During manufacturing, and especially during assembly, there are many possible defects that can lead to a board being scrapped. During assembly, boards will be subjected to repeated thermal excursions, noxious chemicals, cleaning agents, and a round of inspections to ensure quality. During this process, there is one potential defect that is not often discussed: PCB delamination.
During assembly, if conditions are right in the material and soldering parameters, delamination may occur, where portions of the laminated board begin to separate. No PCB is perfect, and it is possible for some small amount of delamination to occur in some areas of the PCB, large scale delamination should be prevented. This involves properly pairing the reflow profile, materials, pre-bake parameters, and storing laminates correctly.
What is PCB Delamination?
When a PCB experiences delamination, the layers of base material begin to separate from each other. If viewed on a fabricated bare PCB or a fully assembled PCBA, the solder mask could appear to have large regions of discoloration and bubbling when delamination has occurred. Related forms of delamination are also known as blistering and measling. The former is nearly the same as large-scale delamination, while measling is the formation of very small delaminated white spots.
An example image showing delamination on the surface layer of a PCB is shown below. The somewhat-circular discolored area is where the top layer of materials are beginning to separate from the base material.
This image shows delamination occurring on the rigid regions of a flex cable. [Source]
The two main factors that can lead to delamination are moisture exposure and repeated thermal excursions.
Moisture trapped in the PCB base materials is the most common cause of delamination. Improper storage conditions of laminate materials can lead to an uptake in humidity, and excess humidity in the PCB substrate during fabrication can cause delamination. PCB base materials are hygroscopic and water will easily adsorb into uncured base materials. This can happen if storage conditions are humid or the materials are exposed to some condensation.
Excess moisture trapped in the board doesn’t always cause delamination, but it will contribute to conductive anodic filamentation (CAF). Water is involved in electrochemical reactions that cause growth of small filaments between conductors in the PCB. This growth can cause conductors to bridge, creating a short circuit that ruins the board, and the reaction rate will be accelerated by the presence of water. CAF can be identified through microscopic observation (normally with a scanning electron microscope) from a microsection or directly on the surface layer (see below).
This image shows the main electrochemical reaction leading to filamentation and an example of filamentation on the surface layer of a PCB. [Source]
Anytime there is moisture in the board, it will turn to steam during later processing stages at elevated temperatures. For example, reflow solder processes are performed above 200 °C, which is well above the boiling point of water. The pressure steam will interfere with board pressing and curing to create a weak bond, which might lead to delamination during subsequent processing steps or during a large thermal excursion when the PCBA is put into operation.
The other area to look into is the period at which a PCB is held at high temperatures. Repeated thermal excursions up to high temperature, particularly if the glass transition temperature is exceeded, can cause delamination during assembly. Even after the board is assembled, if the PCBA undergoes repeated thermal stress above its glass transition temperature over time, it is possible that delamination begins to occur.
To prevent delamination during manufacturing, the reflow profile and wave soldering parameters should be examined to ensure there will not be excessive stress on the base PCB materials. Thermally-induced delamination might begin in the interior layers of the board without inducing blisters or cracking on the external layers. If there will be multiple soldering cycles or the board will be expected to undergo large thermal excursions during operation, then high-Tg board materials should be used.
Prevention Begins With Your Manufacturer
As is usually the case with other manufacturing defects, there is no single cause of delamination. The factors listed above work together to cause delamination and produce the defect shown above. The solution to remove moisture is pre-baking the PCB base materials before fabrication, and possibly before assembly. Baking the material slightly above the boiling point of water in vacuum will force water to desorb from the base materials without activating DICY/phenol curing agents.
There is not much you can do as a designer to guarantee your boards will not experience delamination. It is the job of the fabrication house to ensure materials are stored properly and the proper material pairings are made when planning the stackup for fabrication. The assembler needs to ensure the reflow profile is properly selected so that thermal excursions do not repeatedly exceed the laminate’s Tg value. Finally, a pre-bake step may be needed before fabrication and assembly just to ensure any remnant humidity is removed from the base materials and the assembled board.
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