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由于电子装备集成度和工作频率的迅速提升，基于传统铜导体的电信号互联的寄生效应问题变得十分显著，从而无法高效率地传播电信号，传统电互联已然成为限制电子装备快速发展的瓶颈[1-3]。与电互联相比，光互联不仅能克服电互联的缺点，还具有低损耗、高带宽、大容量，无串扰、抗电磁干扰等诸多优势。针对电子装备内短距离互联技术，全光互联的性能最好，但全光互联技术存在光路设计与实际制造难度大等问题，在今后较长的时间内，互联技术将呈现电互联为主、光互联为辅的光电混合互联方式[4-5]。
挠性光电印制电路板(Flexible Electro-Optical Printed Circuit Board，FEOPCB)采用光电混合互联方式对高速信号进行传输，可有效解决传统电互联发展面临的 “瓶颈”问题，已然受到国内外学者的广泛关注。其光路传输原理为：激光发射器将电信号转化为光信号，光信号经由埋入挠性基板中的光传输介质，传入接收端，再转化为电信号。
FEOPCB一般采用光波导或多模光纤作为传输介质来传递高速信号。由于光波导制作工艺复杂[6-11]，光衰减问题较为突出，聚合物热稳定性难于兼容常规的印制电路板制作工艺，目前仍处于研究阶段。多模光纤具有较低的衰减性和较高的机械鲁棒性，兼容常规层压工艺，还能用于弯曲半径较小的场景，具有较低的弯曲损耗[12-13]，保证传输质量。
高温层压工艺下，埋入挠性基板的光纤会产生热应力，严重情况下将导致光纤裂纹和高温降解等缺陷，对其可靠性和传输性能产生影响。成磊等[14]研究了光纤埋入刚性基板，层压工艺对光纤应力的影响，研究发现光纤应力较大，达到1.49 GPa，且无法弯曲。佘雨来等[15]采用有限元分析的方法对光纤埋入挠性单面覆铜聚酰亚胺（PI）结构的受力情况进行了研究，并未考虑到实际加工中的材料限制等因素，且未进行实验验证。Norio等[16]直接将光纤层压埋入两张PI基材中，并未制作定位槽，导致基板弯曲半径大、容易出现分层等缺陷。
文中对FEOPCB在实际制造过程中的诸多工艺因素进行综合考虑，建立多模裸光纤埋入挠性双面覆铜PI基材结构有限元模型，在层压工艺条件下仿真分析光纤应力及影响因素，指导了FEOPCB制造。FEOCPB完成了温度冲击、低温、高温和湿热环境适应性试验，并在5 mm弯曲半径下完成10万次弯曲疲劳试验，测量了在不同弯曲半径下的弯曲损耗，相邻裸光纤串扰和高速传输性能，对光纤埋入FEOPCB的应用与推广提供了理论与试验依据。
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目前常用光纤埋入槽型有四种，分别为矩型槽、U型槽、梯型槽和三角型槽，考虑到实际制造的难易程度，选用矩形槽结构建立了有/无涂覆层光纤埋入基板结构的数值模型，这两种光纤埋入结构如图1所示。
图 1 (a)无涂覆层光纤埋入结构示意图；(b) 有涂覆层光纤埋入结构示意图
Figure 1. (a) Structure diagram of uncoated optical fibers embedding; (b) Structure diagram of coated optical fibers embedding
对于无涂覆层多模裸光纤埋入结构，其光纤纤芯半径为62.5 μm，半槽宽度值为65 μm，槽间距为250 μm，上下聚酰亚胺(Polyimide，PI)保护层厚度为25 μm，中间PI基板厚度为100 μm，上下填充胶层厚度为50 μm，上下导电层铜箔厚度为18 μm，挠性基板宽度为2 mm。对于有涂覆层多模裸光纤埋入结构，其光纤纤芯半径为62.5 μm，涂覆层为聚酰亚胺材料，厚度为15 μm，能够长期承受300 ℃高温，兼容层压高温工艺；半槽宽度值为80 μm，槽间距为250 μm，上下PI保护层厚度为25 μm，中间PI基板厚度为100 μm，上下填充胶层厚度为50 μm，上下导电层铜箔厚度为18 μm，挠性基板宽度为2 mm。各层材料参数如表1所示[3,15,17]。
网格的剖分对于有限元的求解非常关键，根据挠性基板不同的结构采用不同的网格形状，可在提高仿真精度的同时加快计算结果收敛。对PI保护层与铜箔等形状规则的几何结构采用映射法来划分网格；对于光纤、中间PI基板层及填充胶结构单元采用自由三角形网格来划分，对最大网格单元和最小网格单元尺寸进行控制，最小单元为0.06 μm，最大单元尺寸为10 μm。统计三角形网格数为13141，四边形网格数为10970，平均网格质量为0.8423，求解自由度数为190388。图2为有涂覆层光纤埋入基板结构网格划分后的全局视图。
表 1 材料属性
Table 1. Properties of materials
Material Elastic
modulus/GPaPoisson's
ratioThermal expansion
coefficient/℃Density/
kg∙m−3Heat capacity/
J∙(Kg∙℃)−1Thermal conductivity/
W∙(m∙℃)−1Copper layer 110 0.33 18×10−6 8940 384 398 Protective layer 3.20 0.36 22×10−6 1420 1090 0.12 Silica fiber 71.9 0.16 0.55×10−6 2200 745 1.50 Filled resin 7.84 0.30 27×10−6 970 1600 0.21 Coating layer 3.20 0.36 22×10−6 1420 1090 0.12 
图 2 有涂覆层光纤埋入挠性基板网格划分全局视图
Figure 2. The global view of meshing of coated optical fibers embedding in flexible substrate
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在实际生产制造过程中，需先将挠性基板固定，再对挠性基板施加时间、温度、压力载荷，故在仿真中对基板下表面施加固定约束，对基板上下表面施加温度载荷，对基板上表面施加压力载荷，如图3所示。图4为层压工艺曲线[17]，首先对挠性光电基板从室温（25 ℃）上升到130 ℃，此时压力为0 MPa；然后在15 min内将压力增加到1 MPa （温度为130 ℃）；再将温度升到180 ℃并加压到2 MPa保压0.5 h；接着缓慢升温到220 ℃，在2 MPa的环境下保温固化2 h；最后卸载压力和温度。升温速率和降温速率均为5 ℃/min。
图 3 电路板层压过程
Figure 3. Lamination process of circuit board
图 4 层压工艺曲线
Figure 4. Curves of lamination process
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图5所示为有/无涂覆层光纤埋入双面覆铜矩型槽内受到的应力。从图5(a)可知，无涂覆层光纤的最大应力约为125.04 MPa；从图5(b)可知，有涂覆层光纤的最大应力约为81.30 MPa；图5(c)为层压工艺下涂覆层应力分布图，最大应力约为93.09 MPa。
图 5 (a) 无涂覆层光纤应力分布；(b) 有涂覆层光纤应力分布；(c) 涂覆层应力分布
Figure 5. (a) Stress distribution of uncoated optical fibers; (b) Stress distribution of coated optical fibers; (c) Stress distribution of coatings
图6为中间PI层厚度对埋入光纤应力的影响。由图6可知，随着PI层厚度值从90 μm增大至160 μm，埋入光纤所受应力不断减小，无涂覆层光纤所受应力从129.72 MPa减小至116.80 MPa，有涂覆层光纤所受应力从89.47 MPa减小至52.02 MPa。当PI层厚度值超过140 μm后，有/无涂覆层光纤所受应力趋于稳定。
图 6 中间PI层厚度对光纤应力的影响
Figure 6. The influence of the thickness of the middle PI layer on optical fibers stress
在实际制造过程中，很难保证光纤与铜层槽底直接接触，通常会有填充胶在层压工艺下流动至槽底将光纤与槽底隔开。图7所示为光纤底部填充胶厚度对光纤应力的影响。由图7可知，随着填充胶厚度由0 μm增加到10 μm，有涂覆层光纤应力呈现不断增大的趋势，从81.30 MPa增至84.52 MPa，但趋势较缓；对于无涂覆层光纤，填充胶厚度由0 μm 变为1 μm时，光纤所受应力从125.04 MPa骤减至86.82 MPa，由1 μm增加到10 μm时光纤所受应力不断增大至93.53 MPa。
图 7 光纤底部填充胶厚度对光纤应力的影响
Figure 7. The influence of thickness of optical fiber bottom filler on optical fibers stress
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FEOPCB制造主要分为两步：第一步是在双面覆铜PI基材层上制作高精度光纤矩形槽，以实现埋入光纤的定位；第二步是高温层压，将布置好有涂覆层光纤的PI基材层、半固化片和PI保护层进行层叠定位，然后放到热压设备的层压窗口，加热加压。光纤矩形定位槽与半固化片的使用可降低埋入光纤在层压过程中所受热应力和偏移量，提升FEOPCB的使用寿命和耦合效率。
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由于FEOPCB采用挠性PI基材，无法使用机械方式制作的矩形槽。利用激光的光热烧蚀机理，在双面覆铜PI基材上制作矩形槽结构。制造厂商提供的PI基材尺寸参数如下：基材层厚度约为138 μm （铜层厚度约为18 μm，中间PI层厚度约为102 μm）。首先通过传统的电路板图形刻蚀工艺，在顶层铜上制作出精确矩形槽尺寸宽度Fpw，间距为P，呈现出激光烧蚀PI层的窗口。由于铜箔烧蚀能量大于PI材料，通过设置CO2激光参数值，在未刻蚀掉的顶层铜箔层保护下对PI材料层进行光热烧蚀，直至露出基材层的下表面铜层，从而制造出光纤高精度矩型定位槽，槽深为H。
在挠性PI基材上制作的四条光纤矩形槽在光学显微镜下的横截面如图8所示，光纤矩形槽表面光洁，槽底也无残留基材。光纤埋入精度取决于制造的光纤矩形槽尺寸参数精度。选取某一矩形槽横截面，利用Olympus STM6光学显微镜测量了不同位置的宽度值、深度值及间隔值，如图8中所示，测量结果如表2所示。
图 8 光纤定位槽横截面图
Figure 8. Cross-section diagram of optical fiber positioning groove
表 2 光纤定位槽参数测量结果
Table 2. Measurement results of structural parameters of optical fiber positioning groove
Measurement item Measurement value/μm Design value/μm Error value/μm Fpw （copper layer） 155.95 155 0.95 Fpw （position 1） 155.78 155 0.78 Fpw （position 2） 140.88 155 −14.12 Pitch (P) 244.87 250 −5.13 Depth (H) 118.29 120 −1.71 从测量结果可知，在位置2处，矩形槽宽度误差值最大为−14.12 μm，由于光纤为圆形，该误差对光纤的埋入精度影响不大。位置1宽度误差值为0.78 μm，顶层铜采用传统化学刻蚀工艺，误差值仅为0.95 μm。矩形槽间距P为244.87 μm，误差值为−5.13 μm；矩形槽深度H为118.29 μm，误差值为−1.71 μm。
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将有涂覆层的光纤布置到光纤定位槽中，然后在埋入光纤的双面覆铜PI基材层的上下表面放置半固化片和PI保护层进行叠层与对位，并按照图4所示的层压工艺曲线进行层压。半固化片在高温高压条件下将变为液态，并填充光纤与定位槽中间的空隙，包裹光纤，降低光纤在层压过程所受热应力，保证其不被损坏，半固化片厚度为50 μm。制作的FEOPCB如图9（a）所示，埋入光纤长度为10 cm和15 cm。对FEOPCB进行截面切片金相观察分析，如图9（b）所示，内埋有涂覆层光纤端面光洁，无高温降解、无裂纹及断裂等缺陷。
图 9 (a) 挠性光电印制电路板实物图；(b) 挠性光电印制电路板截面图
Figure 9. (a) Physical picture of FEOPCB; (b) Cross-section diagram of FEOPCB
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虽然半固化片在层压过程中变为液态，包裹光纤，降低其所受热应力，同时也会导致埋入光纤偏移。图10所示为埋入光纤偏移量测量示意图，假设光纤布置在矩形定位槽中心线位置，光纤与槽底层铜相切。光纤中心位置在O点，通过高温层压后的位置变为O'，x和y分别为光纤横向和纵向偏移量。横向偏移量为光纤中心到矩形定位槽中心线距离值，纵向偏移量为光纤中心到底层铜距离值减去77.5 μm （光纤半径值），通过光学显微镜测量并计算FEOPCBA端口和D端口光纤偏移量，如图9（b）所示，结果如表3所示。
图 10 埋入光纤偏移量测量示意图
Figure 10. Measurement diagram of buried optical fiber offset
表 3 埋入光纤偏移量测量结果
Table 3. Measurement values of buried optical fiber offset
Path
numberSection
numberLateral
offset/μmVertical
offset/μm1 A 2.98 7.15 D 3.87 6.94 2 A −0.56 6.26 D −1.24 5.74 3 A 0.89 5.70 D 0.66 6.07 4 A −2.58 6.84 D 2.27 6.71 横向偏移量正负号分别表示埋入光纤相对于矩形槽中心线向左和向右偏移。从测量计算结果可知，埋入光纤横向偏移量最大值为3.87 μm，最小值为0.56 μm，说明制作的光纤矩形定位槽具有较高精度；纵向偏移量最大值为7.15 μm，最小值为5.70 μm，差值为1.45 μm，说明埋入光纤底部填充胶厚度较均匀。根据参考文献[18-19]，埋入光纤横向和纵向偏移量小于±10 μm，均能够保证VCESL激光器和PIN探测器阵列与埋入光纤之间具有较高的耦合效率。在实际应用过程中，可对光纤端面依次经过粒度5、3、1 μm研磨砂纸和抛光垫进行研磨抛光处理，并配合粒度为25~35 nm抛光液，获得平整光洁的端面，可进一步提升耦合效率。
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根据GJB150A中的要求，对光纤埋入15 cm的FEOPCB多个样件开展了温度冲击、低温、高温和湿热环境适应性试验；然后根据GJB7548中耐弯曲的要求，开展了10万次弯曲疲劳试验，来验证FEOPCB的可靠性及环境适应能力。
温度冲击试验条件为：常温升至+125 ℃，保持15 min；再降低至−65 ℃，保持15 min；温度转换时间不超过1 min；100个循环周期。
湿热试验条件为：常温升至+30 ℃；在2 h内，温度由+30 ℃升至+60 ℃，相对湿度升高到95%，并在此状态保持6 h；在8 h内，由+60 ℃降到+30 ℃，此期间相对湿度应大于85%；在+30 ℃及相对湿度95%的条件下保持8 h；完成10次24 h周期试验，循环结束后恢复至标准大气条件。
环境适应性试验后，FEOPCB样件物理外观未出现起泡和分层等缺陷。
将试验后的FEOPCB样件安装于工装上，如图11所示。朝一个方向弯曲90°后，再向另一方向弯曲90°，记为1次，按50次/min的速率完成了10万次，弯曲半径为5 mm。FEOPCB的弯曲部位无分层开裂现象。在弯曲部位做截面切片分析，如图12所示，光纤表面光洁，裸光纤无裂纹及断裂等缺陷，这主要归功于定位微槽和填充胶对埋入裸光纤的保护，提升了其可靠性。
图 11 挠性光电印制电路板弯曲疲劳试验
Figure 11. Bending fatigue test of FEOPCB
图 12 FEOPCB弯曲部位截面图
Figure 12. Cross-section diagram of FEOPCB bending area
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为了验证环境适应性试验和10万次弯曲疲劳试验后FEOPCB样件的性能，对其弯曲损耗、通道间串扰及高速信号传输性能展开了测试。试验测试设备包括：Agilent示波器，型号为86100D；Anritsu误码分析仪，型号为MP2100A；中电科思仪光功率计，型号为6334B；Finisar公司光模块，波长为850 nm，速率为10 Gbps，功耗约为1.5 W。
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对FEOPCB在不同弯曲半径下的损耗开展了测试，其测试示意图如图13所示。首先采用850 nm的光模块作为光源，通过标准LC-MT转接线与FEOPCB一端相连，再通过标准MT-FC转接线与光功率计相连，形成光通路，测量FEOPCB输出功率P。将FEOPCB在不同弯曲半径下分别弯曲90°和180°，分别测量输出功率P0，通过公式（1）计算FEOPCB中埋入光纤所有模式总功率的损耗[20]，每组数据测量10次，并取平均值。
图 13 弯曲损耗测试示意图
Figure 13. Schematic diagram of bending loss test
$$ B L=10 \lg \left(P / P_0\right) $$ (1) 由于4个通道测量计算结果值接近，仅展示了其中某一通道的弯曲损耗值，如图14所示。从图中可知，FEOPCB最小弯曲半径可达2 mm，在90°和180°弯曲损耗约为0.57 dB和1.12 dB。弯曲损耗随着弯曲半径增加而降低，在弯曲半径为5 mm时，90°和180°弯曲损耗约为0.16 dB和0.36 dB。
图 14 弯曲损耗随弯曲半径的变化
Figure 14. Bending loss versus curvature radius
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相邻光纤之间的串扰必须足够低，才能保证FEOPCB埋入光纤中信息的稳定传输。目前，串扰的计算和测试方法主要基于模式耦合理论和功率耦合理论，其中功率耦合理论更适用于FEOPCB中相邻光纤串扰的计算和测试[21]。则相邻光纤之间的串扰计算表达式为：
$$ {XT}_{i j}=10 \lg \left(P_j / P_i\right) $$ (2) 式中：Pi为第i通道的光纤输出功率；Pj为第i通道光纤耦合至相邻第j通道光纤输出功率，其测试示意图如图15所示。
为了与VCSEL激光器和PIN探测器阵列进行对准耦合，FEOPCB埋入裸光纤纤芯间距为250 μm，通过测量，相邻裸光纤之间无芯间串扰。参考文献[22]中多芯光纤的芯间距为38 μm，其串扰值已符合传输要求。目前，国际上多芯光纤的芯间距一般为42 μm[22]。
图 15 相邻光纤串扰测试示意图
Figure 15. Crosstalk test diagram of adjacent optical fibers
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图16为FEOPCB高速信号传输性能测试原理示意图，图17为搭建的测试平台实物图。将误码仪输出码型设置为PRBS31，传输速率为10 Gbps。FEOPCB 4个通道在30 min测试时间的条件下，误码数均为0，误码率均小于10−16，其中某一通道测试结果如图17所示，眼图良好。
图 16 FEOPCB高速信号传输性能原理示意图
Figure 16. Schematic diagram of high-speed signal transmission performance test for FEOPCB
图 17 FEOPCB高速信号传输性能测试平台实物图
Figure 17. Physical picture of high-speed signal transmission performance test for FEOPCB
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FEOPCB在高温高压工艺制作过程中将导致挠性PI基板埋入光纤的应力，造成光纤损坏等缺陷，影响其可靠性和高速传输性能。针对此问题，文中采用有限元分析方法，对有/无涂覆层光纤埋入双面覆铜PI基板的矩形槽高温层压工艺进行建模，并对埋入光纤应力及影响因素进行了分析。结果表明：随着中间PI层厚度值增加，无涂覆层光纤所受应力从129.72 MPa减小至116.80 MPa，有涂覆层光纤所受应力从89.47 MPa减小至52.02 MPa，且厚度值大于140 μm后所受应力值趋于稳定；随着光纤底部填充胶厚度值增加，无涂覆层光纤所受应力从125.04 MPa骤减至86.82 MPa，后不断增大至93.53 MPa，有涂覆层光纤所受应力从81.30 MPa增至84.52 MPa。
基于有限元仿真结果，利用激光刻蚀工艺在双面覆铜PI基材上制造了高精度光纤矩形槽，并对槽型参数进行测量，结果表明，在矩形槽位置1的槽宽度误差值仅为0.78 μm。埋入有涂覆层光纤，在高温层压工艺条件下完成FEOPCB的制作。对FEOPCB进行截面切片金相观察，光纤端面光洁，无高温降解，无裂纹及断裂等缺陷；对FEOPCB两端埋入光纤横向与纵向偏移量，最大值分别为3.87 μm和7.15 μm，能够保证与光电芯片高耦合效率。
FEOPCB完成了温度冲击、低温、高温、湿热和10万次弯曲疲劳可靠性试验，埋入光纤无高温降解和破裂等缺陷。最小弯曲半径可至2 mm，且无芯间串扰，在850 nm波长条件下，通信速率可达10 Gbps，误码数为0，误码率小于10−16。文中的研究结果表明，有涂覆层光纤能降低埋入光纤所有应力，兼容印制电路板层压工艺，制作FEOPCB可靠性高，可满足高速信号传输要求。
Simulation and test of optical fiber characteristics in flexible electro-optical printed circuit board process
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摘要: 挠性光电印制电路板（Flexible Electro-Optical Printed Circuit Board, FEOPCB）在高温层压制作过程中，埋入光纤会产生热应力，造成光纤损坏等缺陷，影响其可靠性和高速信号传输性能。为了降低FEOPCB弯曲半径并提升其可靠性，将在双面覆铜聚酰亚胺(PI)基板上设计制作高精度矩形光纤定位槽。首先建立有/无涂覆层光纤埋入挠性基板有限元仿真模型，对FEOPCB制造工艺进行模拟仿真，并对埋入光纤应力及影响因素进行分析。结果表明，有涂覆层光纤所受应力远小于无涂覆层光纤。针对有涂覆层光纤，采用激光刻蚀技术在双面覆铜PI基板上制作了高精度矩形定位槽，通过高温层压工艺完成了FEOPCB制作。FEOPCB完成了温度冲击、低温、高温、湿热和10万次弯曲疲劳可靠性试验，利用光学显微镜观察分析，埋入光纤无高温降解和破裂等缺陷。FEOPCB最小弯曲半径小至2 mm，弯曲损耗分别为0.57 dB （90°）和1.12 dB （180°），且相邻光纤之间无串扰，在850 nm波长条件下通信速率可达10 Gbps，误码率小于10−16。Abstract:
Objective The traditional electric interconnection method has become the bottleneck to limit the rapid development of high-speed communication electronic products for its inherent physical characteristics in the case of high frequency. Optical interconnection technology can be used instead of electric interconnection technology to realize high-speed, large-capacity, high-density and flexible information transmission in electronic products and eliminate the technical bottleneck. As a new development direction of board-level optoelectronic interconnection technology, FEOPCB can realize flexible interconnection among different subsystems and meet the development trend of lightweight, miniaturization and high performance of high-speed electronic systems. However, during the high-temperature lamination manufacturing process of FEOPCB, the embedded fibers will generate thermal stress, which will cause damage to the embedded fibers, affecting high-speed signal transmission performance and reliability of FEOPCB. Therefore, it is necessary to establish the finite element model with bare optical fibers embedded for simulating and analyzing the thermal force to guide the design and manufacturing of FEOPCB. For this purpose, the research work on simulation and test of fiber characteristics in FEOPCB process was carried out in this paper. Methods In order to reduce the bending radius and improve its reliability, high-precision rectangular positioning grooves for the fibers were designed and fabricated on polyimide substrate with double-sided copper-clad. Firstly, finite element simulation models of fibers with and without coating layer embedded in PI substrate were established and the manufacturing process of FEOPCB was simulated and analyzed with the influence factors of stress for embedded fibers (Fig.1). The results indicate that the stress of the coated fibers is much smaller than that of the uncoated fiber (Fig.5-7). Then, the laser-etching technology was used to fabricate the high-precision rectangular positioning grooves on the double-sided copper-clad PI substrate for the coated fibers (Fig.8). FEOPCB was fabricated by the high-temperature lamination process (Fig.9). Results and Discussions FEOPCB has completed the reliability tests of temperature shock, low temperature, high temperature, wet heat and bending fatigue for 100 000 times (Fig.11). Through the observation and analysis with optical microscopy, the result shows that the embedded fibers have no high temperature degradation and cracking defects under high temperature lamination process (Fig.12). The minimum bending radius of FEOPCB is as small as 2 mm, and the bending loss is 0.57 dB and 1.12 dB respectively under 90° and 180° bending conditions (Fig.14). The measurement results show that there is no crosstalk between adjacent fibers. Finally, the high-speed signal transmission performance was measured which indicated that a 10 Gbps communication rate with bit error rate of 10-16 could be reached at the wavelength of 850 nm (Fig.17). Conclusions In this study, the finite element analysis method is used to establish the model with coated or uncoated optical fiber embedded in rectangular groove of PI substrate, and the stress and influence factors of embedded optical fiber are analyzed. The results show that the stress of uncoated fiber decreases from 129.72 MPa to 116.80 MPa, and the stress of coated fiber decreases from 89.47 MPa to 52.02 MPa with the increase of intermediate PI layer thickness. The stress value tends to be stable, when the thickness value is greater than 140 μm. With the increase of the thickness of the filling adhesive at the bottom of the optical fiber, the stress on the uncoated optical fiber decreases from 125.04 MPa to 86.82 MPa, and then increases to 93.53 MPa, and the stress on the coated optical fiber increases from 81.30 MPa to 84.52 MPa. The transverse and longitudinal offsets of the embedded optical fibers at both ends of FEOPCB were measured, and the maximum values were 3.87 μm and 7.15 μm, respectively. It can ensure high coupling efficiency between bare optical fiber and photoelectric chip. FEOPCB has completed the reliability experiments and performance tests perfectly. The research results show that the coated optical fiber can be compatible with the printed circuit board lamination process. FEOPCB has high reliability and can meet the requirements of high-speed signal transmission. -
图 1 (a)无涂覆层光纤埋入结构示意图；(b) 有涂覆层光纤埋入结构示意图
Figure 1. (a) Structure diagram of uncoated optical fibers embedding; (b) Structure diagram of coated optical fibers embedding
图 2 有涂覆层光纤埋入挠性基板网格划分全局视图
Figure 2. The global view of meshing of coated optical fibers embedding in flexible substrate
图 5 (a) 无涂覆层光纤应力分布；(b) 有涂覆层光纤应力分布；(c) 涂覆层应力分布
Figure 5. (a) Stress distribution of uncoated optical fibers; (b) Stress distribution of coated optical fibers; (c) Stress distribution of coatings
图 6 中间PI层厚度对光纤应力的影响
Figure 6. The influence of the thickness of the middle PI layer on optical fibers stress
图 7 光纤底部填充胶厚度对光纤应力的影响
Figure 7. The influence of thickness of optical fiber bottom filler on optical fibers stress
图 9 (a) 挠性光电印制电路板实物图；(b) 挠性光电印制电路板截面图
Figure 9. (a) Physical picture of FEOPCB; (b) Cross-section diagram of FEOPCB
图 16 FEOPCB高速信号传输性能原理示意图
Figure 16. Schematic diagram of high-speed signal transmission performance test for FEOPCB
图 17 FEOPCB高速信号传输性能测试平台实物图
Figure 17. Physical picture of high-speed signal transmission performance test for FEOPCB
表 1 材料属性
Table 1. Properties of materials
Material Elastic
modulus/GPaPoisson's
ratioThermal expansion
coefficient/℃Density/
kg∙m−3Heat capacity/
J∙(Kg∙℃)−1Thermal conductivity/
W∙(m∙℃)−1Copper layer 110 0.33 18×10−6 8940 384 398 Protective layer 3.20 0.36 22×10−6 1420 1090 0.12 Silica fiber 71.9 0.16 0.55×10−6 2200 745 1.50 Filled resin 7.84 0.30 27×10−6 970 1600 0.21 Coating layer 3.20 0.36 22×10−6 1420 1090 0.12 
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表 2 光纤定位槽参数测量结果
Table 2. Measurement results of structural parameters of optical fiber positioning groove
Measurement item Measurement value/μm Design value/μm Error value/μm Fpw （copper layer） 155.95 155 0.95 Fpw （position 1） 155.78 155 0.78 Fpw （position 2） 140.88 155 −14.12 Pitch (P) 244.87 250 −5.13 Depth (H) 118.29 120 −1.71
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表 3 埋入光纤偏移量测量结果
Table 3. Measurement values of buried optical fiber offset
Path
numberSection
numberLateral
offset/μmVertical
offset/μm1 A 2.98 7.15 D 3.87 6.94 2 A −0.56 6.26 D −1.24 5.74 3 A 0.89 5.70 D 0.66 6.07 4 A −2.58 6.84 D 2.27 6.71
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