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Low Cost Solution for Bonding Wire
Jong-Soo Cho,Jeong-Tak,Moon(R&D,MK Electron Co.,Ltd.Korea,449-812)Abstract:Recently,gold price is going up to over 1,000USD and the price of semiconductor device thatis especially memory device is going down.In semiconductor industry,major concern is cost reduction.To reduce material cost of bonding wire,Au-Ag alloy wire has been tried to be used in electronic de-vices in substitution for Au wire,since several years ago.However,Au-Ag alloy wire could not been ap-plied to such devices due to failures during high humidity reliability test such as PCT(Pressure cookertest).This study investigates failure mechanism during PCT in electronic device of conventional Au-Agwire,the effects of the element(Pd)that was put into Au-Ag wire to prevent the humidity reliability test(PCT)failure.Keywords:Au-Ag wire;PCT(Pressure Cooker Test);Humidity;Reliability收稿日期:2008-04-15·封裝與測試·60電子工業(yè)專用設(shè)備Equipment for Electronic Products ManufacturingEPEMay.2008（總第160期）
1 Introduction
Au has been used as bonding wire for semicon-ductor packaging generally.however,in order to re-duce cost in the semiconductor industries,substitutionfor the gold wire by some other low cost wire has be-come a critical issue.As Gold and Silver form com-plete solid solution,it make it possible that Ag ofhigh contents is alloyed to Au,therefore it can savewire cost for semiconductor package.It has reported,however,that degradation of thebond between Au-Ag ball and Al pad is critical prob-lem.These humidity reliability problem acts as thebarrier for the application of Ag alloyed Au wire tosemiconductor package in spite of low cost.The purpose of current paper is to study(1)whybondability between Au-Ag ball and Al metallizationis weakened during humidity test(2)the influence ofPd element alloying on Au-Ag wire to solve the fail-ure problem during humidity reliability.
2 Result and Discussion
2.1 The shapes of FAB,1st Bond, 2nd Bonds,Looping SEM images were observed to check out whether 。Free Air Ball,1st bonded ball,2nd stitch bond,loop-ing were good or not in Fig.1.As a result,It werechecked out that all conditions were no problem.
2.2 The Bondability after PCTAfter the three kinds of wires(Diameter:20μm)-(1)4N Au,(2)Au-15%Ag,(3)Au-15%Ag-5%Pd-on Al metallization were ball-bonded,the samplewere exposed under JESD 22-A102,PCT condition(at 121’C,204.9×10-3 MPa,100%RH).It is shown in Fig.2 that vertical line representBPT(bond pull test)values,and horizontal line repre-sent PCT duration time.Inspection of this chart re-veal that BPT value was drastically decreased afterPCT for 24 h in case of Au-15%Ag,and it wasaround zero after 48 h PCT.On contrast,in case of4N Au(99.99%Au)and Au-15%Ag-5%Pd,it keptstable BPT values.As a result,we could find out preventing Pd ele-ment from bondability degradation of Au-15%Ag during PCT.
Fig.3 shows ball shear test results.In case ofAu-15%Ag,BST values were dropped down slowly,compared with BPT valued in Fig.2,but overalltrends were almost similar to BPT values in Fig.2
STEM image of interface between Au-15%Agball and Al pad,after PCT(121’C,204.9×10-3 MPa,100%RH,96h)is shown in Fig.4.Inspection of thisimage reveal that intermetallic compounds are formedin the region upper Al pad.It was observed that darkline,that is micro crack between Au-15%Ag andAu-Ag-Al intermetallic compounds.
The Fig.5 shows the line scanning analysis byEDS along the red line in left first STEM image,and the analysis was performed for 4 elements that areOxygen,Aluminium,Gold,Silver.As a remarkable results,it could be detected that there is region whereOxygen peak and Al peak is high around crack.
Judging from existence of Aluminium oxidearound the crack,it was inferred that the crack is gen-erated by Al-Oxide.It’s considered that the reasonfor the oxide layer&crack occurrence is galvanic corrosion,Ag migration phenomena
2.3 TEM,EELS Analysis
Fig.6 shows TEM(Transmission Electron Mi-croscopy)image&oxygen mapping image by EELS(Electron Energy Loss Spectrometer)for the interfacebetween Au-15%Ag wire.The thickness of IMC wasobserved around 50～100 nm,and that of crack wasaround 10～30 nm.
As analyzed by EELS mapping,Oxygen elementis distributed along the crack region,therefore it couldbe acknowledged that the crack occurred by corrosionby this results again.
It was shown that the TEM&EELS analysis re-sults to investigate the influence of Pd element on in-terface between Au-15%Ag-5%Pd ball and Al metal-lization,after PCT 96 h in Fig.7.
Inspection of the TEM&EELS images revealthat it has thinner intermetallic compounds band andless Oxygen distribution compared with case withoutPd element.The thickness of IMC was measured tobe around 20～30 nm,no crack was observed inTEM image.
As a results,It was observed that the less inter-metallic compounds formation,no crack formation,lessOxygen region,in adding Pd element to Au-15%Ag.
3 Conclusions
3.1 The case of bonding Au-15%Ag to Al pad
(1)PCT degrades bond strength drastically.
(2)As a result of Al-oxide layer forming,the mi-cro crack is generated,it makes bondability degradation.
(3)It is considered that the reason for oxide layerforming&crack generation is galvanic corrosion,Agmigration phenomena under high temp.and humidity.
3.2 The case of bonding Au-15%Ag-5%Pd
ball to Al pad
(1)The addition of Pd in Au-15%Ag improves bondability degradation
after PCT considerably
(2)The addition of Pd induces less IMC forma-tion,and it prevents oxide layer
forming
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