbers of the part number are 42, 45 or 46) will use a high-temperature mould material. These products will carry a recommended maximum soldering temperature of 260°C for 10 seconds. If you are unsure about other products, please contact leadfree@harwin.co.uk for confirmation.

Tin Whiskers

The exact cause of tin whiskers is still unclear (see the Useful Links area for further information on this phenomenon). Research is still being carried out on this subject. However, there are certain measures that can be implemented to help prevent whisker growth:

• Plating specification – 100% tin should be used over nickel (standard plating for Harwin tin-plated connectors). Latest research has shown that having an undercoat (usually nickel) below the tin plating has a very significant impact on the likely growth of tin whiskers. In numerous tests it has either eliminated or made insignificant any previous differences shown in matte and bright tin, studies of which were done without this undercoat plating. This is due to the elimination of compressive stress around the formation of tin-copper intermetallics (IMC).
• External stress reduction – can be minimised by consideration of the component design.
• Storage – Parts should be stored in dry, regulated temperatures.
• Solder processing – Solder processing should avoid sudden and rapid changes of temperature where possible. This reduces the risk of compressive stress forming within the metal, due to a mismatch of the Coefficient of Thermal Expansion (CTE) of the base alloy and the plating materials.

These reports carry further information on the latest researches carried out within the industry:

• Tin Whisker Growth – Substrate Effect. Understanding CTE Mismatch and IMC Formation.
• Pure Tin – The Finish of Choice for Connectors.

Discolouration and Dewetting

Discolouration is caused by oxide formation on the plating finish. Although of significant visual impact, it is normally unlikely to affect the solderability of the product. However, Dewetting (the plating finish or solder pools into clusters on the product) is more serious, and can cause poor solder joint quality. There are steps that can be taken to minimise these incidences.

• Plating specification – thin 100% tin over nickel (standard plating for Harwin tin-plated connectors).
• Use of additives to minimise discolouration.
• Solder Preheat – Heat should be applied at a gradual rate of no more than 1°C/sec from ambient. If possible, parts should be held at Preheat Temperature for a short period to allow all components and laminates to equalise to the same temperature
• Solder temperature – for ideal conditions, soldering should be carried out at 240°C ± 5°C. Regardless of oven temperature, component temperature should be checked using a "mole" probe which measures temperature at the pcb surface. Exposure time for the highest soldering temperatures should be kept to the minimum that will ensure a good solder joint.
• Solder oven – Convection ovens give the greatest control of temperature gradients and steady temperatures, and even heating across the PCB surface. Infra-Red (IR) ovens are not recommended – process control is difficult, and surface temperatures can vary by as much as 40°C.

Extensive investigations with our plating solution suppliers has shown that discolouration is not evident until soldering temperatures exceed 300°C. The discolouration is Tin oxide, which forms in the presence of oxygen, and generally presents as a blue reflective surface effect. This effect can be avoided above 300°C by soldering under pure Nitrogen – just 0.5% Oxygen present will allow the development of oxide when molten.