Study On The Reliability Of Halogen-free PCB Materials

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.