Through Holes Introduction
Currently, various types of plated through-holes on printed circuit boards are used and no significant advantages of one type of connection over another have been revealed (Fig. 1).
The most commonly used are plated through holes and caps.
In some cases, the manufacturing methods themselves introduce many factors that reduce product reliability.
These include through holes with gaps in the coating, poor wall adhesion, corrosive pockets, and other defects.
Therefore, it is necessary to consider some data regarding the methods of production, processing, and control of the properties of plated holes.
Through Hole Technology
A plated hole is a means of connecting conductive paths on both sides of a non-conductive substrate if a current path is created in a hole drilled in the substrate. When applying metal, a galvanizing method is used, which ensures good contact.
By this process, a metal film is formed on the wall of a hole made in the non-conductive substrate, and this film is connected to a bonding pad or conductive track of the printed circuit board.
The hole wall is metalized by the reduction of copper under catalysis in a solution containing copper sulfate and formaldehyde.
The catalyst is a palladium chloride metal salt.
Four Methods of Electroplating
After a thin copper film has formed on the walls of the hole due to the reduction of copper, the copper is then grown galvanically to the required thickness.
There are four methods of electroplating, of which three are variations of the main process:
- Exposure of contact pads (used when a large thickness of metal is required on the walls of the hole)
- Use of one resist for single-sided boards
- Use of a double resist (used for miniature boards and in cases when the foil on the substrate does not require additional metal deposition)
- Secondary metallization.
If etching is used in the manufacture, then it is necessary to take into account the possibility of etching the copper inside the hole.
To protect copper inside through holes, use: negative resist and wax.
These methods can handle both multilayer boards and single-sided and double-sided PCBs.
They are especially useful when board densities are high.
The first or second of these methods is advisable to use in cases where, for technical reasons, the secondary metal coating is not required.
Importance of PCB Material and Drilling Technique
PCB material and drilling techniques play an essential role in the production of high-quality plated through holes.
Holes with a rough surface will have roughness after plating.
The most suitable PCB material is glass fiber reinforced epoxy. (In paper-reinforced epoxy resin, it is easier to ensure good drilling, but the quality of the walls of the through-hole after metallization deteriorates.)
The operation of deburring in the hole before metallization should be introduced into the processing cycle, and the metallization technology should ensure the formation of a layer of metal of the required thickness on the walls of the holes.
The possibility of obtaining high-quality metallization depends to a large extent on the ratio of the hole diameter to its height.
Pre Inspection of the Boards
All boards must be inspected during the copper recovery stage prior to galvanization to assess the quality of the conductive copper film on the hole walls.
This requires the use of microscopes. If a tin-lead alloy (solder) is used as a secondary metal, the ferric chloride etchant should be replaced with an ammonium persulfate etchant.
Flying probe test
One of the drawbacks of the conventional method of producing plated through-holes is that the surface of the board must be plated at the same time to provide the required copper thickness on the walls of the holes.
This leads to the fact that the thickness of the conductive Tracks increases AND, therefore, at the stage of final etching, undercutting increases.
In addition, since this would cause a significant proportion of the copper on the board surface to be etched away and go to waste, this method is uneconomical in terms of both material and time costs.
The described two-stage method of creating metallization allows you to create complex highly reliable printed circuit boards.
This method can be implemented using:
- Copper deposition process using chemical deposition and electrolytic metallization
- Copper deposition process using only chemical methods
- A combination of different processes (etching), in which substrates impregnated with a special catalyst are used.
Reliability of Plated Through Holes
At the first stage of manufacturing printed circuit boards, many failures were directly related to the technology of the initial processing of the board.
And the most significant share of them was the result of improper drilling and cleaning of holes (breaks and bends of conductors due to too slow drilling, pulling conductors into holes when drilling too fast, resin residues on conductors).
This caused voids and breaks in metallization.
In addition, the use of a copper pyrophosphate solution can lead to cracking of the metallization in the holes.
This is probably due to the addition of a clarifier to the solution, which introduces additional mechanical stresses.
The etching process can also lead to defects if the etching metallization can be removed from the edges of the holes.
In the reliability of the PCB, the very interconnection of the two conductive layers of the PCB is of paramount importance.
The most likely types of defects are electrical contact breaks or high resistance connections.
Reliability of PCB end-to-end connections is paramount. It is necessary that the end-to-end connections do not have breaks on the way from the soldered connection through the board material to the conductors on its opposite side and be able to maintain their properties both during the further production of the block and during its operation in various environmental conditions when using the equipment.
How to Choose a Reliable Process for Plated Holes?
When choosing a reliable process for making plated holes, the following considerations should be kept in mind.
Metallographic examination of the metalized holes revealed that the defects were caused by cracking of the metallization, which arose during vibration, thermal nipping, or when the attached module was removed.
When the board is heated from room temperature to the soldering temperature, both the substrate and the metal expand, and the expansion rate of the substrate material can be 10 times higher than the expansion rate of copper.
Ductile copper is usually resilient and elongates to compensate for the increase in board thickness.
Consequently, a general requirement for obtaining high-quality metallization becomes obvious: the mechanical strength and ductility of the metal must be sufficient to withstand the stresses due to elongation during heating during the soldering process.
Metallization having high strength per unit length can resist the expansion of the substrate material, since the deformation of the plastic is tolerable and this relieves stress on the metal layer.
The ductile metallization coating is able to withstand elastic deformation, which reduces the stress caused by the expansion of the plastic, so the coating can adapt to the continuous expansion of the board.