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Tips for Reflow Soldering of SMT | Ultimate Guide to PCB Reflow

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What is reflow soldering?

After a solder paste is used to attach thousands of tiny electrical components  to their contact pads, the entire assembly is sent to a controlled heat. Air or nitrogen is heated to a high enough temperature and then blown to the assembly, and the solder on both sides of the component melts and bonds to the motherboard.

reflow soldering

In this article, we will review tips, temperature requirements, critical stepsin reflow soldering, and common reflow soldering problems. Stick around to review in-depth and learn everything you are looking for.

Reflow Soldering Process and Tips

Reflow soldering is the most popular and widely used method of connecting surface mount technology (SMT) components to a printed circuit board (PCB) . The process is focusing on creating acceptable solder joints by pre-heated components, which provides the opportunity to melt the solder without causing damage by overheating.

In order to create an appropriate reflow profile, thermocouples are connected to a simple assembly (often using high-temperature solder), in different locations to measure the range of temperatures across the printed circuit board.

Note
The recommendation here is to have at least one thermocouple on the pad in front of the edge of the printed board, and one thermocouple towards the middle.

The approach to identifying the range of temperatures across the board using thermocouples is known as " Delta T".

The reflow profile includes four main parts of the process that need your careful attention:

  • Proheat- During this phase of the process the printed circuit board, components and solder are heated to a specific dwell or soak temperature, with the main aspect, which is not to be heated too quickly (usually no more than +2 degrees a second). Fast heating could cause component defects such as solder paste  slattering or cracks.
  • Soak- This is the part of the project in which it ensured that the components have reached the required temperature before entering the next reflow stage. The soaks last no more than 60 to 120 seconds, depending on the type of assembly.
  • Reflow- In this general stage, the temperature in the oven is increased above the melting point of the solder paste, forming it into liquid. The time in which the solder is held above the melting point temperature (time above liquidus) is important to ensure that the "wetting" occurs between the printed board and the components. The time is about 30 to 60 seconds and it should not exceed to avoid the creation of brittle solder joints.

    Another thing that you should know is that the peak of the temperature has to be strictly controlled as many of the components could fail if they are exposed to excessive heat. On the other hand, it is important for the temperature to be high enough in order to achieve the desired results. If the reflow profile provides insufficient heat, it will result in bad solder joints which is one of the most common defects.

  • Cooling- It is not healthy to heat the assembly fast, here would be the same. Make sure to never cool down the assembly too rapidly as it could lead to malfunctions. The recommendation here is to keep the rate of cooling above 3 degrees a second.

Temperature Requirements for Reflow Soldering

The PCB industry has many different reflowing components requiring certain temperatures.

With Sn/Pb solders, the melting point temperature typically peaks around 215°C to 245°Cwith around 20-60 seconds above 183 degrees.

For lead-free soldering, the reflow temperature should reach 260°C.

The typical reflow soldering oven has different temperature zones and the temperature occasionally increases according to a pre-set curve that has to be adjusted as per the assembly aspects and temperature requirements.

Advanced printed circuit board assemblers like PCBONLINE  use a reflow soldering oven with 10 temperature zones. While those simple PCB assembler’s ovens have only 8 temperature zones, and they can’t solder lead-free boards.

Suitable Machines and Equipment

reflow soldering oven

The preferred approach to reflow soldering is with an industrial convection oven to SMT components to a PCB.

There are many different types of reflow soldering machines available, specializing in a different design, line of speed, material, and printed circuit boards. The oven has to be a suitable size to seamlessly handle the production rate of the pick and place equipment.

Here is how to calculate the production line speed:

reflow soldering line speed

When you know the line speed, it is easy to determine what chamber and heated length to use in order to meet your workflow requirements. However, if you are not determined to build yourself an assembly line, it’s not recommended to buy such an expensive reflow soldering oven. You can turn to an advanced PCB assembler .

PCB Assemblers Recommendations - Where You Can Expect Perfect Reflow Soldering

If you are looking for a reliable PCB assembler that is familiar with reflow soldering, PCBONLINE  is the solution for you. They are advanced printed circuit board assemblers with rich experience. Their reflow soldering ovens cover all the customer’s requirements. Working with them, you can receive assemblies exceeding your expectations.

Here are some reasons why to choose PCBONLINE:

  • They apply nitrogen reflow soldering to effectively prevent oxidization.
  • They can reflow solder any type of printed circuit board including the lead-free ones.
  • Advanced machines and technology are used, such as 30 Panasonic and Yamaha SMT lines, Germany ERSA selective wave soldering, solder paste detection 3DSPI, AOI, X-ray, etc.
  • Before assembly, they check your bill of materials carefully free of charge.
  • They test the functionality of the PCBA before delivery to you.
  • They can provide all components in your bill of materials for assembly.

Also, for batch assembly, they provide the first article inspection. With PCBONLINE, you will never worry about problems caused by poor wave soldering illustrated in the next part.

Reflow Soldering Common Problems and Their Causes

Reflow soldering requires extensive consideration of the key factors. Although there are many different issues that can appear, we review some of them to illustrate some occasional results.

soldering problems

Oxidization– The issue occurs whenever oxygen finds itself into the reflow oven causing a quick degradation of matter. Oxidization is a common issue that is directly related to the high temperatures and presence of oxygen during the reflow or wave soldering process. The results are usually degradation of metal surfaces and environmental robustness, impacting the electronic hardware lifetime and the components.

The level of oxidization is depending on several factors where the temperature degree is the most important. The second most cormorant is the selection of activation flux that is part of the solder paste. Most manufacturers are excluding the highly activated fluxes from the reflow soldering process due to these reasons.

Outgassing- Non-hermetic, epoxy-based packages used by a number of component designs are featuring a microporous structure that may act as a moisture residual trap. While the components are designed and constructed to operate reliably in accordance with their specifications, some outgassing and fast moisture evaporation can occur during the high-temperature board mounting process.

This may not be a danger to the components, but if the preheat and temperature conditions are not as per the manufacturer's requirements, it often results in a failure. Larger cases in outgassing can blow out the displaced nearby small parts.

Components displacement– During reflow soldering, some components may disconnect from their pads. The component’s displacement is often caused by changes in surface tension during the thermal flow. Here are some other factors that contribute to this problem:

  • Improper pad design layout and position
  • Different heat capacity connected to the pads
  • Wettability differences between the two terminations
  • Solder paste printing misalignments
  • Faulty solder paste application
  • Faulty pick and place process pushing one termination stronger inside the solder paste
  • Misaligned component placement

Tombstoning- Sometimes, some component is not attached properly to the board and is lifting from one side. It occurs more frequently with short, light, thin, and wide terminations devices, such as miniature resistors and capacitors.

Sulfur Contamination– The issue occurs when the silver inside the resistor gets contaminated with sulfur leading to a functionality failure. It is associated with the use and reliability of the thick-film chip resistor with Ag-system as inner termination. The silver existing in the inner termination is likely to be contaminated via sulfur, which produces silver sulfide in chip resistors. The process can be initiated or inhibited by heat-stress while mounting.

How to Make Sure of Successful Reflow Soldering

Here are the essentials that lead to an efficient reflow soldering:

  • Good quality PCB, solder paste, and components
  • Printed boards using a quality stencil
  • Acceptable reflow profile
  • Suitable machines and equipment
  • Reasonable footprint designs for the components

Good Quality PCB, Solder Paste, and Components

PCB

Poor quality of PCB, solder paste, and components will lead to undesirable results. If the printed circuit boards were not stored as per the quality requirements, it often results in poor solderability during the reflow process. Besides, if the PCB manufacturing is poor, it can result in a general defect known as "Black Pad".

PCBONLINE  is a one-stop advanced PCB manufacturer. Sending your Gerber and BOM online to them, you can get a high-quality PCB properly mounted with required components with free design for excellence.

Printed Boards Using Quality Stencil

SMT stencil

One important part of the surface mount assembly is the solder paste application on the printed circuit board. This is generally achieved by using foil or stencil. The stencil is a thin sheet of material (usually stainless steel) with numerous holes. The purpose of the solder paste stencil is to accurately insert the right amount of solder paste onto the mount pads.

Acceptable Reflow Profile

There are several factors that you should pay attention to before reflow solder. To create an acceptable reflow profile, each of the assemblies has to be reviewed as a separate unit, in order to identify how each of the aspects can influence the reflow oven.

The general factors to take under careful consideration are printed circuit board material, quality of the solder paste, printed circuit board thickness, amount of copper in the printed board, number of layers, and quality of surface mount components.

Reasonable PCB Components Footprint Design

The printed board design has a large impact on the assembly reflow. A good example would be the size of the connecting "tracks" to a component footprint. For instance, if the track connecting to one side of the component footprint is larger than the other, this could result in a thermal imbalance causing the part to "Tombstone".

Another good example would be "copper balancing". The majority of PCB designs are using large copper areas and if the PCB is put into a panel to aid manufacturing, it can lead to an improper balance in the copper. This can cause the panel to warp during reflow soldering. The recommendation here is to add "copper balancing" to the waste areas of the panel.


Conclusion

Reflow soldering requires extreme precision and consideration. If you have read the whole article you should now be aware of almost every aspect of reflow soldering.

PCBONLINE provides the best PCB assembly  with precise placement and perfect soldering. Any type of order or amount is available for assembly if you get in touch with PCBONLNE. If you have any hesitation, you can click to get a quote online free of charge.

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