Environmental and technical characteristics of conductive adhesives versus soldering

5. Recycling analysis

5.1 Material contents
5.2 Recycling process
5.2.1 Collection 
5.2.2 Dismantling facility 
5.2.3 Metal recovery - recycling
5.3 Differences in material value
5.4 Differences in environmental load

A recycling analysis has been carried out with the aim of identifying differences connected to recycling of electronics assembled with traditional solder and electrically conductive adhesives, respectively. The Danfoss EST (electronic thermostat) was used as a case. It was tried to answer the following questions:

	What are the contents of the two EST subjects?
	Do the recycling processes differ?
	What are the differences in material value?
	What are the differences in environmental load connected to the recycling?

5.1 Material contents

The complete composition of the two EST case subjects was inventoried by contacting the component suppliers. Data were obtained from most suppliers. Data on the remaining 4-5 components were estimated based on data for the other components. Material composition of the PCB’s were partly calculated and partly obtained from the suppliers.

Most of the supplier information has been forwarded to Danfoss A/S under confidentiality, why only a total can be presented. Differences are due to: The surface treatment of the PCB: An eutectic SnPb HAL (Hot Air Levelling) surface and a solder mask (resist) is used when soldering, whereas adhesive requires chemically applied Ni and Au PCB top layers. The joining material: Solder versus conductive adhesive.
Through-hole components: Two of the through hole components in the soldered version are substituted with SMT components in the adhesive version. NB! Care should be taken not to interpret data on these two components as a general picture of differences between functionally comparable through-hole and SMT components.

Component leading will ideally also differ. Traditional components contain an inner AgPd layer, a Ni-barrier and a SnPb layer on top. Optimum adhesive components only contain a AgPd layer [6]. It was, however, not possible to obtain components with this termination for the test case. Environmental consequences of using different component termination are addressed in the LCA case (Chapter 7).

5.2 Recycling process

For Danish conditions, end-of-life electronics are collected and forwarded to an electronics dismantling facility.

No differences are expected in relation to collection of electronics joined with solder and adhesive, respectively.

5.2.2 Dismantling facility

Danish dismantling facilities typically conduct a preliminary mechanical separation, where the mounted PCBs are separated from major metal and plastic parts (which are forwarded for recycling). The PCBs are then checked for known problematic components such as mercury batteries and electrolytic capacitors containing polychlorinated biphenyls. Problematic components are removed with a pair of tongs. Elektromiljø A/S carried out a test on the two subjects and did not find any differences in dismantling feasibility.

No differences are expected in relation to dismantling electronics joined with solder and adhesive, respectively.

The PCBs are sent for further resource recovery at abroad recycling facilities.

In general, extraction of metals from scrap is a very complex process, because the various types of raw materials used contain a combination of metals and impurities. The processes involve roasting/drying, smelting, converting and refining. After refining of one specific metal, the remaining impurities (sludge) containing several metals are treated in other processes for recovering.

PCB’s may be recovered by different recycling technologies, see e.g. [8]. Elektromiljø A/S has assessed that both subjects would ideally be recycled by the ‘copper process’. The copper process is applied at Boliden Rönnskär, Sweden and will briefly be described as typical for an up to date copper recovery plant.

The Boliden smelter, Rönnskär is one of the largest smelters for electronic scrap. However, the major input to the process is refined copper ore. Other raw materials, incl. electronic scrap consist about 25%.

5.2.3.1 Recovery at Boliden, Rönnskär

The copper process at Boliden is briefly described below. Focus is on metal recovery and especially those metals for which differences are seen between products interconnected with solder and adhesive, respectively.

Electronic scrap (circuit boards from computers, television sets etc.) is inspected to prevent the inclusion of dangerous materials such as mercury and radioactive isotopes. The collectors or dismantling facilities will normally remove electronic components containing dangerous materials before delivery.

The secondary raw materials incl. electronic scrap is flash smelted in a "Kaldo plant", primarily for recovering the copper. The flash smelting recovers energy from burning the organic materials in the scrap at around 1250°C.

The melt is transferred to a copper plant for converting, casting and electro-refining. In a converter, sand is added as a slag-former at about 1200°C for removing iron and zinc. The removed material is further treated and purified in order to give the by-products ‘ironsand’ and ‘zink billets’.

The "white metal" (copper sulphide), formed in the converter, is oxidized (blowing) to 98% pure copper, followed by an injection of liquid ammonia for reducing the level of oxygen. The melt is casted into copper anodes containing 99% copper and 0,5% precious metals.

The copper anodes are electro refined in the copper refinery, using sulphuric acid and copper sulphate as the electrolyte. The sludge in the electrolyte tanks contains precious metal impurities and is forwarded for a separate precious metal plant, where gold, silver, platinum, palladium and selenium are recovered. Boliden has informed that close to 100% palladium and gold is recovered and between 90 and 99% silver. Another by-product from the refinery is a Nickel-fraction, which is treated in a nickel-sulphate plant. The nickel recovery percentage is about 80%.

Tin and lead mainly ends up as dust in gas streams from the Kaldo plant and from the converting. The gas streams are cleaned, capturing the major amount of the tin and lead. This fraction is sent for abroad metal recovery. At present, about 80% may be recovered from an economical point of view.

Metal amounts not recycled end up in waste products, as contaminants in the by-products or leave the plant as emissions.

Very little information is available on these waste streams. However, it is assumed that the far major amount ends up as solid waste products. At present waste materials are stored within the industrial area. A permit from the Swedish National Board of Franchise states that the company shall take measures to store and handle waste material in a manner that prevents nuisance from an environmental point of view via contamination of water, soil or air. The permanent disposal of these waste fractions is currently under investigation. Annually, a public environmental report with full description of the amounts and chemical composition of waste products is sent to the Swedish EPA.

5.3 Differences in material value

Demet Deutche Edelmetall Recycling AG & Co. (Alzenau, Germany) has estimated the following ‘recoverable material value’ for the two EST subjects:

	Soldered version: approx. 7 DM (German Mark)/kg (about 26,50 DKK/kg)
	Adhesive version: approx. 11 DM/ kg (about 41,50 DKK/kg)
	About 2 DM/kg should be subtracted for processing costs.

The results indicate that considerably more precious metal is used for the adhesive version. Interestingly, differences in component termination are not included in the test case. ‘Optimum’ component termination for the adhesive alternative (Ag/Pd) would increase the silver and palladium consumption further and result in an even larger difference.

The result should as such only be taken as indicative, as specific print layouts (PCB area, components applied, packing density etc.) may severely affect the metal content and thereby the value. However, adhesive prints must be assumed in general to contain a higher metal value.

Elektromiljø A/S has looked at the specific case subject and found that the PCB would not be separated from other thermostat parts (the chassis) due to labour cost at the dismantling facility. Therefore, the entire thermostat would be forwarded for recycling. The chassis parts do not contain precious metals and the value per kg thermostat is therefore much lower than the PCB value per kg. This influences the economic benefit as the recycling process costs to some extent are per kg electronic scrap delivered to the recycling facility.

Finally, it should be noticed that it would require a substantial substitution for adhesive technology before differences in material value becomes economically visible for the dismantlers and other interested parties. If only few ‘adhesive PCBs’ are present in the PCB scrap fraction, they would physically and economically ‘disappear’. Possible adhesive substitution potentials are discussed in [3].

5.4 Differences in environmental load

Different recycling facilities have been contacted in order to inventory differences in environmental loads (energy consumption, emissions and waste generation) from recycling solder and adhesive PCBs. However, except for metal recovery percentages (see previous paragraph ‘Metal recovery – recycling’), it has not been possible to obtain data on this issue.

The requested data are not available. It is very difficult to allocate the environmental burdens from the complex recycling processes between the inhomogeneous input materials varying dramatically in composition. Further, an allocation would probably require more detailed measurements than what is presently available.

It can be concluded that it would require a substantial developmental work to map and allocate environmental burdens in a way that would make it possible to differentiate burdens associated with solder and adhesive PCBs, respectively.