LEAD-FREE EUTECTIC SOLDER ALLOY COMPRISING ZINC AS THE MAIN COMPONENT AND ALUMINUM AS AN ALLOYING METAL

The invention is directed to a lead-free eutectic solder alloy comprising zinc (Zn) as the main component and aluminum (Al) as an alloying metal. The invention is also directed to the use of the lead-free eutectic solder alloy.

Legislation driven by environmental and health concerns is still driving the replacement of lead-containing solder alloys with lead-free alternatives. However, the alternative solder alloy's melting temperature, or to be more precise, its solidus temperature must be high enough, especially with regard to applications in which the finished solder joints experience high temperature as the case may be either in later process steps and/or under field conditions. In die-attach applications, for example, a melting temperature of the solder alloy in the range of 280 to 400 °C is required.

US 2013/0045131 A1 discloses lead-free zinc base solder compositions comprising 82 to 96 wt.% (weight-%) of zinc, 3 to 15 wt.% of aluminum, 0.5 to 1.5 wt.% of magnesium and 0.5 to 1.5 wt.% of gallium.

JP2000208533 A discloses a lead-free zinc base die-bonding alloy of 2 to 9 wt.% of aluminum, 0.01 to 0.5 wt.% of magnesium, 2 to 9 wt.% of germanium and a remainder of zinc and inevitable impurities.

US2012/0313230 A1 claims several lead-free zinc base solder alloys, one with 8 to 20 wt.% of aluminum, 0.5 to 20 wt.% of magnesium and 0.5 to 20 wt.% of gallium, a further one with 1 to 30 wt.% of aluminum, 0.5 to 20 wt.% of magnesium and 0.5 to 6.5 wt.% of tin and also one with 1 to 30 wt.% of aluminum, 0.5 to 20 wt.% of germanium and 0.5 to 20 wt.% of gallium, the rest of the alloys in each case comprising zinc.

JP2012228729 A discloses lead-free zinc base solder alloy comprising 0.01 to 9.0 wt.% of aluminum, at least one of germanium, magnesium, silver and phosphorus in ranges of 0.01 to 8.0 wt.% of germanium, 0.01 to 5.0 wt.% of magnesium, 0.1 to 4.0 wt.% of silver, and at most 0.5 wt.% of phosphorus; and a remainder of Zn and inevitable impurities.

JP11207487 A discloses lead-free zinc base solder alloy consisting of 1 to 7 wt.% of aluminum, 0.5 to 6 wt.% of magnesium, 1 to 25 wt.% of tin, and the balance zinc with inevitable impurities.

US 2013/0251587 A1 claims lead-free zinc base solder alloy consisting of 1.0 to 9.0 wt.% of aluminum, 0.002 to 0.8 wt.% of phosphorus, 0.3 to 3 wt.% of germanium and a balance being zinc with inevitable impurities.

There is a desire to find improved lead-free solder alloys.

The invention relates to a lead-free eutectic solder alloy comprising zinc (Zn) as the main component and aluminum (Al) as an alloying metal. Its eutectic temperature, represented by a single melting point, lies in the range of 320 to 390 °C (measured by DSC at a heating rate of 5 °C min^-1).

The abbreviation "DSC" used herein means differential scanning calorimetry.

It has been found that the lead-free eutectic solder alloy of the invention can be used as solder metal or in solder compositions, in particular for use in the electronics and microelectronics field.

The solder joints made from solder compositions comprising or consisting of the lead-free eutectic solder alloy of the invention exhibit surprisingly improved thermal characteristics, microstructural characteristics and reliability. These characteristics result in better performance of products made using these alloys.

The lead-free eutectic solder alloy comprises 6.0 wt.% of aluminum (Al) and 6.0 to 7.2 wt.% of germanium (Ge).

In an embodiment, the lead-free eutectic solder alloy consists of 6.0 wt.% of aluminum (Al), 6.5 wt.% of germanium (Ge), a total of 0 to 0.5 wt.% of one or more doping elements selected among phosphorus (P), nickel (Ni), bismuth (Bi), antimony (Sb) and silicon (Si), and of zinc (Zn) as the remainder to make up 100 wt.%. The alloy exhibits a single melting point at 368 °C (measured by DSC at a heating rate of 5 °C min^-1).

In another embodiment, the lead-free eutectic solder alloy consists of 6.0 wt.% of aluminum (Al), 7.2 wt.% of germanium (Ge), a total of 0 to 0.5 wt.% of one or more doping elements selected among phosphorus (P), nickel (Ni), bismuth (Bi), antimony (Sb) and silicon (Si), and of zinc (Zn) as the remainder to make up 100 wt.%. The alloy exhibits a single melting point at 361 °C (measured by DSC at a heating rate of 5 °C min^-1).

In yet another embodiment, the lead-free eutectic solder alloy consists of 6.0 wt.% of aluminum (Al), 6.0 wt.% of germanium (Ge), 2.0 wt.% of copper (Cu), 1.25 wt.% of tin (Sn), a total of 0 to 0.5 wt.% of one or more doping elements selected among phosphorus (P), nickel (Ni), bismuth (Bi), antimony (Sb) and silicon (Si), and of zinc (Zn) as the remainder to make up 100 wt.%. The alloy exhibits a single melting point at 358 °C (measured by DSC at a heating rate of 5 °C min^-1).

The phrase "zinc (Zn) as the remainder to make up 100 wt.%" is used herein. It shall mean that zinc is the main component in the respective lead-free eutectic solder alloy, as has already been said above. To avoid misunderstandings, the preceding sentence shall not be understood to exclude other elements which due to prevailing technical conditions may have found their way into the lead-free eutectic solder alloy of the invention, for example, as a consequence of an unintentional but inevitable incorporation during manufacture. In other words, such other elements may be present in the lead-free eutectic solder alloy as inevitable impurities, however only in very minor amounts of, for example, > 0 to 0.05 wt.%. In any case such inevitable impurities are not deliberately added or introduced into the alloy forming composition. Insofar, the phrase "zinc (Zn) as the remainder to make up 100 wt.%" means that the wt.% proportion which is missing to make up 100 wt.% of the alloy consists of zinc plus said inevitable impurities, if the latter are present.

The lead-free eutectic solder alloy of the invention can be prepared by conventional processes known to the person skilled in the art of metal alloys, for example, by melting together the zinc, the aluminum and the other necessary components. It is possible to use an induction furnace and it is expedient to work under vacuum or under an inert gas atmosphere. The materials used can have a purity grade of, for example, 99.99 wt.% and above. The metal alloy melt is typically cast in a mold of room temperature in which it cools down and solidifies.

The lead-free eutectic solder alloy of the invention can directly (i.e. as such in metal form) be used as a solder metal. However, from a practical perspective, it must be brought in a form suitable for an intended soldering task. Examples of suitable forms include solder wires, solder rods, solder powders and solder preforms.

The lead-free eutectic solder alloy of the invention can also be used as metal alloy constituent in a solder composition, in particular, as the only metal alloy constituent in a solder composition. Examples of solder compositions include solder pastes and solder wires with fluxes.

The lead-free eutectic solder alloy of the invention or the solder compositions comprising the lead-free eutectic solder alloy of the invention can be used in many applications including mechanical connection and electronic or microelectronic applications. The lead-free eutectic solder alloy of the invention can be used as a brazing alloy in mechanical connection, for example. It can also be used in electronic or microelectronic applications. The solder compositions comprising the lead-free eutectic solder alloy of the invention can in particular be used in electronic or microelectronic applications. Examples of electronic or microelectronic applications include attachment of wafer die to leadframes, attachment of packaged die to heatsink or soldering of leads to printed circuit board.

When performing a soldering task, in particular an electronic or microelectronic soldering task, with the lead-free eutectic solder alloy of the invention (regardless if directly used as solder metal or in the form of a solder composition as mentioned above), it may be expedient to support the soldering process by application of ultrasonic energy to the applied and molten lead-free eutectic solder alloy. Such application of ultrasonic energy may help in preventing formation of voids in the solder and in forming consistent solder joints in terms of homogeneous solder layer thickness as well as solder formation shape regardless of inconsistent solder dot positioning.

Examples

Eutectic zinc-based alloys 1 to 3 with the following wt.-% composition (zinc as remainder) were made:

1. 1. Zn2.2Mg3.8Al
2. 2. Zn6Al7.2Ge
3. 3. Zn0.35Mg5Al1Ge

To this end, the raw materials of ≥99.99% purity were weighed according to their required weight percentage and loaded into a graphite crucible. The graphite crucible was placed in an induction furnace within an enclosed chamber. The chamber was vacuumed to clear the air from the chamber. It was subsequently purged with argon gas before the furnace was started to melt the materials. The average heating ramp rate was 5 °C/sec and heating was performed until the zinc was fully melted. The induction furnace allowed for application of a mild stirring to the melt for homogenization purposes. Each of the alloys were cast into a billet of 48 mm diameter after 1 hour. The billets were directly extruded into 0.76 mm diameter solid wire and subsequently wound up by an automatic winder.

The melting behavior of the alloys was analyzed by DSC (differential scanning calorimeter) at a heating rate of 5 °C min^-1. The microstructure was analyzed by SEM (scanning electron microscopy) with EDS (energy dispersive spectrometry) and the phases were determined by XRD (X-ray diffraction):

Alloy

Microstructural/Phases

Eutectic melting point (°C)

Primary Phases

Secondary Phases

1

Zn (η, HCP)

Mg₂Zn₁₁; Al (α, FCC)

350

2

Zn (η, HCP)

Ge; Al (α, FCC)

361

3

Zn (η, HCP)

Al (α, FCC); Mg0.97Zn0.03; Al₁₂Mg₁₇ and Mg₂Ge

375

α/η = different structural phases

HCP = Hexagonal Close Pack

FCC =Face Centered Cubic

Each of the alloys exhibited a single melting point (same solidus and liquidus) in the DSC curve and was thus considered a eutectic alloy.

The alloys display similar or better electrical conductivity compared with conventional Pb based alloy and exceptional electrical conductivity compared with Pb based and Sn based alloys (compare the following table). The thermal conductivity was determined using a NanoFlash laser thermal conductivity apparatus on a 3 mm thick sample having a diameter of 13 mm. The electrical conductivity was determined according to ASTM E1008-09 standard on 70 mm long rods of 20 mm diameter.

Alloys

Melting Temp. (°C)

CTE^*) (µm/m·K)

Softening Temp. (°C)

Thermal Conductivity (W/m·K)

Electrical Conductivity (%IACS)

Solidus

Liquidus

1

350

350

24

299

42.94

20.82

2

361

361

20

305

60.46

23.15

3

375

375

25

302

65.60

26.60

SnAg25Sb10

228

395

19

TBD

55.00

11.20

PbSn1Ag1.5

313

313

30

304

44.00

8.10

*) thermal expansion coefficient

The low and single melting point of the eutectic alloys shows advantages in process application by allowing a lower process temperature. They are also not prone to microstructure coarsening or shrinkage void typically observed for non-eutectic alloys.

Each of the alloy wires was used for solder-fixing a 0.4 mm thin silicon-carbide die of 3.1 mm · 3.1 mm having a silver contact surface onto a Ni plated leadframe (JEDEC standard TO-220) using a soft solder wire dispensing machine (Besi die bonder Esec 2009 fS^E). Soldering was supported using a programmable ultrasonic module (PUM) from Besi.

The so assembled samples were analyzed for void classification using CSAM (C-mode scanning acoustic microscopy) using a Sonic Echo LS pulse echo with a frequency of 110 MHz. Metallographic samples were prepared by grinding and polishing up to 1 µm diamond using Struers TegraForce-5.

Microstructure was studied using Olympus GX51 inverted optical microscope. Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDX) analysis was performed using Ultra Plus FE-SEM from Carl Zeiss.

The soldered samples were tested for their reliability by conducting high temperature storage test (HTS). The samples were held at 245 °C for up to 500 hours. Evaluation criterion was based on DC electrical connectivity (RDSON) at 0, and every 250 hours.

The soldered samples were subject to a thermal cycling test according to standard JESD22-A104D, Condition C. Evaluation criterion was based on DC electrical connectivity (RDSON) at 0, and every 250 cycles.

The soldered samples were subject to a pressure cooker test according to standard JEDEC-STD-22-A102-A, 121°C, 100%RH, 2atm. Evaluation criterion was based on C-SAM for delamination.

The soldered samples were subject to Moisture Sensitivity Level 1 test based on JES22-A113-D, under condition 85°C/85%RH for 168 hours + 3x reflow, 260°C. Evaluation criterion was based on DC electrical connectivity (RDSON) before and after.

The soldered samples were subject to unbiased HAST (Highly Accelerated Temperature and Humidity Stress Test) based on JESD22-A110B, 130°C / 85% / 96 hours. Evaluation criterion was based on DC electrical connectivity (RDSON) before and after.

The test results are summarized in the following tables.

Alloy

Process Application

HTS Test

Pressure Cooker Test

Visual*)

Voids

0 h

250 h

500 h

2

Ok

10-15%

Passed

Passed

Passed

Passed

3

Ok

<10%

Passed

Passed

Passed

Passed

*) visual inspection using an optical microscope at 10X magnification

Alloy

Thermal Cycling (-65 - 150°C)

MSL Level 1 test

Unbiased HAST test

250 cycles

500 cycles

2

Passed

Passed

Passed

Passed

3

Passed

Passed

Passed

Passed