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The Future
Alternatives of Tin-Lead in PCB Assembly
Felba Jan1, Friedel Kazimierz1, Kisiel Ryszard2
1Wrocław
University of Technology, Institute
of Microsystem Technology
2Warsaw University of Technology,
Institute of Microelectronics and Optoelectronics
key words: conductive adhesives, lead-free solders, electronic packaging
Abstract
Pb-free soldering for the electronic industry is a segment of global trend towards a lead-free environment. This trend was initiated in United States in the early 90s. Now is advanced more rapidly in Japan and Europe. The intent of this paper is to provide the overview the future alternatives of tin-lead in PCB assembly. Special attention was put on selections and technological properties of lead-free solders as well as electrically conductive adhesives. It was also shown how Polish research centres are involved in solving these problems.
Soldering processes with Sn/Pb solders are standard interconnection technologies of electronic components on PCBs. Approximately 60 000 tons of SnPb solder are used every year around the world in electronic assembly [1]. Pb-free soldering for the electronic industry is a segment of global trend toward a lead-free environment.
From the environmental point of view, lead-containing solders are harmful to the environment and human beings. Even small quantities can damage the brain, nervous system, liver and kidney when ingested. When SnPb solders are disposed in landfills, lead can leach into soils and pollute ground water. Most European communities have proposed a ban on the landfill disposal of electronic products containing leaded PCBs. In the USA, consumer electronics were identified as the second largest source of lead (30%) in the municipal solid waste stream after lead-acid batteries (65%), which are already being separated from trash prior to disposal [2]. The attempt to ban lead from electronic solder was initiated in the US Congress in the early 1990s. It was proposed to ban all lead-bearing alloys, including electronic solders, and tax of $ 1.69/kg on primary lead and $ 0.83/kg on secondary lead used in the industry. However, lead solders were removed from the bills after intense lobbying by the US electronic industry [3].
The proposal for new European Directive of European Community obliges the manufacturers to elimination of lead from electronic products by 1 January 2004 [4]. It may be done by investigation of new, lead-free solders, by solder replacement with electrically conductive adhesives and by introducing new technologies of joining. Basing on journals and conference proceedings it can be estimated that about 29% of authors are involved in new solders testing and about 64% in electrically conductive adhesives. Even though probably the statistic does not take into consideration the results of research made by companies which are leaders in packaging and interconnection technologies, it is clear, that in the electronic industry, two groups of materials are being investigated currently as possible alternatives for lead-containing solders: lead-free solders and electrically conductive adhesives.
It is well known that Pb may be present in metals, such as Sn, as an impurity at the level of <0.1% by weight. Obviously this impurity is going to carry over into Pb-free alloys. In addition, it is difficult to have all component converted to Pb-free finishes in given amount of time. It was proposed by manufacturers that a target Pb content of 1% by weight in the interconnect now (1999) would be reasonable with level of <0.1% in several years. When considering the presence of Pb per weight of product, the Pb concentration might be around 100 ppm [3].
The development of lead-free interconnects require the replacement of lead in three key areas of typical solder joint: the solder, the PCB finishes and component leads metallization. The candidates for lead-free interconnections are shown in Tab.1. Many of mentioned systems are based on adding a small quantities of third or fourth element to binary alloy systems in order to lower the melting point and increase the wetting and reliability.
Additionally, the industry would prefer a standard alloy system over a patented system. A standard alloy would be broadly available. A patented alloy could be limited in supply and could increase the price of the material considerably.
Maybe in near future we will see a gradual introduction of lead-free interconnect: lead-free solder first, followed by lead-free PCB finishes and finally the lead-free metallizations [2, 5].
Tab. 1. Lead-free Interconnect selections
|
Solder
Alloy |
PCB
Finishes |
Component
metallization |
|
SnCu SnAg SnAgCu SnAgCuSb SnBi SnAgBi |
OSP, immersion
Ag, immersion Sn Immersion
Au/Electroless Ni HASL Sn/Cu,
Sn/Bi Electroless
Pd/Ni Electroless
Pd/Cu |
Pure Sn, Pd/Ni,
Au, Ag Ni/Pd, Ni/Au,
Ag/Pt, Ag/Pd Pt/Pd/Ag,
Ni/Au/Cu Pd,
Ni |
The favoured Pb-free solder alternatives vary from manufacturer to manufacturer [3, 5]. In the USA four alloys are investigated: Sn/Ag, Sn/Ag/Bi, Bi/Sn and Sn/Ag/Cu. In Europe almost each manufacturer has own candidates. In United Kingdom, the favoured options vary depending on applications: for automotive/military – Sn/Ag/Cu(Sb), industrial and telecoms – Sn/Ag/Cu and Sn/Ag and for consumer Sn/Ag/Cu(Sb), Sn/Ag, Sn/Cu and Sn/Ag/Bi. In Germany, the favoured ones appear to be Sn/Ag and Sn/Cu.
Generally, high Sn alloys are preferred, including Sn/Ag, Sn/Cu, SnAg/Cu, Sn/Ag/Bi and various versions of those alloys with small amount of additions of other elements such as Sb. More detailed the process of alloys selection was described in [3] and [6]. In production only Sn/Ag/Bi systems are used in Japan. Matsushita (Panasonic) is shipping 40 000 MiniDisk players/month with SnAgBi solder since October 1, 1998. Only in Japan market share for this product jumped from 4.6% to 15% in 6 months.
Sn96.5Ag3.5 (m.p.221°C) is an alloy preferred by American companies like Motorola and Ford. There is a long experience of using this alloy.
A soldering alloy Sn99.3Cu0.7 (m.p.227°C) has soldering quality similar to SnPb in telephone manufacturing. It is preferably suggested for wave soldering because low material cost. In air reflow the wettability is reduced, fillet exhibits rough and textured appearance.
An alloy Sn/Ag/Cu has a ternary eutectic temperature at 217°C. Cu is added to Sn/Ag in order to slow the Cu dissolution, to lower the melting temperature and improve wettability, creep and thermal fatigue characteristic.
A Sn/Ag/Cu/X type alloy (Castin Alloy Sn96.2Ag2.5Cu0.8Sb0.5) of melting range 213 - 217°C has greater fatigue performance than eutectic SnPb alloy.
In the case of Sn/Ag/Bi and Sn/Ag/Bi/X systems an addition of 5% Bi lowers the melting point and improves the wettability of Sn/Ag systems. Solderability is the best among a range of Pb-free alloys. Addition of Cu and/or Ge results in strength wettability improvement.
The representatives of major American solder manufactures, end users and various governmental organisations made recommendations for lead-free solder alternative, which have been assessed according to relative strengths and weaknesses. So, the lead-free alloy subgroups were created for reflow and wave soldering. The results of the alloy review are presented in Table 2 [7]. The results of a review indicated that alloys with compositions within the range Sn 3.5-4wt%Ag0.5-1wt%Cu are the best alloys for reflow soldering. The first and second choices for wave soldering was Sn0.7Cu and Sn3.5Ag, respectively.
Tab. 2. Lead-free alloys review
|
Wave
alloy |
Reflow
alloy | ||
|
Alloy |
%
score |
Alloy |
%
score |
|
Sn-0.7Cu |
36 |
Sn-3.9Ag-0.6Cu |
32 |
|
Sn-3.5Ag |
21 |
Sn-3.5Ag-0.7Cu |
29 |
|
Sn-2.5Ag-0.8Cu-0.5Sb |
18 |
Sn-2.5Ag-0.8Cu-0.5Sb |
21 |
|
Sn-3.9Ag-0.7Cu |
13 |
Other |
3 |
|
Sn-3.5Ag-0.6Cu |
4 |
|
|
All of the electrically conductive adhesives consist of polymer binder and a conductive filler. The polymer and its characteristics are mostly responsible for the adhesive’s ability to bond and withstand mechanical stresses. The shape of conductive fillers, ratio between the metal particle sizes and termination geometry, volume concentration of filler and other factors influence the electrical conductivity of the ECA [8]. Generally, criteria for selecting the polymer and filler materials can be as follows [9]:
for polymer:
ú sufficient adhesion to different surfaces,
ú ability to withstand high amount of filler materials,
ú low amount of ionic contamination,
ú suitable for production and rework,
ú low outstanding during cure,
ú ability to withstand commonly used solvents,
ú ability to withstand mechanical stresses,
ú not an environmentally or occupationally harmful substances.
ú sufficient electrical and thermal conduction,
ú does not form insulating oxides,
ú corrosion resistant,
ú good adhesion to the polymer matrix,
ú not an environmentally or occupationally harmful substances.
Polymers are commonly classified as either thermoplastics – typically able to be melted or softened with heat, or thermosets –which resist melting and cannot be re-shaped.
Thermoplastics are macro-molecules that structure is linear or branched. When heated, they become liquid and after cooling they become solid again. These reversible changes of state take place without any chemical changes. The thermoplastic polymer system is not cross-linked, it is meltable, weldable, swellable and soluble. Thermoplastic-based adhesives have the important advantage of fast processing and easy rework. They, especially in the form of solid films, have almost infinite shelf life and can be stored at room temperature.
Thermosets are crosslinked polymers and generally have an extensive three-dimensional molecular structure. Crosslinks are chemical bonds occurring between chains that prevent substantial movement even at elevated temperatures.
Thermoset and thermoplastic polymer adhesive are both suitable for many electronic applications [10, 11], but thermoset polymers are by far the most common ECAs binders. Adhesive binder can be of either type, but each system is very different, especially in terms of storage and processing.
Silver is the most commonly used conductive filler for ECAs. This would seem at first a poor choice because of cost and electrochemical activity. Its most important feature is still high conductivity as silver particles oxidize. Particles of silver are easy to form into controllable sizes and shapes. Usually they are fabricated in the form of balls or flakes.
Copper quickly oxides and becomes non-conductive after exposure to heat and humidity. Nickel oxidises slowly but ECAs with nickel have a much higher resistance than silver-based products. Carbon also can be used as a filler material. But carbon-based adhesives are only used in special applications because of their poor conductivity, The use of non-oxidising metals with high conductivity, like gold, is cost prohibitive.
Silver, gold and nickel are used for coating of conductive or non-conductive particles. Plated plastic particles have low densities and can deform under pressure to make better contact with bonding surfaces.
3.2.2 Main types of ECAs
There are two main types of electrically conductive adhesives, anisotropic conductive adhesives (ACAs), and isotropic conductive adhesives (ICAs).
Anisotropic conductive adhesives provide uni-directional electrical conductivity in the vertical or Z-axis. This directional conductivity is achieved by using a relatively low volume loading of the conductive filler. The low volume loading is insufficient for inter-particle contact and prevents conductivity in the X-Y plane of the adhesive. The ACA, in film or paste form, is interposed between the surfaces to be connected. Heat and pressure are simultaneously applied to this stick-up until the particles bridge the two conductor surfaces. The advantage of this approach is that the material only conducts in the direction in which the force was applied. The main drawback of ACA is the pressure needed for joining adherents.
Isotropic adhesives are electrically conductive in all directions after the materials are cured. The solder replacement goal imposes the dual requirements of both low impedance from DC through operational frequencies and mechanical attachment strength. Two components of adhesives (resin and filler) are separately responsible for these properties. Obviously, electrical conditions are provided by the metal content, and high conductivity requires high metallic content. Similarly, it is the epoxy that provides the mechanical adhesion, and adhesive strength conversely favours low metallic content. The electrical resistance of ICA is dependent on bulk and contact resistances. For the most adhesive systems the threshold on a percolation curve appears usually at about 40 vol.% of a filler [12]
The main reason for electronics manufactures to consider Pb elimination in solders consists in avoiding bio-accumulation of lead. However, both the joints as well as the finishes on boards and components are not Pb-free at present. Thus, to achieve a completely Pb-free solder joining, also new finishes must be found.
Thermal damage can occur, because higher temperatures increase the dissolution rate of metal finishes on board and components, and tend to increase the potential for intermetallic compound and joint failures.
Process difficulties and reliability can be very costly to small and medium assemblers. When selecting an alloy for bar solder and for wire solder, the metal cost is dictated by the raw materials cost and is high (see Table 3). However, for fabricated products such as the solder pastes, the processing cost of manufacturing this material can become a dominant factor and the difference between SnPb and Pb-free materials becomes very small.
Tab. 3. Relative costs of lead-free solder materials [3,6] (relative cost of selected metals: Pb– 1, Cu – 3, Sb – 3.9, Bi – 8.6, Sn – 11, Ag – 260)
|
Solder
Alloy |
Relative bar
cost |
Relative paste
cost |
Patent
(Yes/No) |
|
63Sn37Pb |
1 |
1 |
No |
|
96.5Sn
3.5Ag |
2.29 |
1.07 |
No |
|
91.8Sn
3.4Ag4.8Bi |
2.26 |
1.06 |
Yes |
|
96.1Sn3.2Ag0.7Cu |
2.21 |
1.06 |
No |
|
96.1Sn2.6Ag0.8Cu0.5Sb |
2.06 |
1.05 |
Yes |
The future alternatives to tin-lead in PCB assembly in the case of the adhesives is mainly focused on isotropic conductive adhesives. The most common ICAs are silver flake-filled thermosetting epoxies, which are typically provided as one-part thixotropic pastes. Advantages of ICAs compared to Sn/Pb solders include: environmentally friendliness (lead-free and no flux), low processing temperature (it differs according to the type of the binder and is in the range of 24ºC to 275ºC with the curing time beginning from tens of hours to a few minutes [13]), low thermomechanical fatigue, compatibility with the wide range of surfaces (including non-solderable substrates), finer pitch interconnect capability, simple processing and low cost. There are also drawbacks of adhesive application. Main of them include: lower conductivity, unstable contact resistance with non-noble metal finished components, higher thermal resistance, non self aligning due to the low surface tension (lists of benefits and drawbacks are much longer [11], [14]).
Electrical conductivity of an ICA is lower than that of Sn/Pb solders. Although this conductivity is sufficient for most electronics applications, its improvement is still needed. Contact resistance between an ICA and non-noble metal (such as Sn/Pb, Sn and Ni) finished components increases dramatically especially under an elevated temperature and humidity aging condition.
ICAs are generally designed for working in lower temperature than Sn/Pb solders. But their work at temperatures up to 125ºC is also possible. Further increasing of temperature changes the contact resistance but does not destroy joints [15].
The alloy
should provide sufficient solderability to work with conventional no-clean flux
systems. Traditional flux chemistries are designed for Sn/Pb systems. They effectively reduce the oxides of
solder and promote the reduction of surface tension on substrates such as Sn, Cu
and Au over Ni. New Pb-free alloys require redesigning of the flux to promote
the necessary wetting. Some of them have high potential for oxidation and
generally require nitrogen atmosphere during soldering.
Common soft solders are permissible for operating temperature up to 85°C. There are big demands for solders working in automotive, military and aerospace applications where working temperatures are much higher than 150º or even 180º C. There are three ways to increase the resistance of solder joints on high working temperature[16].
The most obvious way is the use of solders with higher melting point, like SnAg3.5 (m.p. 221° C), SnCu0.7 (m.p. 227°C) or AuSn20 (m.p. 278°C). However, such high soldering temperatures are not acceptable for usual substrates and components.
A more innovative solution for manufacturing of high temperature resistant solder joints is the dispersion strengthening. This strengthening works with very fine distributed particles in the solder joint structure, especially at the grain boundaries. These particles will hamper the moving of grain and grain boundaries. Two different ways of dispersion strengthening are possible: the dispersion hardening and the segregation hardening. Dispersion hardening can be done by micro powders which are mixed in the liquid solder. On the other hand the dispersion particles can be generated during solidification of solder alloys. If some partly soluble additives are added, and the limit of solubility is decreasing with temperature, the additive elements can segregate and form the dispersion particles.
Alloying processes happens during every soldering process by diffusion of base metal into solder and vice versa. The result is a changing of chemical composition of the solder material. So the solder metal in the solder joint can have a higher melting point as the basic solder alloy. Another solution for alloy strengthening is the use of solder mixtures. The solder paste consists of a powder of matrix alloy, which is melting during soldering and a second reactive powder component, which is a solid at soldering temperatures.
In
the case of ICA, composed of a polymer binder and Ag flakes, there is a thin
layer of organic lubricant on the silver particle surface. The layer affects the
conductivity of an ICA because it is electrically insulating. Studies showed
that this organic layer is an Ag salt formed between the Ag surface and the
lubricant which typically is a fatty acid such as stearic acid. To improve
conductivity, this layer have to be partially or fully removed. However, the
viscosity of an ICA paste may increase if the lubricant layer is removed. An
ideal chemical substance (or lubricant remover) should be latent (does not
remove the layer) at room temperature but be active (capable to remove it) at
higher temperature which is slightly lower than the cure temperature. The
lubricant remover can be a solid short chain acid with a high boiling point. As
an example, an ICA with the addition of short chain acids (acetic acid or adipic
acid) changes its electrical resistivity from about 0.0006 W/cm
to 0.0001 W/cm.
In general, an ICA paste has a low electrical conductivity before curing, which increases after an ICA is cured. It is caused by cure shrinkage of polymer binder and more intimate contact between Ag flakes. Therefore, an ICA with a higher cure shrinkage shows better conductivity. For ICAs based on epoxy resins, a small amount of multifunctional epoxy resin can be added into an ICA formulation to increase crosslinking density and thus increase its conductivity [2].
The electrical conductivity improvement can be achieved by applying of the transient liquid phase sintering metallic fillers in an ICA formulations [17]. The filler used is a mixture of a high melting point metal and a low melting point alloy powder (or flakes). When heated to a certain high temperature, the low melting point alloy powder melts and diffuses rapidly into the high melting point filler and eventually solidify (or melts and solders silver flakes). As a result a stable metallurgical network for electrical conduction is formed and higher electrical conductivity can be achieved [2].
It is obvious, that surface material significantly influences the quality of the joints. For example, the resistance as well as nonlinearity measurement points the gold-plated Cu pads better than copper pads [18]. Contact resistance between an ICA and non-noble metal finished components increases dramatically during an elevated temperature and humidity ageing. A recent study indicated that galvanic corrosion rather than simple oxidation of the non-noble metal (at the interface between an ICA and non-noble) was the mechanism responsible for resistance changes [2], [19].
A galvanic corrosion process happens only under wet conditions, with presence of an electrolyte, and oxygen generally accelerates the process. Therefore, one way to prevent galvanic corrosion is to lower the moisture pickup of the ICA. The electrolyte is mainly from the impurity of the epoxy resin. ICAs formulated with resins of high purity and low moisture absorption showed more stable contact resistance.
The second method of preventing galvanic corrosion is to introducing some organic corrosion inhibitors. In general, they act as a barrier layer between the metal and environment by absorbing as a film over the metal surfaces. The corrosion inhibitors must not react with the epoxy resin when it is cured. Otherwise, the corrosion inhibitors are consumed by reacting with the epoxy resin and lose their effect.
As oxygen accelerates the galvanic corrosion, another way to slow down this process is to incorporate some oxygen scavengers into ICA formulations. When oxygen diffuses through the polymer binder, it reacts with the scavenger and is consumed. Unfortunately, it can be depleted completely by the reaction with oxygen, therefore, oxygen scavengers can only delay galvanic corrosion process for some time.
Aging tests
show that ICAs with the corrosion inhibitors are characterized by much smaller
increase of contact resistance than the ICAs without inhibitors. It is also
stated, that inhibitor can stabilize the contact resistance more effectively
than oxygen scavengers [19].
In the case of simple oxidation, the contact resistance stability can be improved by incorporating into the ICA formulations some electrically conductive particles, which have shape edges [2]. The particle is called oxide-penetrating filler. Force must be provided to drive these particles through oxide layer and hold them against the adherent materials. This can be accomplished by employing polymer binders with high shrinkage when cured.
The
most simple way of the joint mechanical strength improvement is decreasing the
filler loading. But it usually means the ICAs electrical conductivity
deterioration. Some compromise is necessary.
The
initial shear strength of the adhesive joints is about three times less than
that of the corresponding solder joints because of the extremely small
interconnection areas [20]. The generally poor wetting behavior of ICAs can be
compensated for by pressing the cube-shaped components in the adhesive dot
during the mounting process. The formation of the interconnection area is mainly
dependent on the mounting pressure, which is subjected to some deviations caused
by semiautomatic placement.
The
shear strength analysis shows, that mechanical properties of the joint depends
also on adhesive formulation [21]. For example, the middle-size chain epoxy
resin is better than the epoxy-formaldehyde hybrid type resin and the mixture of
flakes and semiflakes as filler is slightly better than only flakes. In some
cases the use of nanoparticles is recommended. They improved the electrical
conduction and mechanical strength.
Recently,
a new class of conductive adhesives based on an epoxide-terminated polyurethane
(ETPU) has been developed [2]. This class of conductive adhesives showed
superior impact performance and substantial stable contact resistance with
non-noble metal surfaces such as Sn/Pb, Sn and Cu.
The results of the first research work devoted Pb-free solders in Poland were published in 1995 [22, 23]. The work on developing lead-free alternatives were divided into four basic stages: evaluate the properties of new bulk lead-free alloys, determinate their technological properties in similar to production conditions, e.g. during wave or reflow soldering, evaluate the exploitation parameters during environmental test and estimate the technological properties of Pb-free solder paste. The work was done by the three research centres: Warsaw University of Technology (IMiO), Wrocław University of Technology (ITM) and Tele and Radio Research Institute, Warsaw [24, 25, 26].
During first stage of research the physical and technological properties of some Pb-free solders were defined. It was found that technological properties of Pb-free solders with commercially available classical fluxes are not satisfactory. In the next stage the new special type no-clean fluxes were elaborated and manufactured [27, 28]. Again the technological properties of some type Pb-free fluxes were investigated [29, 30]. During this work the DoE methodology was applied. The results of these works are presented in Table 4. Till now there were no interest of Polish Industry to implement these results. Maybe because till now there are not commercial, legal and technical needs. Only HERAEUS Company from Germany is interested in investigation and estimation of their lead-free solder paste. The results of this investigation performed in Poland are presented in papers [31].
Tab. 4. Physical and technological properties of some lead-free solders produced and investigated in Poland
|
Solder |
m.p. [°C] |
tz
[s] |
F2
[mN] |
l [W/m
K] |
Rm
[MPa] |
r [mWm] |
Ref. |
|
Sn63Pb37 |
183 |
0.33 |
6.9 |
51 |
40 |
0.146 |
22 |
|
Sn97Cu2Bi1 |
225.7 |
0.39 |
5.8 |
74 |
- |
0.141 |
23,33 |
|
Sn99Cu1 |
226.8 |
0.40 |
5.4 |
67 |
- |
0.149 |
33 |
|
Sn99Zn9 |
198.6 |
1.15 |
|
85.5 |
|
0.116 |
23 |
|
Sn91Zn8Bi1 |
197.2 |
1.07 |
2.6 |
73 |
- |
0.126 |
23 |
|
Bi57Sn43 |
|
0.47 |
4.3 |
- |
- |
0.350 |
24 |
|
Sn91Bi7.2Ag1.8 |
|
0.41 |
6.0 |
48 |
74 |
0.160 |
24,26,29 |
|
Sn90Bi9.5Cu0.5 |
|
0.40 |
5.8 |
44 |
72 |
0.140 |
29 |
|
Sn97Ag1.8Cu0.8Sb0.4 |
|
0.41 |
6.1 |
62 |
40 |
0.148 |
29,30 |
|
Sn96Ag3Cu1 |
|
0.36 |
6.1 |
70 |
- |
0.134 |
33,34 |
m.p. – melting point,
tz – wetting time, F2 –
wetting force after 2 s of wetting,
l - thermal conductivity, Rm – Tensile strength,
r - electrical volume resistivity.
There is another group of Polish researches (Warsaw University of Technology, IPBE), which are involved in research of SnPb solders in an European “Copernicus” research project. The aim of this project was to develop and test a new solder alloy system with a common melting point about 180°C and improved high temperature characteristics for solder joints at 120°C operating temperature. Mechanical strength and reliability investigations of THT and SMT joints soldered with standard solder SnPb and new dispersion straightened SnPb solder were done by Polish group [35].
Basing
on available literature authors state that only two research centres in Poland
are involved in adhesives for microelectronics applications. In 1997 Szczepański
and Kisiel from IMiO and Friedel from ITM published in Polish [36, 37] and in
English [14, 38, 39] papers in which some problems of adhesive applying in
electronic devices assembling were discussed. Since 1999 scientists from
Institute of Microsystem Technology have been strongly involved in ICAs for
microwave application [40-43], whereas researches in Institute of
Microelectronics and Optoelectronics have focused on mechanical and electrical
properties of adhesives [21, 44, 45]. Special ICAs formulations for these
researches are prepared by Amepox® Quality Silver Systems from
Łódź.
The
Polish centres activity is quite good base
to be ready to the future alternatives to tin-lead in PCB assembly in our
country. The researchers from Wrocław University of Technology (IMT), Warsaw
University of Technology (IMiO) and Tele and Radio Research Institute have had
the good experience in assessing technological properties of lead-free solders
and estimating wettability of solders in presence of low solid, no clean fluxes.
There are the good base for laboratory scale production of lead-free solders as
well as no clean fluxes.
Now
the works of mentioned above research team is concentrated on estimating the
possibility of applying electrically conductive adhesives as a solder
replacement in SMT.
The
switching to Pb-free processes required the changes in joining materials, PCB
finishes and component leads covering. More advanced are works devoted to
joining materials. The pros and cons of two groups of materials: lead-free
solders and electrically conductive adhesives were analysed in this paper. The
most probable alternative for reflow soldering can be SnAgCu solder paste and
for wave soldering SnCu or SnAg solders. There are no clear suggestion about PCB
and component Pb-free metallization. Alternative materials exists, but still
economical (cost) and technical problems (mainly reliability) are not
solved.
Conductive adhesive technology is still
in its infancy. Main limitations of current ICAs include lower than SnPb
conductivity, unstable contact resistance with non-noble metal finished
components and poor mechanical properties. These problems are now the objects of
works of many research centres, among them there are Polish research
institutes.
References
1.
Biocca Peter “Global Update on
Lead-free Solders” SMT June 1999 vol.13 no 6, p. 64-67
2.
Wong C. P., Daoqiang Lu “Recent
Advances on Electrically Conductive Adhesives for Electronics Applications”
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Technology in Electronic Manufacturing. 18-21 June 2000, Espoo, Finland p.
121-128
3.
Ning-Cheng Lee : “Lead-Free Soldering –
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September/October 1999, p. 29-35
4.
Snowdon Ken:“Towards a Green 2000”
Proc. of 12th European Microelectronics & Packaging Conference,
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5.
Kisiel R.: “Environmentally Friendly
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6.
Chris Bastecki “A Benchmark Process For
the Lead-Free Assembly of Mixed Technology PCBs” September 1999 12
pages
7.
Bradley E., Bath J., Whitten G., Chanda
S.: “Lead-free project focuses on electronics assemblies” Advanced Packaging ,
February 2000 p. 34-42
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Dziedzic A., Snarski A. A., Buda S. I.,
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1999, p. 6
9.
Rusanen O., “Replacing solder with
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thermoplastic and thermoset polymer adhesives in microelectronic assembly
applications”, Microelectronic International, Vol. 16, No. 2, 1999, p.
34
11. Firmstone M. G., Bartholomew P. M.,
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Adhesive in Advanced Electronic Packaging Applications”, Microelectronic
International, No. 43, 1997, p. 16
12. Morris J. E.: “Conduction Mechanisms
and Microstructure Development in Isotropic, Electrically Conductive Adhesives”,
in “Conductive Adhesives for Electronics Packaging”, ed. J. Liu, Electrochemical
Publications Ltd., 1999
13. Mach P., Škvor M., Papež V., “Quality
of Electrically Conductive Adhesive Joints”, 22nd International
Spring Seminar on Electronic Technology, Dresden 1999, p.
1
14. Friedel K. P.: “The guidelines for
selecting electrically and thermally conductive adhesives for use in assembly of
microsystems”, Trans. on the Precision and Electronic Technology, Vol. 3, 1997,
p. 169
15. Kisiel R, “Conductive Adhesives for
High Temperature Applications”, 23rd Conference of IMAPS Poland,
Kołobrzeg 1999, p. 167
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Pape U., Schultz J.: “Solder Joints with Improved High Temperature
Characteristics’ Proc . of 22nd International Spring `Seminar on
Electronics Technology, 1999, May 128-20, Freital-Dresden, Germany p.
62
17. Gallagher C., Matijasevic G.,
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