劉海萍 *,畢四富,王堯

(1.哈爾濱工業(yè)大學(xué)(威海)海洋科學(xué)與技術(shù)學(xué)院,山東 威海 264209;2.哈爾濱工業(yè)大學(xué)(威海)材料科學(xué)與工程學(xué)院,山東 威海 264209)

【研究報(bào)告】

化學(xué)鍍鎳-磷合金層表面化學(xué)鍍金工藝及其性能

劉海萍1,*,畢四富2,王堯1

(1.哈爾濱工業(yè)大學(xué)(威海)海洋科學(xué)與技術(shù)學(xué)院,山東 威海 264209;2.哈爾濱工業(yè)大學(xué)(威海)材料科學(xué)與工程學(xué)院,山東 威海 264209)

采用化學(xué)鍍鎳-磷/化學(xué)鍍鈀/置換鍍金(ENEPIG)工藝獲得鎳/鈀/金組合鍍層,對(duì)比分析了它與化學(xué)鍍鎳/置換鍍金(ENIG)、化學(xué)鍍鎳/化學(xué)鍍金(ENEG)工藝的相關(guān)沉積特征及鍍層耐蝕性能.鍍金過(guò)程中開(kāi)路電位和沉積速率均發(fā)生明顯的變化,反映了基體電極表面狀態(tài)的變化.ENEG工藝的化學(xué)鍍金過(guò)程中的平臺(tái)電位最正,沉積速率最快.與ENIG工藝的置換鍍金相比,ENEPIG工藝中置換鍍金的平臺(tái)電位更正,對(duì)基體的腐蝕也更慢,所得置換鍍金層更致密,具有良好的耐腐蝕性能.綜合對(duì)比 ENIG、ENEG、ENEPIG工藝所得3種鍍層,ENEPIG工藝的鍍層性能最優(yōu).

化學(xué)鍍;置換;鎳-磷合金;金;鈀;耐蝕性

化學(xué)鍍鎳/置換鍍金(ENIG)鍍層具有優(yōu)良的耐蝕性、熱穩(wěn)定性和可焊性,在電子產(chǎn)品表面處理中得到廣泛應(yīng)用.但置換鍍金時(shí)容易對(duì)化學(xué)鍍鎳-磷合金鍍層造成過(guò)腐蝕,導(dǎo)致微電子產(chǎn)品后續(xù)焊接失效,這制約了ENIG技術(shù)的發(fā)展與應(yīng)用[1-3].為了減緩置換鍍金時(shí)對(duì)Ni-P合金的腐蝕,國(guó)內(nèi)外研究者進(jìn)行了許多工作.如采用耐腐蝕性較好的高磷化學(xué)鍍鎳-磷合金,采用中性、低溫的置換鍍金液,在置換鍍金液中添加硫脲等還原劑或聚乙烯亞胺類緩蝕劑[4-5],開(kāi)發(fā)半置換半還原的復(fù)合鍍金層[6-7],等等.這些方法雖然在一定程度上減輕了鎳-磷合金的腐蝕,但并不能杜絕上述問(wèn)題.

化學(xué)鍍鎳/化學(xué)鈀/置換鍍金技術(shù)(ENEPIG)是在化學(xué)鍍 Ni-P層與置換鍍金層之間增加化學(xué)鍍鈀的工藝.化學(xué)鍍鈀層一方面可以避免鍍金液對(duì)Ni-P合金的腐蝕,防止"黑盤"的發(fā)生;另一方面,鈀層作為阻擋層,能夠防止后續(xù)熱處理時(shí)Ni-P層與金層之間的擴(kuò)散,提高鋁線、金線的鍵合能力[8].因此,ENEPIG工藝具有良好的焊接可靠性,能夠滿足RoHS的無(wú)鉛焊接要求,被譽(yù)為"萬(wàn)能"鍍層,在微電子領(lǐng)域具有很好的應(yīng)用前景.本文采用課題組前期開(kāi)發(fā)的較穩(wěn)定的化學(xué)鍍鈀液,以純銅為基體,通過(guò)ENEPIG工藝獲得鎳/鈀/金組合鍍層,并對(duì)比分析了化學(xué)鍍鎳/置換鍍金、化學(xué)鍍鎳/化學(xué)鍍金等的相關(guān)沉積特征及鍍層性能.

1 實(shí)驗(yàn)

1.1 化學(xué)鍍工藝

以20 mm X 20 mm的純銅片為基體,依次進(jìn)行酸洗、微刻蝕、預(yù)浸、活化后化學(xué)鍍Ni-P合金4 ～ 5 μm[6],再分別進(jìn)行置換鍍金(IG)、化學(xué)鍍鈀/置換鍍金(EPIG)、化學(xué)鍍金(EG),分別得到 ENIG、ENEPIG 和ENEG鍍金試樣.

置換鍍金的配方和工藝為:亞硫酸金鈉2 g/L,亞硫酸鈉15 g/L,硫代硫酸鈉17 g/L,配位劑3 g/L,添加劑 50 mg/L,溫度(80 ± 2) °C,pH 7.0.

化學(xué)鍍金液除了增加2 g/L硫脲(還原劑)外,其余參數(shù)與置換鍍金相同.

化學(xué)鍍鈀的配方和工藝為:硫酸鈀2 g/L,硫脲2 mg/L,乙二胺四乙酸20 g/L,次磷酸鈉15 g/L,磷酸二氫鈉12 g/L,溫度60 °C,pH 7.0.

1.2 性能表征

使用上海辰華CHI604E電化學(xué)工作站測(cè)量鍍金過(guò)程中開(kāi)路電位(OCP)隨時(shí)間的變化.研究電極為化學(xué)鍍Ni-P、Ni-P/Pd的銅片(工作面積為1 cm2),輔助電極為鉑電極,參比電極為飽和甘汞電極(SCE).

使用德國(guó)Bruker AXS S4Explorer型X射線熒光光譜儀(XRF)測(cè)量金層厚度,計(jì)算鍍金速率.目視觀察鍍層的外觀和色澤,以德國(guó)Zeiss MERLIN Compact型掃描電子顯微鏡(SEM)觀察鍍層的微觀形貌.

分別采用潤(rùn)濕角法(3.5% NaCl溶液)及塔菲爾(Tafel)曲線測(cè)量,比較不同鍍金試樣的耐蝕性.測(cè)Tafel曲線前,將待測(cè)試片放入丙酮中浸泡5 min,以去除鍍層表面油污,再在3.5% NaCl溶液中浸泡15 min以平衡其電極電位,將此平衡電位作為開(kāi)路電位,以5 mV/s的速率在開(kāi)路電位的± 300 mV范圍內(nèi)掃描.根據(jù)式(1)和式(2)計(jì)算鍍金層的孔隙率ρ.

式中,Rp為極化電阻(單位:Ω.cm2),Rpm為基體的極化電阻(單位:Ω.cm2),Δφ為鍍層與基體之間的電位差(單位:V),jcorr為鍍層的腐蝕電流密度(單位:μA/cm2),βa為陽(yáng)極 Tafel斜率,βc為陰極的 Tafel斜率.

2 結(jié)果與討論

2.1 鍍金時(shí)開(kāi)路電位的變化特征

由圖1可知,Ni-P和Ni-P/Pd合金表面金沉積過(guò)程的開(kāi)路電位隨時(shí)間的變化規(guī)律基本相似.隨金沉積過(guò)程的進(jìn)行,開(kāi)路電位先正移,最后達(dá)到基本穩(wěn)定的平臺(tái)值.ENEG工藝對(duì)應(yīng)的平臺(tái)電位最正,約為-0.16 V,ENEPIG、ENIG的平臺(tái)電位分別在-0.23 V和-0.30 V左右.ENEG工藝到達(dá)平臺(tái)電位所需時(shí)間最短,其次為 ENEPIG.將待鍍電極浸入鍍金液后會(huì)發(fā)生 Au+的還原沉積反應(yīng),使基體表面逐漸被金層覆蓋,導(dǎo)致電極電位正移.到達(dá)平臺(tái)電位的時(shí)間越短,表明基體被金層完全覆蓋所需時(shí)間越短,而平臺(tái)電位越正,則意味著金層的覆蓋率越高[5].因此,在Ni-P合金上化學(xué)鍍金時(shí)所得金層覆蓋率比置換鍍金層要高;而在Ni-P合金上先進(jìn)行化學(xué)鍍鈀也有利于提高金層在Ni-P合金表面的覆蓋率,從而有助于改善Ni-P合金的不均勻腐蝕等問(wèn)題.

從圖2可知,3種工藝的鍍金速率在初始階段都較快,隨沉積時(shí)間的延長(zhǎng)而逐漸降低.沉積時(shí)間相同時(shí),ENEG工藝的鍍金速率最高,ENEPIG工藝的鍍金速率最慢,這與圖1的結(jié)果吻合.一方面,ENEG工藝中的鍍金類型為化學(xué)鍍金,鍍金液中還原劑的存在增強(qiáng)了鍍金液的還原能力;另一方面,因Ni/Ni2+與Au/Au+之間的電極電位相差較大,因此ENEG工藝的初始階段也存在置換鍍金過(guò)程.因此,ENEG工藝的鍍金速率最快.ENIG與ENEPIG兩種工藝都是采用置換鍍金,其驅(qū)動(dòng)力為金屬間電位差,Ni/Ni2+與Au/Au+之間的電位差明顯大于 Pd/Pd2+與Au/Au+之間的電位差,因此 ENIG工藝的鍍金速率比 ENEPIG工藝的鍍金速率大.這也表明,與ENIG的置換鍍金相比,ENEPIG工藝中的置換鍍金過(guò)程對(duì)基體的腐蝕較小,造成基體過(guò)腐蝕的可能性較低.

圖1 Ni-P和Ni-P/Pd合金表面化學(xué)鍍金或置換鍍金的開(kāi)路電位-時(shí)間曲線Figure 1 Open circuit potential vs.time curves for electroless or immersion gold plating on the surface of Ni-P or Ni-P/Pd alloy coating

圖2 ENIG、ENEPIG和ENEG工藝的鍍金速率隨時(shí)間的變化Figure 2 Variation of gold plating rate with time in ENIG,ENEPIG and ENEG processes

2.2 微觀形貌

由圖3可知,采用不同工藝制備的鍍金層均為瘤狀結(jié)構(gòu),結(jié)構(gòu)致密.

圖3 ENIG、ENEPIG和ENEG工藝鍍金層的表面形貌Figure 3 Surface morphologies of gold coatings obtained by ENIG, ENEPIG and ENEG, respectively

2.3 耐蝕性

將3.5% NaCl溶液滴在不同鍍層表面測(cè)其潤(rùn)濕角,結(jié)果見(jiàn)圖4.從圖4可知,NaCl液滴在Ni-P鍍層表面的潤(rùn)濕角為48.92°,在ENIG、ENEG和ENEPIG鍍層表面的潤(rùn)濕角則分別為52.72°、65.88°和77.69°.一般而言,潤(rùn)濕角越大,表明 NaCl液滴在鍍層表面的吸附性越差[1],鍍層的耐蝕性越好.因此,根據(jù)NaCl液滴在不同試樣表面的潤(rùn)濕角可以初步推斷 ENEPIG金鍍層的耐蝕性最好,ENEG金鍍層次之,ENIG金鍍層最差.

圖4 NaCl液滴在Ni-P合金鍍層和ENIG、ENEPIG和ENEG工藝鍍金層表面的鋪展情況Figure 4 Spreading of NaCl droplet on Ni-P alloy coating and gold coatings obtained by ENIG, ENEG and ENEPIG respectively

由圖5及表1可知,與Ni-P合金鍍層相比,ENIG、ENEPIG、ENIEG工藝鍍金層在3.5% NaCl溶液中的腐蝕電位均較正,腐蝕電流密度均較低,表明這3種鍍金工藝均可以提高Ni-P合金的耐蝕性.此外,3種工藝鍍金層中,ENEPIG工藝鍍金層的腐蝕電位最正,腐蝕電流密度和孔隙率最低;ENIG工藝鍍金層的腐蝕電位最負(fù),腐蝕電流密度和孔隙率最高.這同樣表明ENEPIG工藝鍍金層的耐蝕性最好,ENIG鍍層的耐蝕性較差.

圖5 不同鍍層在3.5% NaCl溶液中的塔菲爾曲線Figure 5 Tafel curves of different coatings in 3.5% NaCl solution

表1 不同鍍層的腐蝕參數(shù)Table 1 Corrosion parameters of different coatings

3 結(jié)論

通過(guò)對(duì)化學(xué)鍍Ni-P合金層直接置換鍍金(ENIG)、化學(xué)鍍鈀后再置換鍍金(ENEPIG)和直接化學(xué)鍍金(ENEG)這3種過(guò)程的研究,得出以下結(jié)論:

(1) 采用ENEG工藝時(shí),由于化學(xué)鍍金液中含有還原劑硫脲,其鍍金速率比ENIG工藝快.

(2) 采用ENEPIG工藝時(shí),鍍金過(guò)程的開(kāi)路電位比采用ENIG工藝時(shí)更正,說(shuō)明置換鍍金液對(duì)基體的腐蝕速率明顯降低.

(3) 在3種鍍金工藝中,EPEING工藝所得置換鍍金層最致密,孔隙率最小,耐蝕性最優(yōu).

[1] HO C E, FAN C W, HSIEH C W.Pronounced effects of Ni(P) thickness on the interfacial reaction and high impact resistance of the solder/Au/Pd(P)/Ni(P)/Cu reactive system [J].Surface and Coatings Technology, 2014, 259: 244-251.

[2] MD ARSHAD M K, JALAR A, AHMAD I.Characterization of parasitic residual deposition on passivation layer in electroless nickel immersion gold process [J].Microelectronics Reliability, 2007, 47 (7): 1120-1126.

[3] LIN C P, CHEN C M.Solid-state interfacial reactions at the solder joints employing Au/Pd/Ni and Au/Ni as the surface finish metallizations [J].Microelectronics Reliability, 2012, 52 (2): 385-390.

[4] 劉海萍, 李寧, 畢四富.聚乙烯亞胺對(duì)置換鍍金過(guò)程中鎳基體腐蝕的影響[J].稀有金屬材料與工程, 2009, 38 (6): 1087-1090.

[5] LIU H P, LI N, BI S F, et al.Gold immersion deposition on electroless nickel substrates: the deposition process and the influence factor analysis [J].Journal of the Electrochemical Society, 2007, 154 (12): D662-D668.

[6] KANZLER M.Plating method: US6911230 [P].2005-06-28.

[7] WON Y S, PARK S S, LEE J, et al.The pH effect on black spots in surface finish: electroless nickel immersion gold [J].Applied Surface Science, 2010,257 (1): 56-61.

[8] TECK NG B, GANESH VP, LEE C.Impact of electroless nickel/palladium/immersion gold plating on gold ball bond reliability [C] // 2006 International Conference on Electronic Materials and Packaging.[S.l.: s.n.], 2006: 9858124.

[9] WANG Y, LIU H P, BI S F, et al.Effects of organic additives on the immersion gold depositing from a sulfite-thiosulfate solution in an electroless nickel immersion gold process [J].RSC Advances, 2016 (12): 9656-9662.

[10] NAM N D, BUI Q V, NHAN H T, et al.Effect of Pd interlayer on electrochemical properties of ENIG surface finish in 3.5wt.% NaCl solution [J].Journal of Electronic Materials, 2014, 43 (9): 3307-3316.

Electroless gold plating on electrolessly plated nickel-phosphate alloy coating and the properties of gold coatings

LIU Hai-ping*, BI Si-fu, WANG Yao

A nickel/palladium/gold composite coating was obtained by electroless nickel plating followed by electroless palladium plating and immersion gold plating (ENEPIG) successively.The related deposition properties of ENEPIG, ENIG(electroless nickel plating/immersion gold plating) and ENEG (electroless nickel/gold plating), as well as the corrosion resistance of their coatings were compared.Both open circuit potential and deposition rate during the gold plating change obviously, which reflects the change on the surface of substrate.The electroless gold plating in ENEG process has the highest potential platform and deposition rate.Compared with the immersion gold plating in ENIG process, the immersion gold plating in ENEPIG process has a higher potential platform and a lower corrosion rate of substrate.The obtained immersion gold coating is more compact and resistant to corrosion.It is found that the coating obtained by ENEPIG process has the best performances through a comprehensive comparison among the three kinds of coatings obtained by ENIG, ENEG and ENEPIG processes.

electroless plating; replacement; nickel-phosphate alloy; gold; palladium; corrosion resistance

TQ153.2; TG150

A

1007 - 227X (2017) 19 - 1025 - 04

10.19289/j.1004-227x.2017.19.001

First-author's address:School of Marine Science and Technology, Harbin Institute of Technology, Weihai 264209,China

2017-07-11

2017-10-12

劉海萍(1975-),女,山東汶上人,博士,副教授,主要研究方向?yàn)殡娀瘜W(xué)表面改性及化學(xué)電源等.

作者聯(lián)系方式:(E-mail) hpliuhit@126.com.

[ 編輯:周新莉 ]