Reflow soldering is an important process in printing circuit board
assembly. The method involves melting solder paste to connect components to the
board. It ensures that the components make good electrical contact with the
PCB. Reflow soldering has a number of benefits but has some challenges when
soldering that can lead to defects like insufficient solder (a defect that will
be examined more closely), cold solder joints, blow holes, flux residues, etc.
Soldering defects can disrupt PCB pathways and undermine the performance
capacity of electronic devices and gadgets. In this article, we will go through
various issues in reflow soldering, issues solutions, and best practices that
can be implemented while designing the PCB to avoid failures.
What
is Reflow Soldering?
Reflow soldering is a commonly used process applied for the assembly
of printed circuit boards (PCBs) of SMT. The process consists of applying
solder paste to pads on the PCB where components are supposed to be placed. The
PCB is then moved into a reflow oven where the board is heated in a controlled
process. When the solder paste melts, it bonds the leads of the component to
the pads on the PCB. As the plate cools down, the solder becomes solid, and a
connection is created.
The commonly used process of reflow soldering consists of three
stages. During the preheat phase, the PCB is slowly heated to a temperature
that is suitable for melting the solder paste. The melting period, where the
solder paste reaches melting point; this temperature is around 180°C to 250°C
depending on the solder. During the cooling stage, the solder hardens and forms
the required joints. Maintaining the right temperature profile is important to
achieve quality solder joints.
Common
Challenges in Reflow Soldering
It's very useful and popular, but reflow soldering also has its
challenges. Each step in this process requires maximum management of
temperature including the timing and placement. Minor changes to any of these
parameters can cause solder defects. In the discussion that follows, we’ll go
through the most common issues and challenges in reflow soldering, along with
their solutions.
Cold
Solder Joints
Cold solder joints happen when the solder does not melt in a perfect
manner in reflow, which causes weak and incomplete solder joints between the
component leads, as well as between PCBs. An electrical failure and signal
integrity issue may occur from these joints.
Solution
A cold solder joint can also occur if the setting for the maximum
temperature of the reflow oven is inaccurate or the oven is set on random
settings. In addition, the temperature and PCB output rate must be correct. It
is important to maintain a constant and stable temperature profile to avoid
cold solder joints. Moreover, using the most suitable solder paste for the PCB components
and the right paste application can help make it reliable soldering.
Blow
Holes (Void Formation)
Air pockets trapped in the solder joint are named as Blow holes and
are also called Voids. These voids can weaken the solder joint and cause its
failure. When there is insufficient flux or improper heating, blowholes occur
most frequently and make the reason of blow holes.
Solution
To stop blow holes, the solder paste must have a good flux amount.
This permits the solder to flow better, which limits the possibility of voids.
Besides, optimizing the temperature profile of the reflow oven can prevent
trapped air. For example, the blowing holes can be avoided during the reflow
stage if the printed circuit board (PCB) is heated uniformly.
Insufficient
Solder
The insufficient solder is the reason of less solder paste to the
PCB pads, or solder paste did not flow properly during the heating process.
Solder joints that can easily fail may result from this factor.
Solution
The solder paste must apply at the right amount and there must be no
mismatch of the pads to stop insufficient solder. This can be attained by using
a high-quality stencil and precision paste printing machine. Moreover, by controlling
the heating profile of oven can cause the solder paste to melt completely and
flow properly, achieving a sufficient amount of solder in the joint.
Solder
Bridging
Solder bridging refers to the unwanted connection of two adjacent
pads caused by the excess solder being applied to the pad. Excessive solder
paste, incorrect components' location or improper reflow profiles could be
causing the problem.
Solution
It is important to use the required amount of solder paste and place
the suitable components correctly to avoid solder bridging. To confirm the uniform
solder paste melting without excessive solder spillage onto nearby pads on PCB,
it is an essential to adjust the reflow oven’s temperature profile as well.
Also, checking for excess solder paste on the PCB before reflowing can help
detect bridges.
Flux
Residues
Flux residues are leftover chemicals from the flux used in solder
paste. These residues can affect and decrease the overall performance of the
PCB, also it may cause corrosion, or result in electrical failures if not
properly cleaned.
Solution
The proper flux formulation and the proper cleaning after reflow can
minimize the building up of flux residues. Depending on the flux type used,
cleaning in a solvent or in an ultrasonic cleaning machine may be used.
Besides, it is also essential to ensure to comply with the manufacturer’s
recommendation for the removal of the flux residue.
Inconsistent
Reflow Profiles
During the reflow process, an inconsistent temperature profile can
cause defects like cold solder joints, insufficient solder, blow holes, among
others. The precise solder reflow temperature profile is needed for soldering
uniformly.
Solution
To maintain a consistent, reflow profile and at the same time
prevent the oven from operating in abnormal conditions, the reflow oven shall
be calibrated regularly. Monitoring the PCB reflow temperature with a
thermocouple can enable adjustment of the settings to improve the oven’s
performance. Just like any other production machine, oven requires regular
maintenance.
How
to Design Your PCBA to Prevent Failures During Reflow
Reflow soldering problems can be avoided with proper design
practices as early as possible. Through organizing the placement of components
and board layouts designers can minimize the risks of solder defects during
reflow.
Use
Sufficient Pad Sizes
The size of the pads on the PCB should be right for the components
used. Using pads more than what is required can cause poor solder joints while
unrequired oversized pads can cause solder bridging. Using standards for pad
sizes should minimize the chances of these problems.
Avoid
Large Components Near Small Pads
Large components can impede the flow of solder paste and cause
issues with soldering. When designing the PCB, avoid placing large components
near small pads that require precise soldering.
Consider
Component Orientation
Be sure that components are in the right orientation so that there
are no soldering issues. Components that aren’t correctly oriented may not get
soldered during reflow, resulting in cold solder joints or required solder.
Use
the Correct Solder Paste
Choosing the appropriate solder paste for the application is vital.
Some solder pastes work better than others in certain component types or
high-temperature applications. Choosing the right paste will help you to get
quality soldering operations.
Optimize
Component Placement
Ensure the correct and secure placement of components on the PCB
before reflow. Misalignment of the component leads may cause to damage to the
PCB Assembly due to solder bridging or required solder on the PCB.
Minimize
the Use of Through-Hole Components
Some through-hole components are still used in applications but it
makes the reflow soldering process complex. When possible, use surface mount parts
that are more compatible with reflow soldering, reducing the chance of faults.
Provide
Proper PCB Design for Thermal Management
To maintain a consistent solder reflow temperature throughout the
PCB during reflow, thermal management is essential. Ensuring correct placement
of thermal vias and heat-sensitive components can save your board from thermal
cycling damage.
Reflow soldering is important in PCB assembly, but the process does
have a few challenges associated with it. There can be many defects when reflow
soldering, including cold solder joints & blow holes, insufficient solder,
flux residues & more. However, these challenges can be reduced or even
avoided with proper designs, reflow control, and selection of materials. By
understanding and paying attention to the details of reflow soldering,
fabricators can produce quality PCBs to suit modern electronics.
