【作者】 马丽丽；

【导师】 包生祥；

【作者基本信息】 电子科技大学 ， 材料学， 2011， 博士

【摘要】 电子产品正朝着微型化、多样化和高性能迅速发展,人们的健康和环境保护意识也在不断提高,对电子产品绿色化的呼声也越来越高。随着欧盟三大指令(RoHS、WEEE、REACH)的相继实施,印制电路板(PCB)作为电子产品重要的组成部分,也围绕着无铅化、无卤化和产品的循环再利用化三个方向展开了绿色化的进程。新型无卤PCB的可靠性问题是开发、研究工作的重点之一。鉴于无卤PCB尚属新材料,其可靠性相关资料较少,本文围绕新型无卤PCB是否可以代替传统含卤PCB,是否适宜无铅焊接等问题,从三个方面——实际印制板制造工艺、常规加湿试验和无铅焊接温度循环,研究了PCB材料的可靠性。论文的主要研究内容和成果归纳如下:1.目前可参考文献大多只报道某种无卤PCB的优异性能,不利于PCB设计开发工作者选用材料。本文采用统一的IPC及其他相关PCB材料测试标准,对多种无卤PCB芯板材料的性能进行了分析对比,并对无卤材料进行了整体评价。研究表明,各新型无卤材料的电性能、热性能和物理化学性能均有不同程度的差异。所有材料均符合UL-94 V0要求,具有良好的阻燃性能;热稳定性好,符合无铅焊接温度要求;玻璃化转变温度(T_g)和热膨胀系数(CTE)测试结果与厂家标准数据一致,介电常数(D_k)和损耗因子(D_f)比厂家标准数据略高;具有较高的吸水率和硬度。介电常数和损耗因子高会影响信号传输的完整性,吸湿会影响PCB材料的热性能和电性能等,硬度高则影响材料的加工性能,都将近一步影响PCB的可靠性。改善无卤PCB材料的吸湿性,降低材料的介电常数和介电损耗,在降低热膨胀系数的同时不损害材料的韧性,是无卤PCB材料研究改进的重点方向。2.印制板制作工艺前后PCB材料性能的变化状况,影响设计电路的最终性能,也决定了无卤PCB材料是否适宜工业化生产。鉴于尚无相关工作及文献可查,本文通过分析对比芯板材料与印制板的玻璃化转变温度、固化因子(△T_g)、热膨胀系数和热分解温度等性能参数,研究了印制板制作工艺对无卤PCB材料性能的影响。印制板与芯板材料相比,各性能参数出现不同程度的变化:玻璃化转变温度和固化因子略有下降;在低于T_g点时,印制板材料的面内CTE比芯板材料略高而面外CTE比芯板材料略低,在高于T_g点时,印制板材料的各轴向热膨胀系数都比芯板材料稍高;热分解温度无明显变化。因此,无卤PCB材料的选用及印制电路设计应以印制板性能测试结果为准。3.研究对比了无卤PCB和含卤PCB材料的吸湿、放湿行为。本研究表明无卤PCB材料在室温下、85°C/85%RH和沸水中都比含卤材料更易吸湿。各材料吸湿、放湿过程均遵循费克扩散定律,扩散因子和吸湿含量可用费克扩散定律的一维模型来近似模拟和计算。吸湿后,水分子以两种形态存在于PCB材料内部:自由水分子和键合水分子。材料吸湿后再在105°C烘干去湿,不能去除已键合的水分子。PCB材料中树脂含量越高,吸湿越多,与水分子形成键合的数目越多,烘干去湿时,残余水分子的比例也越高。4.鉴于无卤PCB材料的易吸湿性,对比研究了吸湿对无卤材料和含卤材料介电性能和热性能的影响。常规加湿试验条件下,各PCB材料的介电常数和损耗因子均随材料内部湿度含量的增加而呈线性增加。文中给出了在特定湿度及温度环境下工作的PCB基板材料,在某时刻的介电常数和损耗因子计算公式。吸湿对无卤PCB材料的热膨胀行为曲线影响明显,吸湿后的无卤材料由原来的以玻璃化转变温度为分界点的两段式热膨胀行为,转变为三段式热膨胀,加大了玻璃化转变温度以下的热失配,增加失效几率。原CTE测量方法不能表征湿度对PCB热膨胀性能的影响,本文提出应测试整个加热区间的热膨胀系数。玻璃化转变温度受湿度影响明显,与前人研究结果——T_g随材料内湿度含量的增加而降低最终趋于平衡,略有差异。本研究表明吸湿水分子在PCB材料内部的存在形态不同对T_g有不同的影响,T_g先降后升。饱和前,自由水分子的增塑作用占主导,T_g随吸湿含量的增加而降低;趋近饱和后,自由水分子不再增多,水分子与树脂键合起主导作用,增加了材料的交联密度,T_g随在湿热环境中放置时间的增长而略有回升。吸湿对材料的热分层时间有明显影响,PCB内部材料界面在热失配应力与水蒸气压双重作用下,更易发生分层,热分层时间随PCB材料内部湿度含量的增加而降低。以上参数在易吸湿的无卤材料中变化更为明显。5.评估了相关IPC测试方法对含湿材料测试的适用性。测试标准中预处理的目的是排除干扰因素,使测试结果反映材料的本质性能,具有可重复性。本研究表明IPC-TM-650 2.5.5.9、IPC-TM-650 2.4.24和IPC-TM-650 2.4.25中测试前的预处理方法可以一定程度上降低湿度对材料性能的影响,但不能将具有不同湿度含量的样品处理到标准样品水平,实际测试结果不能反映材料的本质性能,而是吸湿后的性能。因此在按照此三种方法测量的性能参数选用材料时,应注意材料的吸湿历史。6.综合分析研究了无铅焊接工艺对不同阻燃剂不同固化剂等多种PCB材料性能的影响。无铅焊接热曝露降低了材料的热分层时间和面外CTE,增加了材料的吸水性和可燃性,对面内CTE和热分解温度无明显影响。对玻璃化转变温度的影响,因固化剂和材料T_g类型不同而略有差异。无卤阻燃剂的转换对树脂材料的热分解行为有明显影响。相对含卤材料分解迅速,无卤材料的热分解是一个缓慢的老化分解过程。对两种不同固化方式的材料,无卤阻燃剂的添加有着相反的作用,它可以改善双氰胺固化的PCB材料的热稳定性,却会降低采用酚醛固化的PCB材料的热分解温度。各影响因素综合分析表明,使用酚醛固化的无填充剂含卤材料和低T_g类型材料性能相对较为优异。更多还原

【Abstract】 With the development of electronics, the human health and environmental safty get more public concerns. The electronics are droven to GREEN while they are getting smaller, diverse and higher performance. With the release of Directives (RoHS/WEEE/REACH) on the restriction of certain hazardous substances in electrical and electronic equipment, printed circuit boards (PCB), as an essential part of electronics, are on its way by moving to halogen-free, lead-free and recycling. The reliability of new halogen-free PCBs is one of the important concerns in investigation and industry field. However, there are a few studies on the reliability of new halogen-free PCB materials as far. There are several questions are not clear, such as whether the PCBs are still reliable when replacing the halogenated flame retardant with halogen-free alternatives, whether the halogen-free PCBs are suitable for lead-free soldering. Therefore, the reliability of halogen-free PCBs was studied by conducting practical laminating process, moisture absorption and lead-free soldering experiments. The main contents and results are as following.1. As there were only papers which report excellent qualities of halogen-free materials by individual industry or organization, it is not good enough for PCB designers choosing material. Several halogen-free PCB materials were evaluated by testing their properties with IPC and other related test methods and comparing with halogenated materials. There were some differences in electrical properties, thermal properties, physical and chemical properties. All the halogen-free materials reach the UL-94 V0 requirment which means good flame retardancy and have good thermal stability for the lead-free soldering process, but they have higher moisture absorption and stiffness. The test results of glass transition temperature (T_g) and the coefficient of thermal expansion (CTE) were in accordance with their datasheets while the dielectric constant (D_k) and dissipation factor (D_f) were higher than datasheets. High dielectric properties which increase the signal transmission loss, moisture absorption which impact on thermal and electrical properties, high stiffness which affect the drilling process and mechanical process, may affect the PCB reliability further. The halogen-free PCB should be improved by lowering these properties in future.2. The variation of PCB material’s properties in manufacturing process will affact the final performance of circuit and also decide whether the halogen-free PCB materials can be industrialized produced. However, there is no literature related to the variation. The effect of manufacturing process on halogen-free PCB properties was studied by comparing the curing factor (△T_g), CTE and decomposition temperature (T_d) between core material and printed boards. The results showed that T_g and△T_g decreased after manufacturing process. The in-plane CTE of printed boards were higher and out-of-plane CTE were lower than core materials below T_g while both the in-plane and out-of-plane CTE of printed boards were higher than core materials above T_g. The decomposition temperature didn’t show obvious variation. Therefore, the properties of printed boards instead of core materials should be used for choosing halogen-free material and designing circuits.3. The moisture absorption and desorption behavior were analyzed and comparied between halogen-free and halogenated PCBs. The results showed halogen-free PCB materials always absorbed more moisture than halogenated PCBs in room storage condition, in 85°C/85%RH environment chamber and in boiling water. Moisture absorption and desorption followed the Fickcian Law. The diffusion factor and moisture content of PCB materials can be calculated with a one-dimensional Fickian model. The water molecules absorbed in PCBs were classified into two types: bound water and free water. And bound water can not be removed by baking process at 105°C. The moisture content increase with the resin content, result in more bound water in materials which lead to the final moisture content inside the materials is higher after baking process.4. The moisture effect on dielectric and thermal properties of halogen-free and halogenated PCB material was studied. D_k and Df increased linearly with the increase of moisture content under 85°C/85%RH test condition. Two formulas were put forward for calculating D_k and Df in specific environmental conditions. The thermal expansion behaviors of halogen-free material changed significantly after absorbing moisture. It showed three phases in thermal expansion curves while it showed two phases in initial curves before absorbing moisture. The CTE around glass transition region became large which lead to thermal mismatch resulting in failure rate increasing. The original CTE measure method can not characterize the effect of moisture. A total CTE should be measured for whole thermal excursion. The glass transition temperatures were affected obviously by moisture absorption. Previous work has found that the absorbed water in epoxy materials leads to a decrease in the glass transition temperature and finally makes it reach a stable value. However, this study showed that the T_g decreased in the first stage and increased in second stage. Two types water molecules in PCB materials played different role. Before saturation, the plasticizing effect of free water dominated the trends, T_g decreased with the increase of moisture content; After saturation, the cross-linking density increased because water molecule bound with resins, T_g increased slowly with the immersion time. Printed boards more likely delaminated because the thermal stress and water vapor pressure acted on PCB internal interface after moisture absorption. Time to delamination of PCB decreased with the increase of moisture content.5. The suitability of IPC test methods for measuring materials with moisture was evaluated. The purpose of preconditioning steps outlined in these test methods is to exclude confounding factors and bring all the test materials to a standard level so that the test results can be repeated and reflect the nature of materials. However, the preconditioning in IPC-TM-650 2.5.5.9, IPC-TM-650 2.4.24 and IPC-TM-650 2.4.25 can reduce the impact of moisture to a certain extent, but can not bring the samples which have different moisture content to a standard level. The test results were affected by moisture content. Therefore, the material’s moisture absorbing history should get our attention when they were choosed according to the properties measured by those three test methods.6. The lead-free soldering process was applied on some high T_g, middle T_g and low T_g PCB materials which have different additives, such as flame retardant, curing agent and fillers. After lead-free soldering thermal exposure, time to delamination and out-of-plane CTE decreased, water absorption and flammability increased. T_g showed different trends in the materials with different T_g types and different curing agent. There is no obvious effect on in-plane CTE and T_d. However, the resin’s decomposition behavior was affected by replaceing halogenated flame retardant with halogen-free alternatives. Halogenated materials started to decompose at a temperature, subsequently experienced a rapid degradation while halogen-free material experienced a slow degradation. For DICY cured PCB materials, thermal stability were improved by replacing halogenated flame retardant with halogen-free alternatives. On the contray, for Phenolic cured materials, Td was degraded with halogen-free flame retardant. Considering all the factors, the low Tg halogenated materials without fillers, cured by phenolic, had relatively excellent performance.更多还原