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What is drying process in HDI PCB manufacturing?


Measures involved in HDI PCB manufacturing

There are certain measures that are involved in HDI PCB manufacturing and these are as follows:

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Usage of base materials in HDI PCB manufacturing

There is the use of base materials with a high glass transition temperature, the execution of massive polygons of internal layers in the form of grids, the strengthening of adhesive bonds by special treatment of metal surfaces (oxidation). Moisture removal HDI printed circuit base is important.

It is because, the need for this process is not obvious to everyone. And if strict rules are established in soldering, that prevent component’s wetting before soldering, then there are no such rules for mounting substrates in HDI PCB manufacturing

Why optimum volumetric level is important?

Although it is no less important for HDI printed circuit boards to maintain a minimum level of volumetric moisture to prevent their delamination during soldering. This is especially important for group heating methods and is even more necessary for wave soldering of solder.

Of course, standard test methods for basic materials provide for their resistance to thermal shocks that simulate soldering processes. But the path from the semi-finished product to the finished product is so long and diverse that it is difficult to expect the same resistance to soldering from HDI printed circuit.

Drying process in HDI PCB manufacturing

First, in HDI PCB manufacturing it is worth recalling that the greater penetrating power of water is due to the small size of its molecule. In comparison with capillaries and intermolecular cavities in solid materials. The moisture in HDI printed circuit boards has various forms, differing in the bond strength.

During drying, the heating energy and diffusion processes stimulated by heating should break such bonds and remove a certain proportion of moisture from the volume. This proportion significantly depends on the ratio of the strength of various types of moisture bonds (micro and macro structure)

Forms and types of humidification of the bases of the boards

Moisture permeability and water sorption in HDI PCB manufacturing by a material comes by the physical state of the material, surface phenomena, supramolecular structure, and for polymers also by the chemical structure of the structural link and the polymer chain.

Moistened material classification in HDI PCB manufacturing

The properties of the moistened material depend on the form and type of connection of moisture with the dry material substance. The most complete classification of the forms of connection of moisture with the material comes in the theory of drying.

Since the process of removing liquid from a material comes by a violation of its connection with the structure of this material, which consumes a certain energy. The following classification of forms of communication proposes in HDI PCB manufacturing.

1) First and foremost, chemical bonding

2)Secondly, physical, and chemical bonding

3) Above all, physical and mechanical connection

Water-material bonding’s main forms

Let’s consider the definition of the main forms of water-material bonding:

How a chemical bond is formed in HDI PCB manufacturing?


– A chemical bond comes by a bond in strictly defined molecular relationships (stoichiometric bond); it includes an ionic bond formed because of a chemical reaction, and a molecular bond with the formation of crystallohydrates.

During the drying process, the chemically bound moisture  so firmly fixes in the volume of the material that, as a rule, it is not removed at acceptable drying temperatures.

How a physico-chemical bond is formed?

The physico-chemical bond occurs in various, not strictly defined ratios. HDI PCB manufacturing includes the adsorption bond (the bond of the liquid in the hydrate shells) and the osmotic bond. Upon contact of the adsorption-bound liquid with the surface of the material, heat releases.

However, while the greatest amount of heat releases during the formation of the first monomolecular layer. The first layer of water is under high pressure due to the molecular force field, because of which the density of the liquid in this nanolayer is greater than usual.

Properties of Adsorption-bound water

Adsorption-bound water has the properties of an elastic solid, the thin films of which are about 0.1 microns thick have a wedging property. The slightest leaks of HDI PCB manufacturing at the glass fiber-resin interface in composite dielectrics are the concentration of wedging processes.

The sorption form of the bond (in terms of activated sorption) is the physico-chemical form of the bond in microcapillaries. The vapor pressure of the liquid over the meniscus of the macrocapillary is practically (with an accuracy of 1%) equal to the saturated vapor pressure over the free surface of the liquid.

7 types of moisture bonding in HDI PCB manufacturing

There are the following main types of moisture bonding in the material:

  1. ionic
  2. molecular (hydrate)
  3. adsorption
  4. osmotic
  5. structural
  6. capillary
  7. wetting and cathodic.

Categorization of adsorption moisture

The adsorption moisture, in HDI PCB manufacturing comes into:

1)First and foremost, the moisture of monomolecular adsorption

2) Moreover, moisture of polymolecule adsorption.

What is capillary bond of moisture

already mentioned above, comes into the moisture of the microcapillaries and the moisture of the microcapillaries. The moisture of the macro-capillaries divide, depending on the position in the pores, into:
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1) Firstly, capillary

2) Secondly, funicular (wick)

3) Third, pendular (butt)

Moistening process in HDI PCB manufacturing

The process of moistening the boards can last if you want. It is limited only by the time from the shipment of the boards to the customer to the installation on the assembly line. But the evaporation of moisture in HDI PCB manufacturing occurs so quickly that water vapor bursts the material.

HDI PCB manufacturing shows the dynamics of moisture absorption of a composite glass-oxide dielectric of the FR-4 type. It is characteristic that initially moisture absorption is slow: there is a normal penetration of moisture into the volume of the base through capillaries and intermolecular cavities.

Hydrolysis process in HDI PCB

It is this hydrolysis process that is most dangerous for preventing decomposition: cells with hydrated products explode when heated sharply during soldering, creating numerous smallpox in the volume and on the surface of the bases.

Similarly, sorption-bound capillary moisture boils quickly in HDI PCB manufacturing when heated, exposing the structure of fiberglass. To predict the intensity of the flow of the released steam, it is possible using the laws of Avogadro and Boyle.

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