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CONTAMINATION

By Akber Roy - Chief Technical Officer at RUSH PCB

Moisture Always Present

To some extent, moisture is always present inside Printed Circuit Boards (PCBs) and inside components. Usually there’s too little moisture to cause problems.
However, at the time of assembly when it’s wave or reflow soldered, moisture inside the PCB or components can increase the number of failures during assembly, and decrease the circuit assemblies’ total useful life.

Heat of Soldering

Heat during the soldering process vaporizes this moisture. The vapor, being trapped inside, pressurizes the PCB and components.
Low moisture levels mean low pressure, which usually isn’t a problem, however, as the moisture levels increase, the pressure from the heated vapor increases.
It’s the increased pressure during reflow or wave soldering that causes the damage to the PCB or components of the assembly.

Moisture Inside Components

During reflow, as their temperature increases for soldering, moisture inside components causes “Popcorn Phenomena,” bulging sides and sometimes actual popping sounds. This pressure also breaks connections inside the components.
In some cases during reflow soldering, components with higher moisture content “pop” and literally fall off the board.

Moisture Inside the Printed Circuit Board

Inside the PCB, moisture can be:
  • Initially present in the epoxy glass prepreg
  • Absorbed between layers during wet processes while being manufactured
  • Induced into small defects like micro-cracks or voids
  • Diffused into the PCB during storage

If the Soldering of PCBs is not completed within just a few days after the manufacturer’s packaging is removed, baking is required to prevent delamination.

Delamination of the PCB

Inside PCBs during soldering, moisture can be disastrous, it causes delamination during the short-duration high-temperature exposure from soldering. The rapid increases of vapor pressure from the moisture between layers forces the layers apart, or delaminates the PCB.
The reliability of PCB materials is strongly influenced by the presence of moisture, delamination can render a whole PCB assembly useless.
If the rising temperature is low, like in a baking oven, the vaporization is slow and the moisture has a chance to escape through microscopic holes. However, if the temperature rises higher, like for wave or reflow soldering, the residual water vaporizes quickly and doesn’t have time to escape, building the pressure that delaminates.

Moisture Enters Printed Circuit Boards and Components

In essence, the greater the humidity in the air, the faster the PCB or component draws in the moisture. Random agitation of the water molecules in air tends to cause them to go from the greater concentrations of moisture in the air to the lessor concentrations of moisture in the PCB or component.
Opening the factory vapor seal around the PCB or component starts this process.

Problems with Moisture

Moisture can increase the dielectric constant, leading to changes in impedance, switching speeds and propagation delay times.
Long term, because of the delamination and the presence of moisture, metal migration can create conducting bridges (shorts) between conductors on the board.
Delamination of printed circuit boards and bulging of components warp or change dimensions.

Lead Free Soldering and Moisture

Compared to lead-based soldering, the temperature profile for lead-free wave and reflow soldering is more demanding because lead free solder melts at a higher temperature. It’s even more important to solder the printed circuit assembly soon after opening the packages because higher soldering temperatures mean greater vapor pressure from the moisture vapor inside the PCB or components.

Moisture Sensitivity Level Rating

Often, the manufacturer gives components a Moisture Sensitivity Level (MSL) rating. The MSL rating indicates how long a component or PCB can be exposed to air before it has to be re-baked -- the higher the number, the more sensitive to moisture it is.
Lead free solder requires higher temperatures than lead based solder. Because of these higher temperatures, if lead free solder is used, this MSL rating has to be de-rated to indicate greater sensitivity.

Costs of Moisture in a Printed Circuit Board

If the PCB has too much moisture, the costs associated with delamination during soldering can be significant. When expensive components can’t be removed after delamination is discovered, major repercussions occur.

Problems with Re-baking

Re-baking to remove moisture from the printed circuit board may not be straight forward, it affects solder-ability. Making the PCBs harder to solder, heat from baking greatly increases the oxidation on the solder pads. Usually, re-baking of PCBs should be avoided.
In a few cases, if the boards are prone to delamination in the first place, re-baking can also result in bad outcomes.
This all means the re-baking of PCBs may not be an option, they may have to be returned to the supplier for reconditioning.
These are expensive and time-consuming steps.

Preventing Moisture Damage

In most cases, moisture levels inside the PCBs or components isn’t high enough to cause problems, but it’s always something to be concerned with.
The best way to prevent damage during the soldering process of the PCB assembly is the use of proper handling and storage before soldering (leaving the factory’s vapor barrier packaging intact), preventing the moisture from entering in the first place.

Contact RUSH PCB

For further information about manufacturing, contact the engineers at RUSH PCB  .