为了画个通孔焊盘，看了很久的 Thermal relief相关的内容，终于基本上理解了。记录一下。

这篇文章讲得非常清楚：
深入理解Allegro之Thermal Relief与Anti Pad - 百度文库 (baidu.com)

热风焊盘般用在内层，减少一些焊盘和铜皮的连接，防止焊接时散热太快。如下，左侧是全连接，右侧热风焊盘，白色区域的铜皮被删去。

Thermal relief只在negative artwork中使用

正片(positive artwork)中不会用到Thermal relief。——所以如果确定一直用正片设计，其实画焊盘的时候都不用管这个。

Allegro 16.6的文档中如下说明：Thermal relief 和 anti-pad只在 negative artwork layers使用。

Each pin can have any pad type (regular, thermal relief, anti-pads, and custom shapes) defined on each ETCH/CONDUCTOR layer of the design. For negative artwork layers, the layout editor uses thermal reliefs and anti-pads. For positive artwork layers, the layout editor uses just three regular pads. This means a pin can be put anywhere on the design, and the layout editor photoplots the correct pad, whether the location is in an open area or inside a filled shape.

关于Thermal relief和Flash symbol的设置：
20161210-Allegro中的Flash和Thermal Relief_2974786048的博客-CSDN博客
在画Flash symbol的时候，可以看到画的形状是在负片中被挖空的位置，而留下来的就是没被挖空的4小段铜皮。

在正片修改热风焊盘形式

Shape

可以在菜单栏shape-global dynamic shape parameters里面改所有动态铜皮的thermal relief。

修改某个pin的热风焊盘

如下：

举例说明

对同一个焊盘，看了正片和负片下的表现。

正片

正片下，热风焊盘连接形式，由shape parameter或pin决定。

负片

改成负片

显示thermal pad

可以看到，热风焊盘是长方形。

关于Flash symbol

（不一定完全对）我的理解是早期的光绘文件中只有焊盘的位置信息，而焊盘的形状是单独储存在Flash symbol中，这样可以缩小文件体积。而新的gerber文件中已经可以包括焊盘形状信息。

下面是Allegro手册中的描述：

Flash symbols are pads that you create for photoplotting using standard apertures. For raster formats (Gerber RS-274X, Barco DPF, and McDonald Dettwiler MDA), you can include definitions for all apertures in the artwork file. For standard apertures and pads-as-shapes, you can derive the aperture definition from the design. For aperture flashes, however, the design does not contain information about the geometry required for flashes referenced in the padstack data if the design has not been flashconverted from pre- Release 14.0 versions of the editor software (as described in Choosing a Design Methodology for Negative Planes). In designs that have not been flash-converted, the layout editor displays only a circle with a cross-wire for the flash.