International Journal of Adhesion&Adhesives22(2002)471–475
Thermal graft copolymerization of4-vinyl pyridine on polyimide
to improve adhesion to copper
M.B.Chan-Park a,*,S.S.Tan b
a School of Mechanical and Production Engineering,Nanyang Technological University,50Nanyang Avenue,Singapore639798,Singapore
b Megachem Pte Ltd.,132Pioneer Road,Jurong Town,Singapore639588,Singapore
Accepted15June2002
Abstract
The surface of poly?N;N0-(oxydiphenylene)pyromellitimide]?lm,Kapton s HN,was modi?ed to improve its adhesion to copper metal.The polyimide surface was argon plasma activated and then exposed to air.A nitrogen-containing monomer,4-vinyl pyridine,was then polymerized at elevated temperature under constant pressure between the argon plasma activated polyimide?lm and copper foil without any added photoinitiator.Optimization of the argon pretreatment time,curing temperature and curing duration resulted in almost doubling of the single lapshear strength.It is p ostulated that failure occurred mainly between the polyimide and the poly(4-vinyl pyridine).
r2002Elsevier Science Ltd.All rights reserved.
Keywords:B.Plasma;Polyimide;Adhesion;Graft
1.Introduction
Polyimides(PI)are widely used in the microelec-
tronics packaging industry,especially in printed circuit
boards(PCBs),because of their superior mechanical
properties,high-temperature resistance,solvent resis-
tance and low dielectric resistance[1–3].Copper is used
as the metal in PCBs due to its excellent conductivity
and low electromigration property.In making?exible
PCBs,copper is coated onto polyimides by physical
vapor deposition techniques such as vacuum evapora-
tion,metal sputtering or ion sputtering,etc.or
laminated over the copper.The polyimide-copper
laminate so formed has poor adhesion strength,which
is a key issue to be solved currently.A great deal of
effort has been devoted to the surface modi?cation of
polyimides for improvement of their adhesion to metals
[4–9].
Chou and Tang[10]investigated the cause of poor
adhesion between polyimide and copper.They postu-
lated that a metal–oxygen–carbon complex was formed
at the polyimide?lm/metal interface in the metallization
process and this complex dictated the adhesion strength.
The reactivity of copper metal in the formation of the
complex is lower than that of chromium and titanium
metals.This conclusion suggests the importance of a
special functional group that can strongly interact with
copper metal.
Xue et al.[11]reported that the imino groups of
benzimidazole interacted with copper metal at zero
oxidation state to form a complex.Lee et al.[12]
evaluated the simultaneous passivation and adhesion by
incorporating triazole or imidazole functional group
into the polyimide to improve lamination with copper.
Ranby et al.[13,14]demonstrated that initiator-pro-
moted surface graft copolymerization,when carried out
at the surface between two contacting polymer?lms,
was accompanied by the simultaneous lamination of the
polymer?lms.Kang et al.[8,9]showed that the
adhesion of polyimides and copper was improved by
argon plasma activated thermal copolymerization of
1-vinyl imidazole(VIDZ)on PI?lm with simultaneous
lamination.The imino groups of the grafted VIDZ
polymer form charge-transfer complexes with copper
metal,thus enhancing the adhesion strength.Yang et al.
[15]have also recently shown that a poly(4-vinyl
pyridine)layer formed by argon plasma activated UV
copolymerization on PI improved adhesion of copper to
PI.The copper was deposited by electroless plating.
*Corresponding author.Tel.:+65-6790-6064;fax:+65-6791-1859.
E-mail address:mbechan@https://www.doczj.com/doc/0b15470062.html,.sg(M.B.Chan-Park).
0143-7496/02/$-see front matter r2002Elsevier Science Ltd.All rights reserved.
PII:S0143-7496(02)00057-X
In this paper,we explore the effectiveness of thermal graft copolymerization of 4-vinyl pyridine on polyimide ?lm with simultaneous lamination to copper foils.We thermally graft,rather than UV graft the 4-vinyl pyridine monomer onto the argon plasma activated surface.We laminate rather than electroless plate the copper.The simultaneous copolymerization and lami-nation offer a convenient method to make the PI/Cu laminate.
2.Materials and methods
The polyimide (PI)?lm used in the present study was poly ?N ;N 0-(oxydiphenylene)pyromellitimide]with the chemical structures shown in Fig.1(a).It was obtained from Dupont Singapore as Kapton s HN in sheets with thickness of 50m m :The copper used was Olin alloy C7025(96.2%copper)supplied as foils with thickness of 150m m by Olin Corporation.The monomer,4-vinyl pyridine (4VP)with the chemical structure shown in Fig.1(b),was bought from Fluka and used as received.The polyimide sheets were cut into strips of 15?6mm using a sharpknife.The sp ecimens were cleaned with distilled water in an ultrasonic bath for 1min :They were then rinsed with large amounts of distilled water followed by acetone before being dried in a vacuum oven for 5min :These were then ready for use.Copper foils were cut into strips of 15?4mm using a sharp knife.The surface of the copper foil was cleaned with a mixture of aqueous 0:1M HCl =0:1M H 2SO 4=0:1M HNO 3in an ultrasonic bath for 1min :The foils were then rinsed thoroughly with distilled water followed by acetone.The copper speci-mens were then dried in a vacuum oven for 5min :These were then ready for use.
The plasma graft polymerization of 4-vinyl pyridine at the Kapton s HN ?lm surface was carried out in two steps:hydroperoxide generation followed by thermal grafting.The Kapton s HN strips were pretreated with argon before thermal graft copolymerization.The argon plasma pretreatment was carried out in an Anatech SP-100plasma system,equipped with a cylindrical quartz reactor chamber.The glow discharge was produced at a plasma power of 35W and an argon pressure of 0:6Torr :The duration of the glow discharge pretreat-ment was set at between 0and 60s :The plasma-
pretreated ?lms were subsequently exposed to the atmosphere for between 5min to nearly 3h for the generation of surface peroxide and hydroxyl peroxide species [16].
Strips of copper measuring 15?4mm and Kapton s HN measuring 15?6mm were overlapped by 1–2mm as shown in Fig.2.A small quantity of pure 4VP monomer was introduced between the Kapton s HN ?lm and the copper foil at the overlap.Thermal graft copolymerization with concurrent lamination to copper was achieved by heating the PI/4VP/copper assembly under a constant small load of about 5kg =cm 2:The samples were introduced into the oven after the preset curing temperature was reached.The curing tempera-tures investigated were 251C ;1001C ;1101C ;1201C ;1301C and 1501C while the curing time and plasma time were kept constant at 2h and 10s ;respectively.In another set of experiments,the curing time was varied from 1to 5h while keeping the curing temperature at 1201C and plasma time at 10s ;respectively.After the predetermined curing time,the samples were left in the oven to cool slowly.The single lapshear strength was measured with a Zwick 1445Universal Testing Machine with a load cell of 2kN :All measurements were performed at a crosshead speed of 10mm =min :
X-ray photoelectron spectroscopy (XPS)measure-ments were carried out on a VG ESCALAB 2201-Xl spectrometer with a non-monochromatized Mg K a X-ray source (1253:6eV photon).The Kapton s HN ?lm and Cu foil were mounted on the standard sample studs by means of double-sided adhesive tape.They were positioned at a take-off angle of 901with respect to the detector.The X-ray source was operated at a reduced power of 300W (15kV and 20mA).The operating pressure in the analysis chamber was main-tained at 1?10à9Torr or lower during the measure-ments.All binding energies (BEs)were referenced to the C 1s neutral carbon peak at 284:6eV ;in order to compensate for the effects of charging.
3.Results and discussion
Fig.3shows the single lapshear strength of the PI/4VP/Cu assembly,with thermal grafting and
lamination
Fig.1.Chemical structures of (a)poly ?N ;N 0-(oxydiphenylene)pyr-omellitimide]and (b)4-vinyl
pyridine.
Fig.2.Sample preparation for single lap shear test.
M.B.Chan-Park,S.S.Tan /International Journal of Adhesion &Adhesives 22(2002)471–475
472
carried out at 1201C for 2h ;as a function of the Ar plasma treatment time of the PI ?lm.With no plasma treatment,a lapshear strength of 4:67N =mm 2was observed.A maximum lapshear adhesion strength of 8:01N =mm 2was obtained for the bond with 30s of Ar plasma pretreatment time of PI.An increase in the Ar plasma pretreatment time beyond 30s did not result in further increase in the lapshear adhesion strength.The presence of a ?nite adhesion between the untreated PI ?lm and Cu indicates that poly(4VP)homopolymer at the joint also contributed to the observed adhesion.Fig.4summarizes the dependence of the lap shear adhesion strength of PI/4VP/Cu assemblies on the curing temperature of the thermal graft copolymeriza-tion/lamination;the Ar plasma pretreatment time was kept at 10s and the thermal grafting/lamination time at 2h :It appears that higher temperatures of up to 1501C results in increased lapshear adhesion strength.The lap shear strength increased from 5:95N =mm 2at 251C to 8:01N =mm 2at 1501C :In other studies using 1-vinyl imidazole (VIDZ)[8],an optimum lamination tempera-ture of about 1201C for 4h resulted in maximum T-peel strength of the PI/VIDZ/Cu assembly.Beyond this optimum temperature,degradation of the poly(VIDZ)occurred.This is not the case here:the curing temperature of 1501C for 2h did not cause any degradation.Increase in lamination temperature in-creases the degree of conversion resulting in higher
molecular weight poly(4VP)and also increased diffusion of 4VP monomer into PI.The latter will result in an interpenetrating network of poly(4VP)and PI.Finally,Fig.5shows the effect of thermal grafting/lamination time on the observed lapshear adhesion strength of the PI/4VP/Cu assemblies;the Ar plasma pretreatment time of the PI ?lms was kept at 10s and the lamination carried out at 1201C :There is only a small increase in lapshear strength with increase in duration of the thermal grafting/lamination time.From the above,a set of bondings was done at the optimized plasma duration of 30s :The thermal grafting/lamination was done at 1401C for 4h :The shear strength achieved was 8:50N =mm 2which was signi?cantly improved over the value of 4:67N =mm 2with unoptimized conditions.The failure mode of this PI/4VP/Cu assembly with high shear strength was brie?y investigated by XPS.Table 1summarizes the atomic composition of the surface of the layers peeled off from lap joint.For both the PI ?lm side layer and copper metal side layer peeled off from the lapjoint,small amounts of cop p er were detected indicating some cohesive failure within the copper.Diffusion of copper into the polymer as observed by others using a higher lamination tempera-ture of 4001C [17]is unlikely here since the bonding temperature used was only 1401C :The amount of copper detected on the PI side is small (the sensitivity factor of Cu2p3peak is 15.87times larger than that of C1s
peak).
Table 1
Atomic composition of the parts peeled off from the adhesive joint between the graft-polymerized Kapton ?lm and copper metal
Atomic composition at failure surface N/C
O/C Copper side 0.190.16Polyimide side
0.090.14Pure kapton (reference)0.090.23Pure poly(4VP)(reference)
0.14
M.B.Chan-Park,S.S.Tan /International Journal of Adhesion &Adhesives 22(2002)471–475473
Some N-containing organics appear to be left on the copper side indicating either cohesive failure within PI or adhesive failure within the poly(4VP);organic contamination rarely contains nitrogen.The N/C and O/C atomic ratios at the surfaces of the peeled-off layers can be used to determine the failure mode[17].The Kapton?lm should have N/C and O/C atomic ratios of 0.091and0.227and the grafted poly(4VP)0.14and0, respectively.The N/C and O/C ratios of the organics on the PI?lm side layer appear closer to those of pure PI.The N/C ratio of the organics left on the copper?lm is closer to that of poly(4VP).The higher O/C value of the organics on the Cu?lm could be due to contaminant.It is postulated that the failure is partially within the copper but mostly at the interface of PI and poly(4VP). Ideally,Ar plasma treatment of PI results in the generation of radicals on the surface with the subse-quent exposure to air resulting in the formation of various oxygen-containing species,such as peroxides and hydroperoxides[18].The peroxide species can
be
utilized to initiate the surface free-radical thermal polymerization of4-vinyl pyridine[15].Plasma treat-ment can also result in the cleavage of the imide linkages and this will give rise to the formation of carboxyl and secondary amine groups[18].Kang and co-workers[15] observed that only small amount surface grafting of4VP took place on the argon plasma treated polyimide.This coincides with our observation that the failure of the PI/ 4VP/Cu assembly is at the PI and poly(4VP)interface. This also indicates that the interaction of4VP with copper is strong.Fig.6.
4.Conclusions
Improved adhesion between a polyimide?lm and copper foil can be achieved by direct thermal graft copolymerization of4-vinyl pyridine onto PI?lm with simultaneous lamination to the copper foil under atmospheric conditions and in the absence of a polymerization initiator.The adhesion strengths are enhanced by argon plasma pretreatment of the PI?lm prior to grafting and lamination.The single lap shear strength of the copper to polyimide bond with poly(4-vinyl pyridine)as the adhesive was increased from4.67 to8:50N=mm2using the optimized condition of argon plasma pretreatment time of30s;curing temperature of 1401C and curing time of4h:The failure mode is postulated to be partially in the copper but mainly between the polyimide and the poly(4VP).References
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统计热力学基础 一、选择题 1. 下面有关统计热力学的描述，正确的是：( ) A. 统计热力学研究的是大量分子的微观平衡体系 B. 统计热力学研究的是大量分子的宏观平衡体系 C. 统计热力学是热力学的理论基础 D. 统计热力学和热力学是相互独立互不相关的两门学科B 2. 在研究N、V、U有确定值的粒子体系的统计分布时，令刀n i = N,刀n i & i = U , 这是因为所研究的体系是：( ) A. 体系是封闭的，粒子是独立的 B 体系是孤立的，粒子是相依的 C. 体系是孤立的，粒子是独立的 D. 体系是封闭的，粒子是相依的C 3. 假定某种分子的许可能级是0、&、2 ￡和3 &,简并度分别为1、1、2、3四个这样的分子构成的定域体系，其总能量为3￡时，体系的微观状态数为：() A. 40 B. 24 C. 20 D. 28 A 4. 使用麦克斯韦-波尔兹曼分布定律，要求粒子数N 很大，这是因为在推出该定律时：( ) . 假定粒子是可别的 B. 应用了斯特林近似公式 C. 忽略了粒子之间的相互作用 D. 应用拉氏待定乘因子法A 5. 对于玻尔兹曼分布定律n i =(N/q) ? g i ? exp( - ￡ i/kT)的说法：(1) n i是第i能级上的粒子分布数; (2) 随着能级升高，￡ i 增大，n i 总是减少的; (3) 它只适用于可区分的独立粒子体系; (4) 它适用于任何的大量粒子体系其中正确的是：( ) A. (1)(3) B. (3)(4) C. (1)(2) D. (2)(4) C 6. 对于分布在某一能级￡ i上的粒子数n i，下列说法中正确是：() A. n i 与能级的简并度无关 B. ￡ i 值越小，n i 值就越大 C. n i 称为一种分布 D. 任何分布的n i 都可以用波尔兹曼分布公式求出B 7. 15?在已知温度T时，某种粒子的能级￡ j = 2 ￡ i，简并度g i = 2g j，则「和￡ i上 分布的粒子数之比为：( ) A. 0.5exp( j/2￡kT) B. 2exp(- ￡j/2kT) C. 0.5exp( -￡j/kT) D. 2exp( 2 j/k￡T) C 8. I2的振动特征温度? v= 307K，相邻两振动能级上粒子数之n(v + 1)/n(v) = 1/2的温度是：( ) A. 306 K B. 443 K C. 760 K D. 556 K B 9. 下面哪组热力学性质的配分函数表达式与体系中粒子的可别与否无关：( ) A. S、G、F、C v B. U、H、P、C v C. G、F、H、U D. S、U、H、G B 10. 分子运动的振动特征温度?v是物质的重要性质之一，下列正确的说法是： ( ) A. ? v越高，表示温度越高 B. ?v越高，表示分子振动能越小 C. ?越高，表示分子处于激发态的百分数越小 D. ?越高，表示分子处于基态的百分数越小 C 11. 下列几种运动中哪些运动对热力学函数G与
统计热力学基础 题 择 一、选 1. 下面有关统计热力学的描述，正确的是：( ) A. 统计热力学研究的是大量分子的微观平衡体系 B. 统计热力学研究的是大量分子的宏观平衡体系 C. 统计热力学是热力学的理论基础 D. 统计热力学和热力学是相互独立互不相关的两门学科B 2.在研究N、V、U 有确定值的粒子体系的统计分布时，令∑n i = N，∑n iεi = U， 3.这是因为所研究的体系是：( ) A. 体系是封闭的，粒子是独立的 B 体系是孤立的，粒子是相依的 C. 体系是孤立的，粒子是独立的 D. 体系是封闭的，粒子是相依的 C 4.假定某种分子的许可能级是0、ε、2ε和3ε，简并度分别为1、1、2、3 四个这样的分子构成的定域体系，其总能量为3ε时，体系的微观状态数为：( ) A. 40 B. 24 C. 20 D. 28 A 5. 使用麦克斯韦-波尔兹曼分布定律，要求粒子数N 很大，这是因为在推出该定律 6.时：( ) . 假定粒子是可别的 B. 应用了斯特林近似公式 C. 忽略了粒子之间的相互作用 D. 应用拉氏待定乘因子法 A 7.对于玻尔兹曼分布定律n i =(N/q) ·g i·exp( -εi/kT)的说法：(1) n i 是第i 能级上的 粒子分布数; (2) 随着能级升高，εi 增大，n i 总是减少的; (3) 它只适用于可区分的独 8.立粒子体系; (4) 它适用于任何的大量粒子体系其中正确的是：( ) A. (1)(3) B. (3)(4) C. (1)(2) D. (2)(4) C 9.对于分布在某一能级εi 上的粒子数n i ，下列说法中正确是：( ) 10.A. n i 与能级的简并度无关 B. εi 值越小，n i 值就越大 C. n i 称为一种分布 D.任何分布的n i 都可以用波尔兹曼分布公式求出 B 11. 15．在已知温度T 时，某种粒子的能级εj = 2εi，简并度g i = 2g j，则εj 和εi 上分布的粒子数之比为：( ) A. 0.5exp( j/2εk T) B. 2exp(- εj/2kT) C. 0.5exp( -εj/kT) D. 2exp( 2 j/kεT) C 12. I2 的振动特征温度Θv= 307K，相邻两振动能级上粒子数之n(v + 1)/n(v) = 1/2 的温度 13.是：( ) A. 306 K B. 443 K C. 760 K D. 556 K B 14.下面哪组热力学性质的配分函数表达式与体系中粒子的可别与否无关：( ) A. S、G、F、C v B. U、H、P、C v C. G、F、H、U D. S、U、H、G B 15. 分子运动的振动特征温度Θv 是物质的重要性质之一，下列正确的说法是： ( ) A.Θv 越高，表示温度越高 B.Θv 越高，表示分子振动能越小 C. Θv 越高，表示分子处于激发态的百分数越小 D. Θv 越高，表示分子处于基态的百分数越小 C 16.下列几种运动中哪些运动对热力学函数G 与A 贡献是不同的：( ) A. 转动运动 B. 电子运动 C. 振动运动 D. 平动运动 D 17.三维平动子的平动能为εt = 7h 2 /(4mV2/ 3 )，能级的简并度为：( )
第七章统计热力学基础 一、单选题 1．统计热力学主要研究（）。 (A) 平衡体系(B) 近平衡体系(C) 非平衡体系 (D) 耗散结构(E) 单个粒子的行为 2．体系的微观性质和宏观性质是通过（）联系起来的。 (A) 热力学(B) 化学动力学(C) 统计力学(D) 经典力学(E) 量子力学 3．统计热力学研究的主要对象是：（） (A) 微观粒子的各种变化规律(B) 宏观体系的各种性质 (C) 微观粒子的运动规律(D) 宏观系统的平衡性质 (E) 体系的宏观性质与微观结构的关系 4．下述诸体系中，属独粒子体系的是：（） (A) 纯液体(B) 理想液态溶液(C) 理想的原子晶体 (D) 理想气体(E) 真实气体 5．对于一个U，N，V确定的体系，其微观状态数最大的分布就是最可几分布，得出这一结论的理论依据是：（） (A) 玻兹曼分布定律(B) 等几率假设(C) 分子运动论 (D) 统计学原理(E) 能量均分原理
6．在台称上有7个砝码，质量分别为1g、2g、5g、10g、50g、100g，则能够称量的质量共有：（） (A) 5040 种(B) 127 种(C) 106 种(D) 126 种 7．在节目单上共有20个节目序号，只知其中独唱节目和独舞节目各占10个，每人可以在节目单上任意挑选两个不同的节目序号，则两次都选上独唱节目的几率是：（） (A) 9/38 (B) 1/4 (C) 1/180 (D) 10/38 8．以0到9这十个数字组成不重复的三位数共有（） (A) 648个(B) 720个(C) 504个(D) 495个 9．各种不同运动状态的能级间隔是不同的，对于同一种气体分子，其平动、转动、振动和电子运动的能级间隔的大小顺序是：（） (A)△e t >△e r >△e v >△e e(B)△e t <△e r <△e v <△e e (C) △e e >△e v >△e t >△e r(D)△e v >△e e >△e t >△e r (E)△e r >△e t >△e e >△e v 10．在统计热力学中，对物系的分类按其组成的粒子能否被分辨来进行，按此原则：（） (A) 气体和晶体皆属定域子体系(C) 气体属离域子体系而晶体属定域子体系 (B) 气体和晶体皆属离域子体系(D) 气体属定域子体系而晶体属离域子体系 11．对于定位系统分布X所拥有的微观状态t x为：（B） (A)(B)