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表面处理是印制电路板(Printed Circuit Board, PCB)制作过程中的关键工艺,能够有效防止焊盘表面氧化,提高PCB的可焊性。目前,PCB制作过程中常用的表面处理技术有热风整平、化学镀锡、化学镀镍、化学镍金和有机可焊保护剂(Organic Solderability Preservative, OSP)等[1-5]。其中,OSP具有工艺简单、价格低廉、性能优异和绿色环保等优点,因此,越来越受到关注[6-8]。
OSP主要成分包含成膜剂、有机酸、过渡金属离子和水等[9]。其中,成膜剂可以在铜焊盘表面生成一层有机化合物保护膜(OSP膜),对PCB的储存和焊接起着重要作用。PCB放置在自然环境中保存时,OSP膜可以阻挡焊盘与空气接触,防止焊盘氧化;另外,PCB在焊接时,助焊剂能够除去焊盘表面的OSP膜,使裸露的铜面与熔融的焊料快速结合,形成牢固的焊点。文献[10-12]提出咪唑类化合物是目前最常用的OSP成膜剂,且不同成膜剂生成的OSP膜在耐热性方面存在较大的差异。
近年来,随着RoHS标准的实施,PCB焊接工艺要求选用无铅焊料来取代锡–铅焊料,这使得焊接温度提高到约260 ℃[13]。此外,随着电子产品向着小型化、高集成度和高可靠性方向发展,表面贴装技术(Surface Mount Technology, SMT)和回流焊工艺在PCB制作中得到广泛应用。然而,对于SMT组装的PCB,至少需要经过两次回流焊才能完成元器件的装配,这使得焊接时间延长。针对无铅高温回流焊工艺,传统成膜剂生成的OSP膜耐热性较差,在焊接过程中容易分解,造成焊盘表面氧化,影响元器件的焊接性能[5, 14]。相比传统成膜剂,文献[8-9]提出苯基苯并咪唑类化合物作为成膜剂具有与铜面结合牢固,生成的OSP膜耐热性更好等优点。因此,研究苯基苯并咪唑类化合物耐高温有机可焊保护剂(High-Temperature-Resistant Organic Solderability Preservative, HT-OSP),对于克服无铅高温回流焊工艺具有重要意义。
2−[(2,4−二氯苯基)甲基]−1H−苯并咪唑(C14H10Cl2N2)是苯基苯并咪唑的一种衍生物。本研究选用C14H10Cl2N2分子作为HT-OSP成膜剂,研究了C14H10Cl2N2分子在铜层表面生成HT-OSP膜机理,分析了HT-OSP膜的抗氧化性和耐热性。
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2−[(2,4−二氯苯基)甲基]−1H−苯并咪唑(C14H10Cl2N2, 98.0%)购买于阿拉丁(上海)有限公司;甲酸(CH2O2, 99.0%)、乙酸(C2H4O2, 99.5%)和正庚酸(C7H14O2, 98.0%)购买于上海麦克林生化科技股份有限公司;乙酸铜(C4H6CuO4·H2O, 99.0%)购买于天津市瑞金特化学品有限公司;氨水(NH3·H2O, 25.0%~28.0%)购买于天津市华东试剂厂,用于调节HT-OSP的pH值。
HT-OSP(A)和HT-OSP(B)用作对比实验,其配方如表1所示。
表 1 HT-OSP(A)和HT-OSP(B)的配方
HT-OSP
标号试剂浓度/ mol·L−1 pH值
氨水C14H10Cl2N2 甲酸 乙酸 正庚酸 乙酸铜 A 0.01 0.45 1.0 0.02 ─ 3.0 B 0.01 0.45 1.0 0.02 0.01 3.0 -
铜原子外层电子排布为3d104s1,HT-OSP中成膜剂C14H10Cl2N2分子是一种常用络合剂[10],在其作用下Cu原子容易失去电子变为Cu+或Cu2+。借助量子化学计算去探究HT-OSP在铜层表面的成膜机理。选用密度泛函B3LYP方法,模拟HT-OSP中成膜剂C14H10Cl2N2分子与Cu+的络合反应[15-17],计算C14H10Cl2N2分子中不同基团与Cu+的吸附,对比分析C14H10Cl2N2分子与Cu+形成不同结构络合物的稳定性。络合物结构优化时,对于N、Cl、C和H元素选用6-311G+ (d, p)基组进行计算,Cu元素选用LANL2DZ基组进行计算[18-20]。
络合物中C14H10Cl2N2分子与Cu+之间的键能根据式(1)进行计算[21]:
$$ {{E}}_{\text{bonding}}\text={{E}}_{{\text{C}}_{\text{14}}{\text{H}}_{\text{10}}{\text{Cl}}_{\text{2}}{\text{N}}_{\text{2}}}\text+{{E}}_{{\text{Cu}}^{\text+}}-{{E}}_{\text{complex}} $$ (1) 式中,
$ {{E}}_{{\text{C}}_{\text{14}}{\text{H}}_{\text{10}}{\text{Cl}}_{\text{2}}{\text{N}}_{\text{2}}} $ 是C14H10Cl2N2分子的焓;$ {{E}}_{{\text{Cu}}^{\text{+}}} $ 是Cu+的焓;$ {{E}}_{\text{complex}} $ 是C14H10Cl2N2分子与Cu+形成络合物的焓。 -
图1是HT-OSP在双面覆铜板(3 cm × 5 cm)的铜层表面生成HT-OSP膜的工艺流程图,步骤如下。1)表面清洗:把覆铜板浸入盛有乙醇的烧杯中,室温条件下超声波清洗3 min,除去铜层表面的油渍;然后将覆铜板放入0.5 mol·L−1稀硫酸溶液中5 min,除去铜层表面的氧化层,表面清洗有助于在铜表层面生成均匀的HT-OSP膜。2)预浸:35 °C条件下,把覆铜板浸入YT-36预浸液3 min,促进HT-OSP在铜层表面快速生成HT-OSP膜。3)生成HT-OSP膜:45 °C条件下,把覆铜板浸入HT-OSP,成膜剂C14H10Cl2N2分子会与Cu原子发生化学反应,在铜层表面生成一层致密的HT-OSP膜。
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红外光谱仪(Thermo Fisher Scientific, Nicolet iS50)用来表征HT-OSP膜中的特征官能团;HT-OSP膜中Cu元素的化合价,选用X射线光电子能谱仪(THERMO ESCALAB 250XI)进行测试;台阶仪(Dektak XT)用来测量HT-OSP膜的厚度;扫描电子显微镜(HIEACHI S3400)用来表征HT-OSP膜的表面形貌;X射线衍射仪(PHILIPS X’PERT MPD)用来表征存放一定时间后铜层的成分;HT-OSP膜表面的粗糙度选用原子力显微镜(dimension ICON)进行测试;同步热分析仪(Mettler Toledo TGA-DSC-1)用来测试HT-OSP膜的分解温度。
Research on Coating Formation Mechanism of a High-Temperature-Resistant Organic Solderability Preservative and the Coating Performance in PCB
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摘要: 印制电路板铜焊盘表面生成耐高温有机可焊保护剂(HT-OSP)膜是克服无铅高温回流焊工艺并获得良好焊点的关键。选用2−[(2,4−二氯苯基)甲基]−1H−苯并咪唑(C14H10Cl2N2)作为成膜剂,在铜层表面生成了HT-OSP膜。理论计算结合对比实验,研究C14H10Cl2N2分子与Cu原子反应生成HT-OSP膜机理。基于量子化学密度泛函理论,模拟C14H10Cl2N2分子与Cu+之间的络合反应;利用红外光谱对HT-OSP膜中的特征官能团进行表征;借助X射线光电子能谱测试HT-OSP膜中Cu元素的化合价;设计对比实验分析Cu2+对生成HT-OSP膜的影响。结果表明:HT-OSP膜生成机理是C14H10Cl2N2分子与Cu原子发生反应生成HT-OSP膜并沉积在铜层表面,Cu2+通过络合反应促进HT-OSP膜生长。另外,HT-OSP膜的分解温度高达531 °C,HT-OSP膜保护的铜层放置在自然环境中180 天没有被氧化,证明HT-OSP膜具有优异的耐热性和抗氧化性。
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关键词:
- 表面处理技术 /
- 耐高温有机可焊保护剂 /
- 成膜机理 /
- 密度泛函理论 /
- 印制电路板
Abstract: To overcome lead-free high temperature reflow process and obtain satisfactory solder joints on printed circuit board, the key issue is to generate a High-Temperature-Resistant Organic Solderability Preservative (HT-OSP) coating on the surface of copper solder pads. Selecting C14H10Cl2N2 as a coating formation agent, a HT-OSP coating is formed on copper surface. Theoretical calculations combined with comparative experiments, coating formation mechanism of C14H10Cl2N2 molecules reacting with copper atoms generating the HT-OSP coating is investigated. Based on the density functional theory in quantum chemistry, the complexation reactions between C14H10Cl2N2 molecules and cuprous ions are simulated. The typical functional groups in the HT-OSP coating are characterized by infrared spectroscopy. The valences of copper element in the HT-OSP coating are tested by X-ray photoelectron spectroscopy. The influence of copper ions on forming HT-OSP coating is analyzed by designing comparative experiments. The results indicate that the mechanism of generating HT-OSP coating is as follows: C14H10Cl2N2 molecules react with copper atoms generating HT-OSP coating deposition on copper layer surface. Copper ions promote the HT-OSP coating growth by complexation reaction. Moreover, the decomposition temperature of the HT-OSP coating is up to 531 °C. The copper layer protected by the HT-OSP coating is not oxidized after 180 days in the atmospheric environment. It is proved that the HT-OSP coating possesses excellent antioxidant and heat resistance. -
表 1 HT-OSP(A)和HT-OSP(B)的配方
HT-OSP
标号试剂浓度/ mol·L−1 pH值
氨水C14H10Cl2N2 甲酸 乙酸 正庚酸 乙酸铜 A 0.01 0.45 1.0 0.02 ─ 3.0 B 0.01 0.45 1.0 0.02 0.01 3.0 -
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