Printed Circuit Boards

电子元件通常在称为电路板的平坦表面上组装和互连。几种类型的现有电路板可以分为两种广泛类别：用于原型或实验电路的那些，以及用于生产和/或商业销售的那些。用于实验工作的电路板通常被称为面包板， 要么Protoboard.s. Breadboards allow engineers to construct circuits quickly, so that they can be studied and modified until an optimal design is discovered. In typical breadboard use, components and wires are added to a circuit in an ad hoc manner as the design proceeds, with new data and new understanding dictating the course of the design. Since breadboard circuits exist only in the laboratory, no special consideration need be given to creating reliable or simple-to-manufacture circuits—the designer can focus exclusively on the circuit's behavior.

相比之下，用于生产或商业销售的电路板必须具有高度可靠的电线并与所有组件互连永久粘合，以及适用于大规模生产和彻底测试的地形。并且，它们必须由可靠，低成本且易于制造的材料制成。玻璃纤维基板具有铜线（蚀刻层压铜板）是过去几十年的印刷电路板（PCB）材料的首选材料。Digilent Board是这样一个板的简单示例。请注意，大多数情况下，生产电路板设计仅在广泛的面包板阶段完成。组件使用焊接工艺永久地固定在生产板上。图1显示了PCB的图层。

生产电路板通常开始为thin sheets of fiberglass (about 1mm thick) that are completely covered on both sides with very thin sheets of metal (typically copper). A “standard” circuit board might use a 1 ounce copper process, which means that one ounce of copper is evenly spread across 1 square foot of circuit board. During the manufacturing process, wire patterns are “printed” onto the copper surfaces using a compound that resists etching (hence the name Printed Circuit Board, or PCB). The boards are subjected to a chemical etching process that removes all exposed copper. The remaining un-etched copper forms wires that will interconnect the circuit board components and small pads that define the regions where component leads will be attached.

在使用通孔技术的PCB中，通过焊盘钻孔，使得部件引线可以插入并然后固定（焊接）到位。在使用表面贴装技术的PCB中，部件引线直接焊接到表面上的焊盘上。PCB中的每组焊盘（或孔）旨在接收特定的组件。要识别必须加载哪些组件在其中，请使用丝网过程紧邻焊盘紧邻的电路板上打印参考指示符。零件列表通过描述组件并将其分配特定参考指示符，将指定的一组焊盘链接到物理组件。参考指示符在使用PCB时帮助指导汇编器和测试人员。必须将许多组件放入特定方向上的PCB中。根据惯例，需要特定方向的组件具有指定为引脚1.在PCB上的一个引线，方形垫或丝网印刷器通常表示引脚1。

On all but the simplest PCBs, wires must be printed on more than one surface of fiberglass to allow for all the required component interconnections. Each surface containing printed wires is called a layer. In a relatively simple PCB that requires only two layers, only one piece of fiberglass is required since wires can be printed on both sides. In a more complex PCB where several layers are required, individual circuit boards are manufactured separately and then laminated together to form one multi-layer circuit board. To connect wires on two or more layers, small holes called vias are drilled through the wires and fiberglass board at the point where the wires on the different layers cross. The interior surface of these holes is coated with metal so that electric current can flow through the vias. Most Digilent boards are simple four or six layer boards; some more complex computer circuit boards have more than 20 layers.

The unloaded PCB appears green because thin sheets of green plastic have been applied to both sides (otherwise the PCB would appear pale yellow). Called solder masks, these sheets cover all exposed metal other than the component pads and holes so that errant solder can't inadvertently short (or electrically connect) the printed wires. All metal surfaces other than the exposed pads and holes (i.e., the wires) are underneath the solder mask. Not infrequently, blue or even red solder masks are used.

Circuit components are manufactured with exposed metal pins (or leads) that are used to fasten them to the PCB both mechanically (so they won't fall off) and electrically (so current can pass between them). The soldering process, which provides a strong mechanical bond and a very good electrical connection, is used to fasten components to the PCB. During soldering, component leads are inserted through the holes in the PCB, and then the component leads and the through-hole plating metal are heated to above the melting point of the solder (about 500 to 700 degrees F). Solder (a metallic compound) is then melted and allowed to flow in and around the component lead and pad. The solder quickly cools to form a strong bond between the component and the PCB. The process of associating components with reference designators, loading them into their respective holes, and then soldering them in place comprises the PCB assembly process.

检查数字板，并注意两侧的印刷电线。一侧的电线在很大程度上“北方和南”，而另一边的电线主要“东西和西”。在多层PCB上，交替层上的垂直或曼哈顿布置在交替层上非常常见。找到一些通孔，并注意它们在相对侧连接电线。定位各种组件，它们的孔图案和相关的参考指示符。识别各种组件的垫1。注意，通孔略大于通孔，并且该组件可以很容易地插入到它们的通孔中，但不能进入通孔。

• 电子元件通常在称为电路板的平坦表面上组装和互连。本节中讨论的两种类型是Protobood / Breadboard和印刷电路板。
• Printed circuit boards were discussed in detail as those are the type of boards students will be working with.
• 在电路板上打印参考指示器，以帮助识别哪些部件附在PCB上的位置。