Remove dirt/dents from PCB
The step of removing stains is to use chemical methods to remove resin stains on the inner copper layer. This kind of grease was initially caused by drilling holes. Concave corrosion is a further deepening of stain removal, which involves removing more resin, allowing copper to "protrude" from the resin and form a "three-point bonding" or "three sided bonding" with the copper plating layer, improving interconnection reliability. Permanganate is used to oxidize resins and 'etch' them. Firstly, the resin needs to be swollen for permanganate treatment, and the neutralization step can remove the residual permanganate. Glass fiber etching uses different chemical methods, usually hydrofluoric acid. If not properly stained, it can cause two types of voids: resin stains on rough pore walls may contain liquid, which can lead to "blowing pores".
The residual dirt on the inner copper layer can hinder the good bonding of copper/copper plating layer, leading to "hole wall pullaway" and other issues, such as separation of copper plating layer from hole wall during high-temperature treatment or related testing. Resin separation may cause detachment and cracking of pore walls, as well as voids on the copper plating layer. If the potassium permanganate residue is not completely removed during the neutralization step (specifically in the case of a reduction reaction), it may also lead to voids, and reducing agents such as hydrazine or hydroxylamine are often used in the reduction reaction.
PCB drilling
Worn drill bits or other inappropriate drilling parameters may tear the copper foil and dielectric layer, forming cracks. Fiberglass may also be torn instead of cut. Whether the copper foil will tear from the resin depends not only on the quality of the drilling, but also on the bonding strength between the copper foil and the resin. A typical example is that the bonding between the oxide layer and the semi cured sheet in multi-layer PCB boards is often weaker than the bonding between the dielectric substrate and copper foil, so most tearing occurs on the surface of the oxide layer of multi-layer PCB boards. In the gold phase, tearing occurs on the smoother side of the copper foil, unless "reverers treated foil" is used.
The weak bonding between the oxidized surface and the semi cured sheet may also lead to worse "pink circles", where the copper oxide layer dissolves in acid. Rough borehole walls or rough walls with pink circles can lead to cavities at multi-layer junctions, known as wedge cavities or blow holes. The "wedge cavities" are originally located at the junction interface, and their name implies that they have a shape like a "wedge" and can be retracted to form cavities, which can usually be covered by electroplating layers. If the copper layer covers these grooves, there is often moisture behind the copper layer. In subsequent processes such as hot air leveling, moisture (moisture) evaporates and wedge-shaped cavities usually appear together. Based on their location and shape, it is easy to confirm and distinguish them from other types of cavities.
Catalytic steps before PCB chemical copper deposition
The mismatch between decontamination/pitting/chemical copper deposition and the insufficient optimization of each independent step are also issues worth considering. Those who have studied voids in pores strongly agree with the unified integrity of chemical treatment. The traditional pre-treatment sequence for copper deposition is cleaning, adjustment, activation (catalysis), acceleration (post activation), followed by cleaning (rinsing), pre soaking, which is fully suitable for the Murpiy principle. For example, adjusters, a cationic polyester electrolyte used to neutralize negative charges on glass fibers, often need to be applied correctly to obtain the desired positive charge: too few adjusters result in poor activation layer and adhesion; Too much adjusting agent can form a film, leading to poor adhesion of copper deposition; Causing the hole wall to pull off. Insufficient coverage of the adjusting agent makes it most likely to appear on the glass head.
In the gold phase, the opening of voids is manifested in poor copper coverage or absence of copper at the glass fiber site. Other causes of voids in glass include insufficient etching of the glass, excessive resin etching, excessive etching of the glass, insufficient catalysis, or poor activity of the copper sink. Other factors that affect the coverage of Pd activation layer on the pore wall include activation temperature, activation time, concentration, etc. If the cavity is on the resin, there may be the following reasons: manganese oxide residue from the decontamination step, plasma residue, insufficient adjustment or activation, and low activity of the copper sink.