Soldering method in component mounting process

Difference in soldering method

Soldering is the process used to mount electronic components on printed circuit boards. Soldering methods are largely divided into three types: reflow soldering, wave soldering, and manual soldering.

Recent component mounting is done using a combination of the methods shown in the above figure.
For example, local or manual soldering is used for each part when soldering parts that are not heat resistant or have large thermal capacities and cannot pass through the reflow process.

Regardless of the soldering method used, flux cleaning (PCB cleaning) after soldering is effective to improve the reliability of the PCB.KAKEN TECH develops cleaning liquid and cleaning equipment to remove flux residue generated on printed circuit boards. Please feel free to consult with us about cleaning methods according to your soldering method.

We will now explain in detail the differences between reflow and flow soldering and the processes of each method.

Soldering for surface mount (reflow process)

Shows the surface mounting process by reflow soldering.

①Print solder paste on printed circuit board

②Mounting components

③Soldering in reflow oven

④Flux Cleaning

Solder paste printing

Although precoating (plating) and solder balls are also available, the most commonly used method is to print solder paste on a printed circuit board using a metal stencil. It is said that "about 70% of product defects in the mounting process are caused by the initial solder paste printing process". Therefore, a process to inspect the amount of solder paste applied and the printed position may be added after printing. Cleaning the metal stencil used for printing is also important to maintain and improve printing accuracy.

▼▽Solder paste printing process and metal stencil cleaning are explained in the following page!▽▼
	PCB Mounting Columns Printing process and trouble with metal stencils In the reflow process of electronics packaging, the most common method of supplying solder is to print solder paste using a metal mask. This section introduces the explanation of metal masks, the printing process, possible problems and cleaning.
	Products information Stencil　cleaning/screen plate cleaning Many materials such as solder paste (cream solder/solder paste), thick film paste, conductive adhesive, resist ink, and temporary fixing bond (red bond) are printed using stencil masks and screen plates. In the management of these printing plates, aperture cleaning is indispensable to reduce printing defects.

Components mounting

Equipment called a mounter is used. The nozzle in the mounter picks up the electronic components on the tape reel or tray and places them at the appropriate position on the printed circuit board.

Reflow soldering

The reflow soldering process uses an oven-like device called a reflow oven. The reflow oven has a tunnel-like structure that integrates the three zones of preheating, main heating, and cooling. While the printed circuit board passes through the reflow oven on a conveyor, the heat necessary to melt the solder paste is supplied for soldering.

When soldering, all soldering points must be heated above the melting point of the solder paste and below the heat resistance temperature of the electronic components and printed circuit boards to be mounted. When soldering in a reflow oven, the temperature is controlled according to a "temperature profile" set so that the temperature inside the reflow oven is appropriate for the type of solder paste, mounted components.

For example, if the temperature rises too quickly, the solvent component in the solder paste may boil, resulting in voids or solder balls. Conversely, even if the temperature in the reflow oven is the same, some electronic components may have difficulty in raising the temperature of the bonding area depending on their size and shape, and insufficient heating may result in a decrease in bonding strength. Therefore, it is important to set the temperature profile after checking the characteristics of the solder paste and electronic components.

Heating methods of reflow oven include hot air heating, infrared heating, and steam heating, etc. Some reflow ovens combine multiple heating methods to maintain temperature uniformity inside the oven and prevent over-temperature rise.

The inside of the reflow oven needs to be cleaned regularly because the heating process causes atomized flux from the solder paste to accumulate inside the oven.

▼▽Cleaning agents for reflow oven are introduced in the following pages.▽▼
	Products information Reflow oven cleaner KakenClean series KakenClean DF-555U is a water-based cleaning agent for reflow ovens and parts inside the ovens that powerfully removes adhered flux contamination. In addition to reflow ovens and reflow parts, it can also be used for flux cleaning applications such as flow tank transfer jaws, carrier pallets, and fluxers.

Flux cleaning

After reflow soldering is completed, the PCB is cleaned to remove flux residue (flux cleaning). Flux residue left on PCBs can lead to physical and electrical problems, and flux cleaning is an effective way to prevent and solve such problems. When selecting a cleaning method, it is necessary to understand not only the cleaning performance of flux, but also its environmental impact and safety, the burden on workers (frequent liquid replacement and waste liquid disposal), the impact on the target PCBs and electronic components, running costs, and other advantages and disadvantages.

▼▽Flux cleaning is explained in the following pages!▽▼
	Trouble solution Tips on flux cleaning Flux cleaning is a process to remove flux residue that is generated at the same time as soldering when mounting electronic components. This page explains why flux is cleaned, what cleaning methods are available, and what is required for efficient cleaning.

Insertion mount soldering (flow)

We will introduce the insertion mounting process by flow soldering.

①Insert components into the PCB.

②Apply flux

③Pass through solder bath and soldered.

④Clean flux

Component Insertion

The lead is bent to match the pitch of the through-hole to be inserted, and the lead of the component is inserted into the through-hole using an automatic insertion machine called an inserter. For stable quality and mounting cost, it is desirable to use components that can be inserted automatically, but if there are special components that cannot be inserted automatically, they are inserted manually at the end of the process.

Flux applying

	Flux can be supplied by ・Immersion method The mounting surface of the printed wiring board is immersed in a tank containing flux, ・Foaming method Flux in the form of bubbles is applied to the printed board by contact ・the spray method in which atomized flux is discharged from a nozzle. The spray type, which can stably supply flux to finer areas, has become the mainstream.

▼▽Flux is explained on the following page!▽▼
	PCB Mounting Columns What is flux, the leading actor of soldering ? In addition to what flux is, its role, composition, and types, we will also explain the problems caused by flux residue left after soldering and how to prevent them.

Flow soldering

As shown in the figure below, soldering is performed on the leads by passing the printed circuit board over the solder that has been melted at high temperature. The most common specification is to make two waves (peaks) so that the molten solder can easily contact the leads as the PCB passes through.

Jet types include jets and waves with varying strengths and heights, and jets with waves that move left and right to make it easier for solder to reach electrodes and through holes and to produce a good finish.

Flux cleaning

Many insertion-mounted components to be flow-soldered are not resistant to cleaning agents, and in many cases, only the backside of the printed circuit board (soldering area) is manually cleaned. Considering the burden on workers (odor, health hazards, etc.), and in addition, if cotton swabs or brushes are used, it is necessary to select those that do not easily shed lint or hair.

▼▽Flux cleaning is explained in the following pages!▽▼
	Trouble solution Tips on flux cleaning Flux cleaning is a process to remove flux residue that is generated at the same time as soldering when mounting electronic components. This page explains why flux is cleaned, what cleaning methods are available, and what is required for efficient cleaning.

Double-sided mounting

There are also examples of double-sided mounting, in which one side of a PCB is surface mounted (reflow) and the other side is inserted (flow). An example of a double-sided mounting process is shown below.

①Print solder paste	②Mounting components	③Soldering in reflow oven
④Turn the PCB over.	⑤Cover the surface mount section with a carrier pallet.	⑥Insert components
⑦Apply flux	⑧Soldered on solder bath	⑨Clean flux

Carrier pallets

In double-sided mounting, soldering is performed in the order of reflow and flow. Before moving to the flow process, the PCB is set on a carrier pallet (also called a pallet or flow pallet) made of heat-resistant resin to cover the parts already mounted in the reflow process.
There are two main reasons for using a carrier pallets.

	Protects surface mount components from the heat of the solder bath.
	The carrier pallet protects previously mounted surface mount components from being damaged by heat as they pass through the flow solder bath.
	Prevents solder from adhering to surface mount components.
	The carrier pallet is open only where the leads of insertion-mounted components protrude so that they can be soldered. By setting the PCB on the carrier pallet, flux and solder do not adhere to previously mounted surface mount components.
Periodic cleaning of the carrier pallet is important because solder can creep up due to flux adhering to the edge of the carrier pallet opening (edge), and solder can also adhere to the masked area.

▼▽Cleaning the flow pallet is explained on the following page!▽▼
	Products information Carrier pallet cleaning system, PALLETCLEANER Regular cleaning of carrier pallets is necessary because flux adheres to the open edges of carrier pallets, and the adhered flux can cause solder to creep up and adhere to the masked areas. PALLETCLEANER & PALLETCLEAN, specialized for cleaning carrier pallets, greatly contributes to stable quality.

Manual soldering

Although the soldering process is increasingly automated, manual soldering (also called hand soldering) using an electric soldering iron is used in the following cases.

	Modification and Repair of Components
	When some components mounted on PCBs are not in good soldering condition, or when component failure or design changes occur, it is necessary to correct them by manual soldering.
	Resistance of components
	In some cases, reflow soldering or flow soldering is avoided due to the heat resistance of the component or the solvent resistance of the component. Manual soldering is performed while understanding the reason why the relevant component is to be retrofitted and giving individual consideration.

Manual Soldering Procedure

References

Standard Micro Soldering Technology, 3rd Edition
Japan Welding Engineering Society, Micro Soldering Education Committee [ed.]
Nikkan Kogyo Shimbun