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In reflow soldering, a small electrical component attaches thousands of pads to the contact pads of a circuit board. As part of this process, solder paste is used which is a mixture of powdered solder and flux that is used to create permanent solder joints. 

You can also use reflow ovens to solder through-hole components simply by filling the through-hole holes with solder paste and inserting the component leads through the paste. Wave soldering, on the other hand, is generally preferred for attaching multi-leaded through-hole components to circuit boards that are designed for surface-mount components.

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Step by Step Reflow Soldering Process

Assembly of electronic components on printed circuit boards (PCBs) is known as an electronics assembly. Surface mount technology (SMT) is the electronics assembly process where electrical throughout the PCB, individual pad connections are used to connect component leads.

Here’s main the basic SMT process consists of the following steps:

Step 1 (Solder Paste Printing) — With a screen printer, solder paste is applied to the PCB. In addition to stencils with individual holes over individual pads, boards are designed with solder holes for the application of solder.

Step 2 (Component Placement) — Electronic components are positioned on a PCB with placement equipment (pick-and-place machines, chip shooters, etc.). Typically, solder-pasted pad leads are placed directly in contact with component leads.

Step 3 (Reflow Soldering) — Solder paste is heated until liquidus (reflowed) and cooled until hardened, creating a permanent connection between the component leads and the PCB. In a SOLDER REFLOW OVEN, this process is carried out.

The finished circuit board can then be tested, cleaned, or assembled into a final product.

What are the types of Reflow Ovens?

SMT components or surface mount technology components are most commonly soldered with long convection ovens are reflow soldered to a PCB or printed circuit board. Reflow soldering is the process of applying heat to solder paste applied PCBs (Printed Circuit Boards) with assembled components.

Here are three main types of processes used for mass reflow of PBA’s:

Convection ovens — PBAs are usually soldered in high-efficiency convection ovens that use high amounts of air and nitrogen turnover to achieve the best soldering profile and provide fairly even temperature distribution across the board. Using nitrogen (nitrogen) for convection ovens has the disadvantage that gas consumption is relatively high

IR ovens — In IR ovens, the temperature difference between the IR element and PBA can be large, for instance, it depends on the color of the component and the large temperature difference between the IR element and PBA. This means that certain components are at risk of overheating. For lead-free soldering, IR ovens are not recommended.

Vapor phase (condensation) ovens — In the vapor phase (or condensation) soldering, a liquid is used as the heat transfer medium to transfer heat.. Heat transfer liquids melting at a maximum temperature and thus provide a very low DeltaT even for large masses with uneven mass distribution across the PBA.

Reflow soldering thermal profile zone

There are mainly two different reflow profile types, Ramp-to-Peak and Ramp-Soak-Peak. It is primarily a matter of following the instructions for the solder paste manufacturer when choosing between ramp-to-peak or ramp-soak-peak reflow profiles.

Here are four types of reflow soldering zones used for the reflow soldering process

Preheat zone — In the preheat zone, solvent evaporation begins. It is advisable to keep the temperature increase low in order to prevent the solvent from exploding and solder balls from forming. The risk of moisture-sensitive components “popcorning” will also be reduced with a low-temperature increase.

Soak zone — In the soak zone, flux in the soldering paste is activated. The flux wets both the component leads and the PCB pads to remove oxides. Too short time in the soak zone may give unsoldered joints because of oxides remaining on the soldering surfaces. A too long time may exhaust the flux, leaving too little flux activity for the peak zone.

Reflow zone (Peak zone) — Reflow occurs in the reflow zone when the surface of the metal melts and watts. To reduce the risk of thermal damage to the components, the solder should be in a liquidus state for as little time as possible.

Cooling zone — In the cooling zone, the PBA is cooled to solidify the solder and then to an even lower temperature so that the PBA can be handled outside the reflow oven (e.g. on conveyors, in PBA racks etc.). Too fast cooling can damage components and PCB.

Reflow soldering defects

In this section, we will discuss the main smt reflow soldering defects that can be caused by inefficient reflow profiles and the reflow soldering process. We will also discuss how extreme temperature and humidity conditions affect crop performance. 

1. Cold Solder Joint (dull joint)

In the Cold Solder Joint reflow soldering defects, A dull, grayish appearance is caused by poorly wetted solder connections after soldering. It could be caused by not enough heat being present to adequately reflow the solder.

Possible solutions to this problem include Setting a maximum reflow temperature high enough for reflow of the material, After reflow, accelerate the cooling rate, and Vibrations of an assembly during reflow and immediately afterward are minimized.

2. Non-wetting

This condition occurs when molten solder contacts a surface and parts of or none of the solder adheres to it. Basically, this is a phenomenon that could be associated with any process. Using long soak time in the reflow process may also be causing the problem.

It may also be caused by insufficient heat during the reflow process, in which case the flux will not be able to reach the right activation temperature.

3. Solder Balling

The term solder balling describes the formation of very small spheres of solder that separate from the main body of solder that forms the joint. What causes solder balls during reflow? Solder balls may be caused by moisture-contaminated solder paste. Solder spheres are left behind when moisture splatters during the reflow

4. Tombstoning

Tombstoning refers to a soldering defect resulting from force imbalances during reflow soldering that causes a chip component to pull into a vertical position and have only one terminal connected to the PCB.

An uneven heating process can result in tombstoning by causing a difference in temperature at the component terminals. The solder melts at different rates, causing one side to reflow before the other, causing the other lead to standing upright.

Reflow soldering double sided PCB

Because double-sided SMT reflow is highly in demand due to cost-saving and compact PCBA size of advanced technology products. There are some risks and defects involved with this process when it comes to double-sided printed circuit boards. The following steps can be taken by designers to prevent double-sided boards from exhibiting these defects.

There are two alternatives for reflow soldering of double-sided PBA:

Alt 1: — On the first side to be soldered, the component bodies are attached with glue to hold them in place during the second reflow. Besides adding the extra sub-process of glue dispensing, there is also a risk of getting adhesive on the soldering surfaces.

Alt 2: — When soldering without attaching the component with adhesive, ensure that only components with satisfactory weight per footprint area are used on the side that is being soldered first [5, 20]. Through-hole connectors, components with large mass and/or unsymmetrical weight or lead placement should be located on the same side of the PCB so that they can be soldered during the second reflow pass. If done correctly, the bottom side components will be held in place by the solder’s surface tension during liquidus.

The reflow profile should be adapted to the board side that will be soldered. The top temperatures of the package should also be paid attention to when performing the second soldering operation.