PCB ASSEMBLY REFLOW SOLDERING

The predominant method used for attaching surface mount components to printed circuit boards during PCB assembly is reflow soldering.

Reflow soldering is a four-step process that involves:

1. The initial heating of the components, PCB, and solder paste (preheat)

2. Thermal soak

3. The careful melting of the solder (reflow)

4. Cooling

Preceding the reflow soldering process, a stencil is created and used to apply the solder paste to the contact pads on the PCB. A pick-and-place machine then places the components on the board. For double-sided boards, the board is flipped and the process is repeated.

The board then goes into the reflow oven to form the solder joints and bind the components to the board.

Reflow Step One: Initial Heating / Preheat

The initial heating step is to ensure the assembly and solder reach a specified temperature at a specific rate, also known as ramp rate. The heating rate during this step is defined by the solder paste’s datasheet and guidelines but usually is 2oC/second or less.

Heating the solder too quickly can cause splattering, leading to solder balls/beads. Solder beading will fail minimum gap requirements, affecting the board’s electrical reliability. Components can also be damaged if heating guidelines are not properly followed during these steps.

Reflow Step Two: Thermal Soak

The thermal soak step lasts just 1 to 2 minutes and will get all the components to the exact required temperature for reflow in step 3. It’s important to note that the initial heating step is critical prior to the soak step to ensure the heating rate is sufficiently gradual. Likewise, it’s critical the soak phase is done at the perfect temperature or the flux (flowing agent in the solder paste) can suffer from defects like graping and/or head-in-pillow (HIP or HNP).

Reflow Step Three: Reflow

The reflow step brings the oven, board, components, and solder to the maximum temperature during the entire process. In this step, the solder paste is melted to fuse the solder joint. The solder flux activates and the powder melts, forming the joint, while the flux evaporates away.

It is critical that the maximum temperature is established properly during the reflow profile definition. This maximum should be below the component that has the lowest maximum temperature allowance, but above the reflow melting point of the solder paste used. This maximum temperature is a delicate point that needs to be quite precise - it can not be too hot (to avoid thermal damage to board and/or components) or too cool (to avoid poor coalescence / solder joints).

“Time above liquidus” (TAL) is also established - the length the solder is held above its melting point. This time is critical to ensure appropriate “wetting” materializes between the components and board. This time is typically 30 seconds to 1 minute. Too much time at this temperature can cause damage to the components or can lead to brittle solder joints.

At the melting point, the flux within the solder paste reduces surface tension at the juncture point of the metals to achieve metallurgical bonding. At this point, the solder powder spheres combine with the board and components.

Nitrogen Reflow Soldering

A reflow environment with nitrogen is often recommended or required by assembly specifications to reduce surface oxidation. Oxygenated reflow environments are often not compatible with the flux in solder paste, leading to non-solderable oxidized surfaces and wettability issues. Many soldering defects can arise from oxidation, like the head-in-pillow defect.

An oxygen-free environment, with the use of nitrogen reflow soldering, will prevent or drastically slow down oxidation, improving wettability, to reduce these types of defects.

During the reflow process, the chamber of the oven is filled with nitrogen, forcing the oxygen to the bottom of the chamber and reducing its exposure with the solder paste.

Nitrogen reflow soldering, however, does have some negatives including high cost and some other potential effects like increased tombstone phenomenon and solder wicking / solder drainage.

Reflow Step Four: Cooling

The final step is to simply cool the reflowed board at a controlled rate. It is important to cool it at a governed rate, typically 2 to 4°C per second, to properly solidify the solder joints, while avoiding thermal shock. Generally, a faster rate will result in a finer grain structure in the metals, minimized intermetallic growth, and stronger solder joints, but too fast can result in thermal shock. Note that a max of 6°C per second is often specified for boards but this rate does offer significant shock risk to the assembly.

This step is also challenging as the cooling rate is not easily controlled. Some reflow ovens have the capability to control the cooling rate by varying a cooling fan. Other variables also come into effect, such as ambient air temperature and the thermal mass of the board (higher thermal mass will cool more slowly).