[Repost] Several basic factors affecting PCB electroplating hole filling process

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[Repost] Several basic factors affecting PCB electroplating hole filling process [Copy link]

The global electroplating PCB industry output value accounts for a rapidly growing proportion of the total output value of the electronic component industry. It is the largest industry in the electronic component sub-industry and occupies a unique position. The annual output value of electroplating PCB is 60 billion US dollars. The size of electronic products is becoming thinner and smaller. Directly stacking holes on through and blind holes is a design method to obtain high-density interconnection. To do a good job of stacking holes, the flatness of the bottom of the hole should be done first. There are several typical methods for making flat hole surfaces, and the electroplating hole filling process is one of the representative ones.

The electroplating hole filling process can not only reduce the necessity of additional process development, but also be compatible with existing process equipment, which is conducive to obtaining good reliability.

Electroplating hole filling has the following advantages:

(25)](1) It is conducive to the design of stacked vias and via on pads; (2) It improves electrical performance and is conducive to high-frequency design; (3) It is conducive to heat dissipation; (4) Via plugging and electrical interconnection are completed in one step; (5) The blind hole is filled with electroplated copper, which has higher reliability and better conductivity than conductive glue.

Physical influencing parameters

The physical parameters that need to be studied include: anode type, cathode and anode distance, current density, agitation, temperature, rectifier and waveform.

(1) Anode type. When it comes to anode types, there are soluble anodes and insoluble anodes. Soluble anodes are usually phosphorus copper balls, which are prone to produce anode mud, polluting the plating solution and affecting the performance of the plating solution. Insoluble anodes, also known as inert anodes, are generally composed of titanium mesh coated with tantalum and zirconium mixed oxides. Insoluble anodes have good stability, do not require anode maintenance, do not produce anode mud, and are suitable for pulse or DC plating; however, the consumption of additives is relatively large.

(2) Anode-cathode spacing. The spacing design between the cathode and the anode in the electroplating hole filling process is very important, and the design of different types of equipment is also different. However, it should be pointed out that no matter how it is designed, it should not violate Faraday's first law. (3) Stirring. There are many types of stirring, including mechanical shaking, electric vibration, gas vibration, air stirring, jet (Eductor), etc. For electroplating hole filling, people generally tend to add jet design based on the configuration of traditional copper cylinder. However, whether it is bottom jet or side jet, how to arrange the jet pipe and air stirring pipe in the cylinder; how much jet flow per hour; how much distance between the jet pipe and the cathode; if side jet is used, is the jet in front of or behind the anode; if bottom jet is used, will it cause uneven stirring, weak stirring at the top and strong stirring at the bottom of the plating solution; the number, spacing and angle of the jets on the jet pipe are all factors that must be considered when designing the copper cylinder, and a large number of tests are also required.

In addition, the most ideal way is to connect each jet tube to a flow meter to achieve the purpose of monitoring the flow rate. Due to the large jet volume, the solution is easy to heat up, so temperature control is also very important.

(4) Current density and temperature. Low current density and low temperature can reduce the deposition rate of surface copper while providing enough Cu2 and brightener to the hole. Under this condition, the hole filling ability is enhanced, but at the same time it also reduces the electroplating efficiency. (5) Rectifier. Rectifier is an important part of the electroplating process. At present, the research on electroplating hole filling is mostly limited to full-board electroplating. If graphic electroplating hole filling is taken into account, the cathode area will become very small. At this time, very high requirements are placed on the output accuracy of the rectifier. The selection of the output accuracy of the rectifier should be determined according to the size of the product's lines and vias. The thinner the lines and the smaller the holes, the higher the accuracy requirements for the rectifier should be. Generally, a rectifier with an output accuracy of less than 5% should be selected. If the selected rectifier has too high accuracy, it will increase the investment in equipment. When wiring the output cable of the rectifier, the rectifier should be placed as close to the plating tank as possible. This can reduce the length of the output cable and the rise time of the pulse current. The selection of the rectifier output cable specification should meet the requirement that the line voltage drop of the output cable is within 0.6V at 80% of the maximum output current. The required cable cross-sectional area is usually calculated based on the current carrying capacity of 2.5A/mm. If the cross-sectional area of the cable is too small or the cable length is too long, or the line voltage drop is too large, the transmission current will not reach the current value required for production.

For plating tanks with a slot width greater than 1.6m, the use of bilateral power supply should be considered, and the lengths of the bilateral cables should be equal. In this way, the bilateral current error can be controlled within a certain range. A rectifier should be connected to each side of each flying bar of the plating tank, so that the current on the two sides of the component can be adjusted separately.

(6) Waveform. At present, from the perspective of waveform, there are two types of electroplating hole filling: pulse plating and DC plating. Both of these electroplating hole filling methods have been studied. DC electroplating hole filling uses traditional rectifiers, which are easy to operate, but if the board is thick, it is powerless. Pulse electroplating hole filling uses PPR rectifiers, which have many operating steps, but have strong processing capabilities for thicker boards.

The influence of substrate

The influence of substrate on electroplating hole filling cannot be ignored. Generally, there are factors such as dielectric layer material, hole shape, thickness-to-diameter ratio, and chemical copper plating.

[color=rgb(25, 25, (25)](1) Dielectric layer material. The dielectric layer material has an impact on hole filling. Compared with glass fiber reinforced materials, non-glass reinforced materials are easier to fill holes. It is worth noting that the glass fiber protrusions in the hole have an adverse effect on chemical copper. In this case, the difficulty of electroplating hole filling lies in improving the adhesion of the chemical plating seed layer (seed layer), rather than the hole filling process itself.

In fact, electroplating hole filling on glass fiber reinforced substrates has been applied in actual production.

(2) Thickness-to-diameter ratio. At present, both manufacturers and developers attach great importance to hole filling technology for holes of different shapes and sizes. The hole filling capacity is greatly affected by the hole thickness-to-diameter ratio. Relatively speaking, DC systems are more widely used in business. In production, the hole size range will be narrower, generally with a diameter of 80 pm to 120 um, a hole depth of 40 um to 80 um, and a thickness-to-diameter ratio of no more than 1:1. (3) Chemical copper plating. The thickness, uniformity and placement time of the chemical copper plating layer after chemical copper plating all affect the hole filling performance. If the chemical copper is too thin or the thickness is uneven, the hole filling effect is poor. Generally, it is recommended to fill the hole when the chemical copper thickness is > 0.3 pm. In addition, the oxidation of chemical copper also has a negative impact on the hole filling effect. Source: Internet, if infringed, please delete.

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