PCB Tech - Circuit board green manufacturing process (2) through-hole plating and copper plating

Circuit board green manufacturing process (2) through-hole plating and copper plating

In the production of circuit boards in the electronics industry, circuit board soldering has always been the user of lead-containing solder. In the past few decades, this technology has been widely used in countless assembly and packaging products, and all circuit boards can also adapt to this mature soldering technology. Various quality and reliability standards, test methods, and management procedures are all based on this lead-containing soldering technology.
The ban on lead led by ROHS (European Union's Directive on Restricting the Use of Hazardous Substances) has brought a great impact on the entire circuit board in terms of plates and processes, and the main focus is on the changes in soldering technology. The impact caused by this limitation is not only the welding technology, but also the piping of the material of the circuit board. In other words, even if the circuit board material does not contain lead, it does not mean that it is compatible with lead-free technology. Most of the new soldering methods have preferred the so-called SAC305 alloy (tin, silver, copper), which has a melting point about 34°C higher than the current tin-lead eutectic solder. The current task is how to use this lead-free solder to achieve the welding performance of the old lead alloy. In order to keep up with the latest developments in board, reflow, and flux, the industry must invest a lot of manpower and material resources to avoid any gaps in the transition.

1. Desmear and plated through holes
(1) Removal of glue residue on the circuit board
When the circuit board adopts the green process, it will bring a lot of impact to the current processes such as de-smear and through-hole plating. The current connotation of the so-called green manufacturing process is mainly as follows:
◆Halogen-free base material ◆Base material resistant to lead-free soldering ◆Cyanide-free chemical copper manufacturing process
◆E D TA-free chemical copper manufacturing process ◆Formaldehyde-free chemical copper manufacturing process
Halogen-free board refers to those that do not contain odorant and flame retardant. When the de-smear or PTH process is carried out, many incompatibility will inevitably occur. At this time, the cooperation of circuit board substrate manufacturers is also needed to develop new flame retardants that can match the subsequent process, as well as additives and other boards. For these brand-new components, the downstream process must be re-evaluated to find the best working conditions. As a result, process suppliers and circuit board manufacturers will inevitably increase a lot of work. Another important challenge is that green production will inevitably cause premature aging and poisoning of certain processes. For example, the filler (Fi11er) in the sheet will shorten the active period of the gel-removing liquid, and may also affect the te activation reaction before chemical copper. These annoying things need to be evaluated and further researched.
As for the plates that can withstand lead-free soldering, some halogen-free materials also have to face the question of adaptability. How can they still have better performance after changing the composition of the base material? How can they pass the T260 and T288 heat resistance The rigorous test of the time requires continuous efforts and continuous improvement. Many new lead-free substrates on the market are almost all based on phenols (PN) instead of dicyandiamide (Dicy) as the hardener of epoxy resin, or other special resin mixtures. For lead-free and halogen-free materials, the use of fillers to reduce the number of swelling and improve electrical properties is also very important. All new boards must be compatible with the potassium permanganate de-smear system, and the possible poisoning and susceptibility must also be discussed in depth. Atuo Technology is conducting such research work. For many traditional standard substrates, halogen-free substrates and lead-free base materials are awaiting in-depth discussion. The following is part of the work.
◆Standard and powerful permanganate slag removal liquid with three kinds of leavening agents
◆The absorptivity of tin metal in the activation reaction before copper smelting
◆Coverage and adhesion of chemical copper process
◆ Pollution control of key tank liquids, etc.
Part of the results can clearly show the adaptability of each substrate, as well as the needs of individual substrate evaluation.

(2) The production process of chemical copper without cyanide.
Cyanide has been used as a stabilizer in the chemical copper process for a long time. Although the content of cyanide in the chemical copper solution is very small, because cyanide is highly toxic, it is best if it can be used less or not. Under the pressure of environmental protection, the new chemical copper supplied by syrup manufacturers emphasizes that they are no longer cyanide-free, and these new products are now being used in horizontal and vertical manufacturing processes.
(3) Process of chemical copper without EDTA
EDTA is a chelating agent. NtYm has always been used in the industry and in the chemical copper process. Because of its strong thawing characteristics, it will capture the divalent metal ions in the bath and become complex ions, so that it can be suspended in the liquid. middle. Therefore, in the process of wastewater treatment, the lead or other metal ions to be precipitated will chelate with E D T A and cannot be precipitated, resulting in difficulties in wastewater treatment. Once the waste water treatment cannot destroy the thalidomide, heavy metals will be released into the environment and cause pollution.
In order to eliminate the harm of E D TA to the environment, the green process hopes to use other weaker thalassic agents, such as natural grape products, tartaric acid, etc., as substitutes, so that it can be easily reprocessed and biodegraded. As early as when the level of chemical copper was introduced, Ato’s company decided to use the environmentally friendly tartaric acid chemical tank; since the installation of the first level system in 1999, it has continued to use EDTA-free chemicals. Copper system, this trend has now become the trend of all potion suppliers.
(4) Formaldehyde-free chemical copper manufacturing process
It has recently been confirmed that formaldehyde has been upgraded from possibly carcinogenic to a real carcinogenic substance. These newly announced hazardous substance standards mean that the industry should seek alternatives to formaldehyde as soon as possible. At present, there are many syrups advertised for direct electroplating without formaldehyde in the market. Therefore, improved chemical copper has become an urgently needed process for finding F. Nowadays, one or two modified chemical coppers are in the trial phase, and the latest generation of formaldehyde-free systems have been applied in the soft board manufacturing process. The trend of green circuit boards as the theme has been discussed. The hole manufacturing process has an impact, and the biggest impact is currently the halogen-free and lead-free substrates. The on-line use of these brand-new boards will inevitably have an impact on the existing manufacturing process. Care must be taken to introduce them gradually. It is best to establish a set of optimized practices in order to achieve the best results.
Second, electroplating copper
For electroplating copper, it is best to control the thickness ratio of hole copper and surface copper. The pretreatment and copper plating itself must be adjusted for the new lead-free soldering plate, such as the hole aspect ratio or the line width and spacing in the copper plating of the circuit. The characteristics are considered in detail. The electroplated copper coating must have good copper thickness uniformity and excellent mechanical properties, such as ductility (Elongation) and ductility (Ducility), etc., in order to withstand the test of various subsequent processes. Only then will the finished circuit board have sufficient reliability and meet customer quality standards. There is now a relatively new thermal cycling (Thermal Cycling) measurement technology, which can have a more accurate assessment of the quality of PCBs. Compared with the production of traditional circuit boards, the production of the so-called green PCB is related to the use of lead-free soldering. This will have a great impact on copper electroplating.
The differences between lead-free soldering of circuit boards and traditional soldering are as follows:
◆The soldering temperature is about 34 degree Celsius higher than the original lead.
◆The high temperature time in welding operation is longer.

The higher welding temperature and longer retention time cause the expansion of the plate in the Z direction to increase, and the tensile stress of the electroplated copper layer in the through hole increases. This increased tensile stress on the copper layer is very similar to the traditional measurement method for the reliability of electroplating copper. The general method is to use the bleaching or immersion tin method on the test piece in a tin-lead alloy at a specified temperature. Thermal stress test. In order to get the results quickly, the S T method of interconnecting thermal stress has been adopted. It can be seen from various tests that the sequence of factors affecting the reliability of electroplated copper is as follows:
◆The thermal expansion coefficient CTE of the PCB substrate in the Z direction.
◆Board thickness.
◆Through hole diameter.
◆Plating copper thickness.
The PC B base material has the greatest influence on the reliability of the current soldering, so it is foreseeable that the impact on the reliability of lead-free soldering should also be the greatest.
(1) The influence of lead-free reflow times on electroplated copper
There have been a series of test methods for the influence of lead-free soldering of circuit boards on the reliability of electroplated copper. First, use direct current to plate copper on the vertical line production, and make the copper thickness reach 3 Oum. The following is the key description of the test board production:
◆The thickness of the plate is 1.5 m m, and the aperture is 0.4 m m and 1.2 m m
◆Six-layer board with copper plating
◆Material characteristics: Tg1: 131.1 degree Celsius, Tg2: 137.8 degree Celsius.
The board used is a standard: FR 4 low T g material. This type of board material is expected to fail in the first stage of the reliability test. T g = 6.6 degree Celsius has indicated that the substrate is in the process of pressing. The polymer may not be fully polymerized, so there is a potential for explosion in the subsequent various thermal processes.
After the copper electroplating is completed, the test piece cut from the test board needs to be subjected to a thermal stress (T h er m a l S t r e s) test of Solder Floating at 288°C X 10 seconds for a total of 6 times, and then microsection inspection. Other test pieces from the same source have also been subjected to the thermal stress test of Industrial ST, and a total of four test solderings are performed according to the lead-free soldering temperature curve of the Industrial PC standard, and then subjected to 6 times of bleaching heat at 288°C for 10 seconds each Stress test, and finally compare and interpret each other.