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Feb 04, 2021

Using Manual Soldering in SMT Assembly Manufacturing Process

Several tutorials and instruction manuals are available claiming to teach do-it-yourself enthusiasts how to manually solder their electronic Surface Mount Technology or SMT kits. However, most of these involve using medium-sized Surface Mount Devices or SMDs, and boards fairly light in component density. As the number of SMDs on board increases, and their size tends to the miniature, manual soldering becomes increasingly difficult to execute.

Electronic industry has developed the SMT assembly manufacturing process mainly for improving the efficiency of the task of assembly to ultimately achieve high production throughputs with optimum quality and reliability. The sheer number of assembled boards that the automated processes of SMT assembly can achieve also helps to drive down the cost per board significantly. Add to this the near absence of errors in the automated processes in SMT compared to the several possibilities in manual soldering, and you begin to wonder why the question of manual soldering in SMT processes? Yet, there are instances where the industry requires manual soldering even in highspeed production lines using SMTs—mainly for repairs, rework, touchups, and for special components.

Differences Between Automated Assembly and Manual Soldering of SMDs

The major difference between the two processes is equipment. While automated assembly requires many large machines, manual soldering comparatively requires almost none, barring a few handoperated instruments.

To begin with, the automated assembly process for SMT components begins with dispensing machine depositing solder paste on the printed circuit board using a stencil as a guide. The next step involves pick-and-place machines mounting SMDs on the solder paste deposits on the PCB. The combination of PCB and SMDs then travels further on a conveyor to enter a reflow oven, wherein high temperature melts the solder paste and bonds the components to the PCB to create a PCB assembly or PCBA.

In comparison, manual soldering requires a human operator to pick the right SMD using tweezers or a suction pick and place it on its intended position on the PCB. The operator then must use a hot soldering iron and solder wire to apply molten solder simultaneously to the PCB pad and the component terminal to bond them. He/she must then repeat the process for all the terminals of the component individually

As the process of holding a soldering iron and solder wire requires two hands respectively, the component is free to move on the board during the soldering process. Therefore, the operator must put in a lot of effort and skill to solder the component properly in place.

Some operators prefer to manually deposit a small amount of solder paste on the pads of the board before manually placing the component. This allows them to use tweezers to stabilize the component with one hand, while they apply the hot soldering iron with the other. While this is process is acceptable for discrete SMDs, accurate placement of fine pitch components becomes tricky as solder paste covers the pads.

Why Manual Soldering is Necessary?

Although the automated processes of SMT manufacturing produces extremely good results, there are occasions where one cannot do without manual soldering. Some of these include:

Touchups : Oftentimes, small components are in the shadow of a larger component on the PCB, resulting in incomplete melting of solder paste during the reflow process for the former, leading to dry solder. Although the designer must correct this error in the next revision of the PCB, rather than discard the present lot, a minor touch up with a manual soldering iron can allow them to be useful.

There can be several other causes requiring a manual touchup. For instance, a mishandled component with a bent pin, a tombstoned component, and components requiring rework or repair

Rework and Repairs : This is mainly necessary for replacing a damaged or wrongly placed SMD with the correct one. It may be a field failure, a wrong component, or a component mounted in reverse. The operator must remove the component from the board and replace it with a new component or place it with the proper orientation.

Operators generally use hot-air guns for locally heating the solder joints of the component. Most hot-air guns come with nozzles of various sizes and temperature settings to suit the type of the component. Once the hot air melts the solder joints, the operator can lift the component away from the board. To replace the component, the operator must clean the pads of excess solder, apply solder paste manually on the pads, replace the proper component. The operator then must use the hot air gun again to melt the solder to form the joints. This manual technique is suitable for both discrete SMDs as well as SMD ICs such as QFPs. However, manual rework of BGAs and gull-winged components is not possible as the pins and solder balls remain hidden and inaccessible between the body of the IC and the PCB.

Special Components : Some fragile components are temperature sensitive and cannot withstand the prolonged high temperature exposure inside the reflow oven. Operators must solder such components by hand and within a very short duration, so that heat from the soldering operation does not damage them.

PCBs often contain a mix of large and small components on board. During reflow, the smaller components heat up much faster while it takes longer for the larger components to heat up enough to let the solder melt on their pads. The overheating on the smaller components tends to drive off the flux leaving them insufficiently soldered. Often the remedy for such cases is to hand-solder the larger components after completing the reflow with the smaller components.

Resources for Manual Soldering of SMDs

Manual soldering of SMDs requires some special resources. These include:

  • Experienced Operators
  • ESD regulations
  • Constant-temperature Soldering Irons
  • Special Heating Tips for Soldering Irons
  • Vacuum Pump
  • Hot-Air Guns
  • Special Tips for Hot-Air Guns
  • Leaded/Lead-Free Solder Wire/Paste
  • Solder Flux
  • Solder Wick
  • Tools — Tweezers, Pliers, Cutters, Magnifying Glass, Vise, etc.

StencilPrinting

For manual soldering of SMDs, experienced operators are an absolute necessity. They must possess adequate knowledge and technique on SMT soldering and inspection along with sufficient working experience. They must not be color-blind.

As operators will be manually handling boards with electronic components, implementation of rigorous ESD regulations will prevent inadvertent damage to the PCBA.

Handling small components requires being careful when heating them. Soldering irons with variable tip temperature settings are essential as they can maintain their tip temperature to allow quick soldering and de-soldering, while not overheating the component.

Rather than use a single tip on the soldering iron for all components, it is preferable to use special heating tips. These are available in various sizes and helpful in heating small to large components evenly.

Hand-operated vacuum pumps are helpful in removing molten solder. However, many operators prefer to use hot-air guns that operate both in the blowing and sucking modes. The tips of such guns are hollow and a pump blows air through the hot tip. When operated in reverse, the pump sucks air and molten solder through the heated tip. Special tips are available that allow heating pins on all four sides of quad ICs for soldering or de-soldering them.

Working with SMDs requires use of leaded/lead-free solder in the form of wires or paste and matching solder flux. Depending on space available when soldering, solder wicks are very helpful in removing excess solder.

Apart from the above, several tools help with the manual soldering operation. For holding small SMDs, a tweezer is a handy tool, as is a needle-nosed plier. A proper vise is inevitable for holding the board steady during soldering. For inspection after the soldering process, an illuminated magnifying glass is a very helpful tool.

Conclusion

Contrary to popular belief, manual soldering methods still play an important role in the SMT assembly manufacturing process in the electronic industry. Although the SMT process of PCBA offers several advantages such as high density, high efficiency, low cost, and high reliability, there are some places on the board where only manual soldering will work.