简介概要

Semisolid forging electronic packaging shell with silicon carbon-reinforced copper composites

来源期刊：Rare Metals2013年第2期

论文作者：Kai-Kun Wang

文章页码：191 - 195

摘要：To fabricate electronic packaging shell of copper-matrix composite with characteristics of high thermal conductivity and low thermal expansion coefficient, semisolid forming technology, and powder metallurgy was combined. Conventional mechanical mixing of Cu and SiC could have insufficient wettability, and a new method of semisolid processing was introduced for billets preparation. The SiC/Cu composites were first prepared by PM, and then, semisolid reheating was performed for the successive semisolid forging. Composite billets with SiC 35% volume fraction were compacted and sintered pressurelessly, microstructure analysis showed that the composites prepared by PM had high density, and the combination between SiC particles and Cu-alloy was good. Semisolid reheating was the crucial factor in determining the microstructure and thixotropic property of the billet. An optimised reheating strategy was proposed: temperature 1,025 °C and holding time 5 min.

Rare Metals 2013,32(02),191-195+2

Semisolid forging electronic packaging shell with silicon carbon-reinforced copper composites

Kai-Kun Wang

School of Materials Science and Engineering,University of Science and Technology Beijing

作者简介：Kai-Kun Wang e-mail:kkwang@mater.ustb.edu.cn;

收稿日期：11 March 2012

基金：supported by the National Natural Science Foundation of China(No.51174028);the Beijing Natural Science Foundation(No.2102029);

Semisolid forging electronic packaging shell with silicon carbon-reinforced copper composites

Abstract：

To fabricate electronic packaging shell of copper-matrix composite with characteristics of high thermal conductivity and low thermal expansion coefficient, semisolid forming technology, and powder metallurgy was combined. Conventional mechanical mixing of Cu and SiC could have insufficient wettability, and a new method of semisolid processing was introduced for billets preparation. The SiC/Cu composites were first prepared by PM, and then, semisolid reheating was performed for the successive semisolid forging. Composite billets with SiC 35% volume fraction were compacted and sintered pressurelessly, microstructure analysis showed that the composites prepared by PM had high density, and the combination between SiC particles and Cu-alloy was good. Semisolid reheating was the crucial factor in determining the microstructure and thixotropic property of the billet. An optimised reheating strategy was proposed: temperature 1,025 °C and holding time 5 min.

Keyword：

Semi-solid forming; Silicon carbon reinforced copper composites; Electronic packaging shell; Microstructure;

Received： 11 March 2012

Abstract To fabricate electronic packaging shell of copper-matrix composite with characteristics of high thermal conductivity and low thermal expansion coefficient,semisolid forming technology,and powder metallurgy was combined.Conventional mechanical mixing of Cu and Si C could have insufficient wettability,and a new method of semisolid processing was introduced for billets preparation.The Si C/Cu composites were first prepared by PM,and then,semisolid reheating was performed for the successive semisolid forging.Composite billets with Si C 35%volume fraction were compacted and sintered pressurelessly,microstructure analysis showed that the composites prepared by PM had high density,and the combination between Si C particles and Cu-alloy was good.Semisolid reheating was the crucial factor in determining the microstructure and thixotropic property of the billet.An optimised reheating strategy was proposed:temperature1,025°C and holding time 5 min.

Keywords Semi-solid forming;Silicon carbon reinforced copper composites;Electronic packaging shell;Microstructure

1 Introduction

Metal matrix composites are rapidly turning into hot spot materials in producing electronic packaging shells.For example,aluminum reinforced by Mo and Si C is widely studied because the fabricated composite possesses high thermal conductivity and low thermal expansion coefficient due to the excellent combination of metal matrix and ceramic-reinforced particles[1–3].Recently,Si Cp/Cu composites with high Si C volume fraction have become another focus for thermal management applications for electronic packaging because of better thermodynamic properties of Cu than those of Al,thus becoming a more competitive candidate as packaging material[4,5].

Semisolid processing is an innovative near net-shape technology,and thixoforming as a technique of the semisolid forming could shorten the production line and make manufactured products with good surface quality and excellent comprehensive properties[6–9].The concrete process includes billets preparation,reheating to the temperature between solidus line and liquid line of the studied alloys or composites,and final extrusion.With the optimized reheating temperature,expected liquidus fraction could be obtained.

Conventional mechanical mixing or electromagnetic stirring for billets preparation is not applicable for Si C and Cu because of poor wettability.Moreover,the most important reason is that the density of Cu alloy and that of Si C has a very large difference[10–12].In this study,powder metallurgy method was adopted to prepare billets so that homogeneous chemical composition of the composite could be prepared.After billets reheating,phase segregation and flow mechanism were utilized to manufacture electronic packaging shell.

2 Experimental

Cu₉₀Sn₁₀powder was used as matrix material and could be heated into semisolid state.The solidus temperature of the alloy was 880°C,and the liquidus temperature of the alloy was 1,035°C.Therefore,the range of semisolid temperature was 155°C.The reinforcing material was Si C particles.The detailed compositions are shown in Tables 1 and 2.

Table 1 Composition of Cu₉₀Sn₁₀ 下载原图

Table 1 Composition of Cu₉₀Sn₁₀

Table 2 Physical properties of Si C particles 下载原图

Table 2 Physical properties of Si C particles

Fig.1 Complete flow chart of fabricating Si Cp/Cu electronic pack-aging shell

Powder metallurgy was adopted for billets preparation.Cylindrical Si Cp/Cu billets from cold compacting were reheated to a state in which copper matrix was in semisolid state,while Si C particle remained in the liquid phase.Since the liquid flows in front of solid phase,electronic packaging shell with high Si C volume fraction was obtained.Flow chart was shown in Fig.1.

2.1 Si Cp/Cu composites preparation

The wettability between Cu and Si C not only determines the uniformity but also determines the final mechanical and thermodynamic properties of the produced part[13,14].In solid state,Cu and Si C lack wettability and have no interface reactions.At high temperature,combination reaction may occur,destroying the reinforced body.Therefore,it is necessary to improve adhesion ability by refining copper matrix into the powder size and employing powder metallurgy technique to make powders.Mechanical alloying method was chosen for its simple process condition.In addition,under room temperature,mixing Cu and Si C was preserved from reaction as seen in Eqs.(1)and(2),which usually happened in high-temperature coating method in which oxide decreased the thermal conductivity and increased the CTE;besides,oxygen may cause voids[15].

In the test,Al as an active element was added to release the surface tension of Cu and formed a new phase between Si C and the liquid copper reducing the Gibbs of system These three kinds of powders were wet mixed in a ball mill.The focused parameter of powder mixture was mixing time,and with the extension of powder mixing time,the uniformity of the mixture was improved continuously,and the defect of uneven reunion was reduced.After repeated experiments,powder mixing time was set as 1 h.In the subsequent powder compacting,one-direction compaction was carried out to make the experiment simple and practical.A cylindrical cavity was used for cold compression:with the action of upper punch,billets were gradually formed.The prefabricated billets analysis showed that improvement of density distribution could be achieved via reducing the height–diameter ratio of the billet.Finally,billets with Si C volume fraction 35%were compacted.

Before pressureless sintering,billets were dewaxed for5 h at 500°C.Two main parameters should be considered in sintering:sintering temperature and sintering time,both of which seriously affected microstructure,phase composition,and density of billets,thereby affecting the overal performance of billets.The sintering temperature in our study was 800°C,and sintering time was 1 h.

2.2 Reheating and thixoforming

Reheating is an important step deciding the solid-phase volume fraction for die filling and mechanical properties Reheating temperature and dwelling time are two main parameters.Temperatures of 990;1,010;and 1,030°C with two different holding times of 5 and 10 min were used for the tests under controlled experiments.Figure 2 shows the schematic diagram of the forming die.The forming pressure was 400 k N.With the downside movement of the punch,the semisolid billet was squeezed into the shell cavities in the two sides of the die.

3 Results and discussion

3.1 Billets microstructure before reheating

Optical microstructures of Cu/Si Cp composites are shown in Fig.3.It can be seen that Si Cp/Cu billet prepared by PM has uniform microstructure.SEM images(Fig.4)explicitly illustrate the uniform and dense microstructure of Si Cp/Cu composites.Si Cp particles are dispersed continuously in grain boundaries.The matrix grain is also small and dense The interface between Si C and bronze is shown in Fig.4b After pressureless sintering,the boundary is clean and dense.This compact combination is beneficial to achieve effective load transfer and share expansion force from the matrix to the reinforcement.In this way,the CTE would be dominated by both Cu and Si Cp since Si Cp restricted copper expansion due to the interlock function between reinforcement and matrix.

Fig.2 Schematic diagram of semisolid forging of electronic packaging shells.1 punch,2 upper die,3 billet,4 core,5 lower die,6 excess material and 7 electronic packaging shells

Fig.3 Microstructure of Si Cp/Cu composites

Fig.4 SEM images of microstructure of billets:a microstructure of Si Cp/Cu composites,and b the interface between Si C and bronze

Fig.5 Two different models of Si Cp/Cu phase configuration:a continuous ductile phase model,and b interpenetrating model

3.2 Billets microstructure after reheating

Kuen-Ming Shu[4]assumes two common models(Fig.5)for Si Cp/Cu composites phase configuration.Figure 5a presents an ideal situation occuring in coated-copper composites in which copper restricts the Si C particle from expanding.In the research,after the process of mechanical mixing,the phase configuration could be considered as the interpenetrating model(Fig.5b):both the copper and Si C restrict the thermal expansion.The metal and Si C powders are uniformly mixed,and the Si C plays a role to form a structure due to higher hardness.Bronze powders distribute in this scenario as matrix.When reheated to the semisolid range,the melted matrix phase begins plastic flow,and the Si C frame hinders this trend.With the increase of hardening capacity resulted from the original interface bonding,the Si C structure would crumble which could be the cause of holes(Fig.6).Density difference then leads to phase segregation in the reheating process.

Fig.6 Microstructure of billets at the beginning of reheating

The higher the temperature is,the faster the matrix melted,but the faster the grain grows.The grain growth,on the other hand,is obstructed by the second phase of Si C particles.Smaller particles mean more number of grains and larger interfacial areas,thus enhancing the resistance of grain boundary migration.Therefore,grain growth rate decreases with the increase of second-phase volume fraction and refinement of particles.At the beginning of reheating,grain morphology transforms from rosette to sphere.With the increased proportion of liquid phase,spherical grain grows.Reheating temperature determines the ratio of two phases and the flow ability,and the holding time controls the grain growth.Controlled experiments involving three different reheating temperatures of 995;1,015;and 1,025°C and two different holding times of 6and 12 min were conducted to investigate the influence of reheating parameters on semisolid microstructures.The optical microstructures of composites after quenching are shown in Fig.7.

Elevating temperature and prolonging dwelling could both increase the proportion of liquid phase.Because there exists an eclectic temperature for both better grain size and morphology,it could be known that grain was relatively small when reheating temperature was 1,025°C and holding time was 6 min.However,an expansion in holding time could obviously form larger dendrite arm and spacing which would affect the mobility of the liquid and the performance of the final product.Therefore,a recommended reheating strategy is that heating temperature is set as 1,025°C and holding time is 6 min.

4 Conclusion

Si C/Cu composites were fabricated by powder metallurgy and used for semisolid forging.The microstructure of composite billets is dense,and Si C particles are homogeneously disperse in the refined matrix grain.The interface is clean and smooth.

Microstructures of different temperatures at different holding times show the influence of heating parameters on the composites.Heating temperature of 1,025°C and holding time of 6 min could assure a better microstructure for semisolid forging.