背钻 backdrill

2dmin

如果PCB的层数超过4层，PCB的走线从其中的某两层走线（除从表层到表层外），总会产生类似于下图这种情形的stub：产生多余的镀铜部分，当电路信号的频率增加到一定高度后，多余的镀铜就相当于天线一样，产生信号辐射对周围的其他信号造成干扰，严重时将影响到线路系统的正常工作，Backdrill的作用就是将多余的镀铜用钻孔的方式钻掉，从而消除此类EMI问题。其操作步骤和注意事项如下：

<IMG alt=给net设定Backdrill src="http://pagesadmin.ednchina.com/images/article/0427846e-8d62-4c99-8081-c35c88091f98/1.jpg" _max_pth_stub属性>

1.给net设定Backdrill_max_pth_stub属性

　　
Property　　BACKDRILL_MAX_PTH_STUB

Purpose　　Identificationofnetstargetedforbackdrilling

Usage　　Required

Value　　Maxallowablestubinlength(databaseunits)

Objects　　Nets

Backdrillanalysisflag　　S

　　2.给net设定Backdrill_min_pin_pth属性，Backdrill_min_pin_pth考虑可靠性,必须保证BACKDRILL去掉多余镀铜后,保证一定剩余一定长度.

<IMG alt=给net设定Backdrill src="http://pagesadmin.ednchina.com/images/article/0427846e-8d62-4c99-8081-c35c88091f98/2.jpg" _min_pin_pth属性>

Property　　BACKDRILL_MIN_PIN_PTH

Purpose　　Ensuresminimumpinplatingdepthisnotcompromised

Usage　　Optional

Value　　Length

Objects　　Symbols,pins

Backdrillanalysisflag　　P

　　3.压接的接插件backdrill：要保证良好的接触,必须保证下面的值：

　　

　　Property　　BACKDRILL_PRESSFIT_CONNECTOR

　　Purpose　　Use on pressfit connectors to permit backdrilling from both sides of the board. Backdrill depth will not protrude into contact range as defined by the property string. .

　　Usage　　Optional

　　Value　　<value1:value2>wherevalues=pincontactrange;thisvaluemustbeobtainedfromthemanufacturer

　　Objects　　Symbols

　　Backdrillanalysisflag　　n/a

　　4.有些场合不能用此功能的可以用下面的命令排除.

　　Exclusions   It may be necessary to exclude certain objects from backdrilling even though stub violations are present. The BACKDRILL_EXCLUDE property can be applied to symbols, pins or vias at both the library and design level using Edit - Property. Common examples might include solder tail connectors or a dense pin escape pattern in BGA areas. The PCB designer should consult with a manufacturing engineer before assuming all pin/via objects are suitable for backdrilling.

　　Property　　BACKDRILL_EXCLUDE

　　Purpose　　Excludesobjectsfrombackdrillingeventhoughstubmaybepresent

　　Usage　　Optional

　　Value　　Boolean(true/false)

　　Objects　　Symbols,pins,vias

　　Backdrillanalysisflag　　X

背钻与否，不仅取决于数据速率还取决于板厚，下面简单的公式
: B# ?: O. `! u. L: P  o2 e/ P板厚（mil）<300/数据速率，不需要背钻；
9 g, T$ n0 L0 k" V, l& Z6 q300/数据速率<板厚（mil）<600/数据速率，根据具体情况决定是否背钻；
, r; a! {3 k8 U5 |  b- l0 L板厚（mil）>600/数据速率，必须背钻。

补充一下，backdrill工艺已经比较成熟，成品率也能够达到要求，只是采用backdrill会增加10%~20%的成本