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Through Hole Assembly

The operation of component mounting through the holes in a printed circuit board (vias) is termed through-hole

(TH) assembly. TH assembly involves two main processes: inserting the component parts, and then soldering the component's leads to the PCB tracks.

The PCB type, such as single-sided, multi-sided, or multilayer, has a significant impact on the board's density and component layout. The method of production, however, depends predominantly on the number of assemblies being fabricated -low volume assemblies tend towards manual procedures, though quickly moving to more economically viable automated methods as production increases.


A soldered joint formed between a component lead and copper track on a plated through-hole PCB. 

Most considerations regarding component insertion into TH PCBs are generated by the components themselves, including the component shape, position of wire leads, component sensitivity, mechanical stress levels, and any special requirements.

Manual insertion is the simplest and most time-consuming method of assembly.    It involves personnel bending the component leads by hand to fit the PCB, then inserting the necessary components. The individual also has to trim the leads after soldering.

By using semi-automatic methods, some manual insertion operations can be improved greatly. Specific machinery can be introduced to bend and fit the component leads (preforming or prepping), show the component location (PCB layout projection through slide and illumination techniques), and to execute the trimming and clinching of the component leads (before soldering). Although semi-automatic insertion reduces production time, it still demands the considerable attention of human assemblers.

Automated component insertion, on the other hand, aims to reduce human involvement to a minimum, thus reducing labour time/costs. And, by avoiding human handling, damage to static sensitive units is prevented. It is achieved by bringing machinery into the production line to place the components and crimp/cut their leads.

To accommodate different component types (axial, radial, DIL packaged),
TH machinery comes principally in two forms:

  • axial inserters, accommodating components with even leg configurations
  • radial inserters for components such as electrolytic capacitors that tend to be three-Iegged.    

The choice of insertion machine depends largely upon the assembler's demands. Soldering of components secures the mechanical and electrical connections between elements on the PCB. TH joints rely on the single fact that a wire lead is inserted through a hole in the board, then soldered to bond to a metal track. Four different through-hole joints can be defined:

    1. non-plated through-hole, straight lead
    2. non-plated through-hole, clinched lead
    3. plated through-hole, straight lead
    4. plated through-hole, clinched lead

Hand soldering and wave soldering are the two prevailing soldering methods for TH assembly; drag soldering and dip soldering methods becoming increasingly uncommon.

a) Hand soldering:
    This is an individual, joint-by-joint technique used for small runs     
    with typically limited component variation.

b). Wave soldering:
Provides compatibility with the continuous electronics assembly
      production line, and supplies speed and reliability. This soldering
      methods the accepted norm for through-hole assembly, especially in
      large scale productions.

In general, although there was a wholesale shift towards SM technology during the 1980s, which continues today, TH assembly still exists. Any decline in production methods is being countered by the explosive growth in the electronics market. And, although impractical for modern gadgets like mobile phones and digital watches where board space is at a premium, TH practicality does stretch to those products that do not aspire to be portable, such as televisions and videos.




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