What is Reflow Solder Process (Ultimate Step By Step Guide)

In reflow soldering, a small electrical component attaches thousands of pads to the contact pads of a circuit board. Solder paste is part of this process, and a mixture of powdered solder and flux creates 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.

On the other hand, wave soldering is generally preferred for attaching multi-leaded through-hole components to circuit boards designed for surface-mount components.

Step by Step Reflow Solder Process

The Assembly of electronic components on printed circuit boards (PCBs) is 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) — Solder paste is applied to the PCB with a screen-printer. 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 or surface mount technology components are most commonly soldered with long convection ovens and reflow soldered to a PCB or printed circuit board. Reflow soldering applies heat to solder paste applied PCBs (Printed Circuit Boards) with assembled components.

Here are three main types of processes used for the mass reflow of PBAs:

IR ovens — The temperature difference between the IR element and PBA can be significant in IR ovens. For instance, it depends on the component’s color and the significant temperature difference between the IR element and PBA. This means that specific components are at risk of overheating. For lead-free soldering, IR ovens are not recommended.

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

Vapor phase (condensation) ovens — A liquid is used as the heat transfer medium in the vapor phase (or condensation) soldering. Heat transfer liquids melt 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 of 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

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 PCBs.

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

Preheat zone  In the preheat zone, solvent evaporation begins. I am keeping the temperature low to prevent the solvent from exploding and solder balls from forming. The moisture-sensitive components’ “popcorning” risk will also be reduced with a low-temperature increase.

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.

Reflow soldering defects

This section will discuss the primary smt reflow soldering defects 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 insufficient heat to reflow the solder adequately.

Possible solutions to this problem include Setting a maximum reflow temperature high enough for the 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 or none of the solder adheres to it. That is a phenomenon that could be associated with any process. Using a 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 tiny 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 to lead to stand upright.

Reflow soldering double-sided PCB

Because double-sided SMT reflow is highly in demand due to the cost-saving and compact PCBA size of advanced technology products, some risks and defects are involved with this process for double-sided printed circuit boards. Designers can take the following steps 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 being soldered first [5, 20].

Through-hole connectors, components with large mass and unsymmetrical weight or lead placement should be located on the same side of the PCB to 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.

Fish-bone diagram for reflow soldering

Conclusion – Wrapping it up

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Hello, this is Manoj, A Soldering and DIY Tool expert. Writing is one of my hobbies. With more than 20 years of broad experience. I love researching, discovering, and sharing new products with others, I hope you’ll enjoy the greatest featured products to make life easier, more fun, and more productive.

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