PCB ASSEMBLY EVOLUTION

From THT to SMT and THR

1. Introduction

      Today the progress in electronic manufacturing consists primarily in implementing environmentally friendly technologies. Properties like high quality, reliability, cost-effectiveness, simplicity and high integration density are features marking the evolution of PCB assembly, too. THR or Through-Hole Reflow technology combines all these characteristics. It is the point the evolution of PCB assembly has reached today. It combines in itself tradition and innovation by combining conventional Through-hole components with the soldering method typical for the Surface-mount technology- the reflow soldering. Thus PCB assembly demonstrates a spiral -like development from the THT to SMT and from SMT to THR.

2. From THT to SMT
Through-hole technology of PCB assembly implements the conventional through-hole components the leads of which are inserted through the mounting holes of the printed circuit board and are soldered at the back side of the board. The assembly is carried out manually or with special placement equipment. The components are soldered to the PCB manually or with wave soldering machines. The need of defeating the disadvantages of the Through-hole technology like high manufacturing costs, low density, assembly faults due to the manual processes involved have resulted in development of the technology called SMT.
3. SMT
SMT is today's standard of electronics manufacturing. SMT stands for Surface-Mount Technology that has largely taken the place of Through-hole technology because of the improvements made in both – the component construction and the process of the PCB assembly. The drawbacks of the THT have been overcome: the components have become miniature – as small as (0.01 in x 0.005 in) in SMD of 01005 - size. The high density has been achieved thanks to the small size of the components and the possibility of mounting them on the both sides of the board. The process of PCB assembly has been completely automated in the SMT assembly line. The solder paste is dispensed onto the PCB in the first machine of the line- the Screen/Stencil Printer. The The Pick and Place machine, fed with components in the state in which they are delivered , places or shoots the components onto the PCB. The shooter of the Pick and Place machine is often equipped with machine vision tools for the accurate placing of the smallest parts. The reflow soldering process that takes place in the reflow oven melts only the solder paste deposited onto the PCB contrary to the wave soldering process. In-line AOI machines or alone-standing ones secure the quality of the processes involved by 2D or 3D inspection of assembled PCBs. The advantages of SMT are high quality, low production costs, high density, possibility of one-sided or double-sided PCB assembly . The SMT is also environmentally friendly due to the ROHS compliant components and lead-free solder materials used.
4. From SMT to THR
There are still heavy electronic components like coils, connectors and transformers which have to be joined to the PCB with stable joints. So there are applications with mixed component construction, such with both through-hole and surface-mount components. In that case mixed technology is applied: at the first stage the PCB is assembled using surface-mount technology, then, at the second stage, the through-hole elements are placed and soldered manually in a manual or semi-automated process. This process is expensive and non-reliable. There is a need of uniforming the process by eliminating the manual operation and the wave soldering. The innovative technology that has been developed in meeting this need is known as Through-Hole-Reflow (THR) , Pin-in-Paste, or Pin-in-Hole-Intrusive-Reflow.
4. THR is a technology with through-hole and surface-mount components being soldered together in the reflow soldering process. In the stencil printer, the solder paste is dispensed onto the pads and into the holes for the through-hole components. Then the THR Placing machine inserts the pins of the through-hole components into the holes filled with paste prior to placing the surface-mount elements or vice versa depending on the PCB design . The solder joints of the heavy components are stable. Wave soldering is eliminated, and only one soldering process is applied . The process is completely SMT compatible and thus- environmentally friendly.
5. Conclustion
For more than 50 years, the PCB assembly has been the core process in electronics manufacturing. It has been constantly improved for achieving high quality, cost-effectiveness, high integration density and environmental friendliness. The innovative THR of today has focused in itself all these advantages in accumulating all improvements achieved in PCB design, component construction, lead-free soldering materials and automated environmentally friendly processes.

March 22, 2012

Marlena Yurukova, 

Dipl.-Ing. In Radio electronics