PCB Tech - Green manufacturing process of circuit boards (1) Lead-free soldering of multilayer boards

Green manufacturing process of circuit boards (1) Lead-free soldering of multilayer boards

1. Introduction
   Lead-containing solder has always been the user of circuit board soldering. 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 specifications 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 favored the so-called SAC305 alloy (tin, silver, copper), whose melting point is 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 achieve this goal, usually the board must be changed to a resin that can withstand strong heat and a board with good moisture resistance.
   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. The collection of key knowledge and reliability data will allow fully prepared suppliers to win a valuable place in the new welding market.
   2. Multilayer PCB with lead-free soldering on circuit board
   Green products have a high demand for high T g and halogen-free materials. The lead-free soldering temperature will cause the plate to expand in the Z direction, which will negatively affect the reliability of the plated through holes and the integrity of the inner layer bonding. However, there is still not much research done so far on the impact of the increase in assembly welding temperature on the inner lamination process, and it is still awaiting further in-depth research by the industry. In this article, the new grain boundary etching (Intergrain Etch for short as IGE) on the inner copper surface, combined with alternative treatment (ie tin sinking treatment), makes this revolutionary improvement have a stronger synergistic effect on the multilayer board structure. It will also be explained in depth later in the article.
   These two hybrid treatments (trade name Secure HTg) of boundary erosion and tin sinking have completed the inner surface treatment with multiple advantages; such as improved tear strength and reliable heat resistance. The degree of enhancement to eliminate pink circles, avoid wedge-shaped breaks, and facilitate the horizontal manufacturing process of large-area thin plates, etc. will be introduced in detail.
   (1) Description of the new hybrid process (Secure HTg) of the inner copper surface
   The adhesion treatment of the inner layer deliberately depositing a metallic tin layer on the copper surface has proven to be able to withstand the strong heat test of lead-free soldering. The flow of this new process is as follows:
   (1) Cleaning treatment
   Before performing micro-etching on the copper surface, a strong cleaning process must be completed to remove the residues of dry film etching resists and heavy pollution fingerprints.
   (2) Initial processing
   This site can protect the next site's micro-etching solution from pollutants, and can provide a suitable surface etching potential, so that the subsequent micro-etching effect can be improved.
   (3), micro-etching
   The improved "sulfuric acid/hydrogen peroxide" micro-etching fluid can attack the grain boundaries of copper materials to obtain the surface topography required for strong adhesion. Such ultra-deep micro-etching can obtain the required surface roughness and make the subsequent mechanical bonding strength better. Compared with the poor shear strength of traditional black oxidized fluff, the structure produced by the copper material here has shown better shear strength.
   (4), enhanced processing
   After the microetching of the inner copper surface is completed, a gray metallic tin layer will be deposited immediately, and then the MLB pressing process that is resistant to lead-free soldering will be completed.
   (2) Examination board and results
   For this alternative oxidation treatment test board, a six-layer board and a twelve-layer board are selected. The whole board is composed of a variety of different substrates and is matched with 35μm copper foil. These test boards can be used for a variety of reliability tests. Each batch of test boards must undergo a tear strength test as a reference value for other subsequent tests. After this initial tear strength test, the same batch of samples will be subjected to multiple infrared lead-free reflow processes. After that, another tear strength test is carried out to compare the possible deterioration of the bonding strength of the plate caused by reflow. Generally, it can be seen from this kind of information that lead-free assembly has a negative impact on circuit boards.

(3) Test results and discussion
The above-mentioned tin-copper mixing process (Secure HTg) on the inner copper surface has proven to be quite strong and practical after many tests. Each DOE sample is operated in the "steady state" bath of the mixing process to simulate the standard production process.
From the performance of pre-reflow, it is known that the traditional blackening reaction has shown good results in standard FR4 and halogen-free materials; however, the results of the pre-heat stress test are not very good, and after such thermal stress test The adhesion is more important. As a result, it is unable to withstand multiple lead-free reflow operations. In fact, it has been found that the adhesion strength has been reduced by more than 50%. At the same time, it was also discovered that the alternative blackening method (AO), grain boundary etching (IGE), and AO plus reinforcing agent, and the new hybrid process (SecureHtg) of the inner copper surface, etc., will appear after the test welding of the completed multilayer board. Loss of adhesive strength. However, the loss of the hybrid process is less than that of other methods. When the hybrid process is applied to halogen-free materials, there is only a 6% reduction in adhesion strength, which is better than the standard alternative blackening and the 28% and 54% of the orthodox blackening.

It was also found that the hybrid process can pass lead-free reflow in a variety of circuit board substrate materials, and even the heat resistance test of T260 can pass. As for some samples that failed to pass the T288 test, it was found that the failure was due to the problem of the substrate itself, which had nothing to do with the treatment of the copper surface. Due to extreme heat conditions and subsequent film failure, T288 may not be considered a reliable test method. The excellent performance of the hybrid process should be derived from the interfacial co-gold compound formed between tin and copper, which has been completely transformed into an interfacial co-gold compound layer during the high-temperature pressing process. However, strong heat can cause changes in the microstructure between copper and tin. The density of the tin layer decreases with the increase of the thickness of the co-gold compound at the interface, which inevitably results in the formation of indeterminate micro-holes near the grain boundary between copper and tin. All results show that tin atoms will migrate into the copper layer, leading to the growth of interfacial co-gold compounds, and there will also be a kind of "protruding fingerlike structure" (ProfiliCfingerlike structure) between copper and IMC. It will greatly increase the mechanical bonding force.

PCB manufacturing, It is still difficult to understand the characteristics of the chemical bond between the copper surface and the epoxy resin before 17t. There have been many related theories in the past, but the first thing to know is what kind of film is formed on the copper surface? Is it the earlier black oxidation? Or the later reductive black oxidation? Or is it substituted black oxidation? Generated and stored in a vacuum, otherwise some copper oxide will inevitably be hydrolyzed in the environment due to moisture absorption, and further form Hydroxyl Group (Hydroxyl Group), these Hydroxyl Group will continue to be slightly acidic with the epoxy resin It will react in the middle, and then form a chemical bond between each other.
The difference in the adhesion strength of the surface in different oxidation places is determined by the surface charge, which is determined by the "surface isoelectric point" (ISO-electric point of Thesurface; IEPS). When using halogen-free or standard FR4 sheets, the adhesive strength provided by the new hybrid oxidation process will be even better. The high adhesive strength that appears after this kind of pressing, after repeated lead-free reflow soldering, its strength value can still be maintained above 41b/in. Therefore, the hybrid process will provide better process stability in the future lead-free demand.