| 电磁屏蔽（EMI）性能 |
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| 最开始这些数据出现在海德鲁镁业公司的技术数据表格中。 |
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| 介绍 |
| 对于电子设备的外壳来讲，一个最为关键的性能要求就是，所要选择的材料是否有能力阻止电磁干扰（EMI）。电磁干扰总是不期而至，从而对电子系统或其子系统造成伤害。射频干扰就是电磁干扰的一种，尽管其波段非常窄。 |
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| 任何设施或电路，只有有电流存在，都有可能造成电磁干扰。因此，电子系统设计的目标就是所有系统的子系统之间，还有该系统和未来工作环境中的其他电子设备之间，必须可以实现电磁兼容。 |
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| 在电磁波发射器和感受器之间，任何一个障碍性物体，可以削弱这种电磁辐射的，都可以做为电磁屏蔽装置。当然，该装置屏蔽性能的好坏取决于它本身使用的材料和对材料的设计，同时，电子设备本身的条件也是一个因素。当辐射抵达屏蔽物体，要么被吸收，或被反射，再或者被穿透。在高阻抗场环境里，大多的辐射能力被金属屏蔽反射；如果是磁场环境下，吸收就变得很重要。下表中是对一些干扰屏蔽的性能比较。 |
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| 屏蔽性测量: |
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| 屏蔽效果，分贝 |
削弱比率 |
穿透屏蔽的比率% |
| 20 |
10:1 |
10.0 |
| 40 |
100:1 |
1.0 |
| 60 |
1,000:1 |
0.1 |
| 80 |
10,000:1 |
0.01 |
| 100 |
100,000:1 |
0.001 |
| 120 |
1,000,000:1 |
0.0001 | |
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| 金属和塑料在电磁屏蔽性能方面的比较 |
| 大多金属本身都是可导电的，也会吸收和反射一部分电磁辐射。电磁屏蔽的效果与很多因素有关，包括金属的属性，辐射的频率，辐射里所包含的容易被吸收或反射的波段，金属的传导率和导磁率，以及辐射源和屏蔽物体的距离远近等。 |
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| 因为塑料本身是绝缘的，因此对电磁辐射基本上是透明的，塑料材料作为电磁屏蔽必须以来对其表面的处理以及添加一些金属颗粒，才能满足要求。但以上俩种方法都会有额外的成本，后者还会影响模具和加工工具的寿命。 |
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| 目前塑料和金属对作为电子屏蔽材料的竞争，取决于什么应用环境下这些材料本身的优缺点，以及不断发展的对外壳材料的审美/电子/机械性能要求。 |
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| 对金属类材料来讲，相对优势在于： |
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固有的传导性能和因此具有的电磁屏蔽能力 |
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材料成本低廉 |
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薄壁设计的结构强度较高 |
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更加经久耐用 | |
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| 随着电子行业的设计正朝着越来越小，但功率越来越大的趋势发展，易加工性和金属的耐热性能正在变地越来越重要 |
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| Advantages of Magnesium for Die Cast
Shielding Enclosures |
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Very low density, leading to light
weight, portable units. |
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Low heat capacity, a significant
factor in achieving high production rates. |
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Very low solubility for iron,
providing a major basis for superior tooling
life. |
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Excellent fluidity, an important
contributor to castings with thin walls, minimum
draft and dimensional accuracy. |
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For shielding applications dependent
upon reflection, the weight saving benefit of
magnesium enclosures extends over the full
frequency spectrum. |
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For shielding by absorption, die
cast enclosures of magnesium and aluminium provide
nominally equivalent shielding effectiveness on an
equal weight basis (the higher conductivity of the
aluminium is offset by the lower density of the
magnesium). As frequency increases, however, the
wall thickness required for a given level of
shielding effectiveness becomes progressively
smaller. Above approximately 1 MHz, the required
thickness of the enclosure becomes defined by
castability (fluidity) limits and structural
integrity requirements. In this portion of the
frequency spectrum, which encompasses most
commercial applications, the thinner wall casting
capability and lower density of magnesium provide
significant advantages in weight and cost
reduction over die cast aluminium. The Figure
illustrates these
considerations. | |
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| Shielding at Joints |
| Joints, cable entries and other openings
in electronic enclosures represent important possible
sources of radiation leakage. Gasketed joints require
special attention, with the objective of maintaining a
uniform low resistance joint over long service periods,
sometimes in severely corrosive environments. Shielding
of these joints normally involves the use of dissimilar
metals in one form or another (fingers, mesh, conductive
elastomers, conductive flanges, etc.). This creates the
potential for galvanic corrosion and therefore the
possible development of a high resistance joint. |
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| The maintenance of low resistance joints
starts with flange coatings, which vary with the
enclosure metal and the service environment. These can
include conductive conversion coatings, conductive epoxy
paints or metallic plating deposits. Interior
environments normally dictate only a conversion coating,
whereas severely corrosive environments might require
the use of duplex gaskets, viz., an inner conductive
gasket to provide shielding and an outer non-conductive
gasket to seal out the corrosive atmosphere. The
conductivity of conversion coatings on magnesium tends
to vary inversely with the corrosion protection
value. |
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| Summary |
| Die cast magnesium alloy enclosures for
EMI shielding provide significant advantages over both
plastic and alternative metal housings. Effective
shielding at joints or openings is maintained through
established protection measures. |
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