TECH TALK

A journal of bus architecture tips & techniques

Rugged Enclosure Construction: Dip Brazing

Constructing a rugged system is more difficult than building a system for a lab.  You must design and test for shock, vibration, temperature variations, EMI / RFI, and other nasty elements.  An important component is the enclosure.  Manufacturing enclosures capable of withstanding the environment and meeting military specifications are usually limited to one of the following techniques:  dip brazing, welding, milling, or Electrical Discharge Machining.

Of available techniques for joining metal, dip brazing may be the most versatile.  It is a popular method of building light weight, sturdy enclosures and ATRs (Air Transport Rack) for both aerospace and military industries, and is ideal for packaging VMEbus, CompactPCI, PC/104, and custom electronics.

What is Dip Brazing?
Brazing is a process that joins similar or dissimilar metal joints using a third filler metal (also called flux).  A metallurgical bond is created between the flux and the surfaces of two metal pieces being joined. Because metals to be joined are not melted, unlike welded joints, they retain their original metallurgical properties.

Enclosure pieces are first cleaned, then mounted in a stainless steel fixture.  The fixture is dipped into a bath of molten flux whose temperature has been raised to around 1100° F.  The alloy flows by capillary action between the metal pieces, forming fused braze joints.  Upon removal from the bath, the assembly is cooled, cleaned, and age-hardened to restore temper and strength.

Why use Dip Brazing?
Dip brazed enclosures are lightweight, strong, rugged, and versatile – a good fit for flight, shipboard, or ground mobile applications.  They meet military specifications such as MIL-E-5400T, MIL-STD-810, MIL-STD-461, MIL-STD-704, and MIL-STD-454. Brazed joints also have a clean, well finished appearance. 

Dip brazing offers several major advantages: 

• Structural Integrity.   A brazed joint has great tensile strength, often as strong as or stronger than the metals being joined.  It repels gas and liquid and withstands vibration, shock, and temperature fluctuations.  The braze process heats all components uniformly yielding minimal distortion and maintaining the integrity of the base metals.
  
  
• Better Conductivity.  Conductivity is better than enclosures assembled mechanically because dip brazed joints are leak-tight and EMI-shielded.
  
  
• Cost Savings.  Compared to other processes, dip brazing yields the lowest cost given its superior characteristics.  The dip brazing process requires little, if any, special tooling.  Common fixtures allow for quick, inexpensive configuration changes; thus, the cost of implementing an engineering change is reduced.
  
  
• Time Savings.  A dip brazed design can be completed quickly compared to other processes.  All joints are brazed at once and more than one assembly can be brazed at the same time.

Macrolink designs and manufactures rugged enclosures and ATRs using dip brazed aluminum construction.  They commonly house VMEbus, CompactPCI, and custom electronics.  Convection cooled and conduction cooled enclosures are available in multiple sizes.  Two examples are illustrated here:

                         
Conduction Cooled ¾ ATR                                                                     Convection Cooled 1 ATR

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