Common quality problems and solutions of copper clad laminates
I. Resistance to diffuse soldering
Dip soldering resistance is a common problem in China. It is also a process technology issue that many manufacturers pay great attention to. The performance reliability of electronic products depends to a large extent on the quality reliability of printed circuit boards. After the electrical components are inserted into the printed circuit board, automatic soldering (wave soldering or dip soldering) is required. If blistering of copper foil, even the lifting of pads and copper tube lines and the falling off of wires occur during these processes, it is related to the unreasonable processing technology of printed circuit boards and the resistance to dip soldering of copper clad laminates.
The upper limit of the time and temperature of domestic wave soldering is 5 5/260 “c, usually 2.5~4.5
5/230 – 250 “c, and the best condition is generally around 3 5/240 “c. According to my country’s national standards, the index of the resistance to dip soldering of paper-based copper clad laminates is 10 5/260 “c, that is, no delamination and no blistering within 10 5.
The importance of improving and stabilizing the solder resistance of copper clad laminates is as follows.
① Most components such as televisions and tape recorders are installed on printed circuit boards. If the solder resistance of the board is poor or unstable, the entire component and printed circuit board will be damaged or scrapped and affect the normal operation of the entire assembly line.
② If the quality problem of the printed circuit board is not detected after automatic welding, the quality problem of the printed circuit board will not be detected after the whole machine is assembled, or even after automatic welding, resulting in greater waste and loss.
③ Some whole machine factories have found that some printed circuit boards have this quality problem during wave soldering, and sometimes they have to take measures such as lowering the welding temperature and reducing the depth of the wave peak. The solder cannot fully contact the pad, the solder has poor fluidity, and causes poor wetting defects, resulting in reduced quality stability of the whole machine. All copper clad laminate manufacturers attach great importance to the quality of solderability. The stability and flatness of this performance have almost become two important items for measuring the quality of similar paper-based copper clad laminates. It directly and seriously affects the product reputation of each manufacturer.
2.Under thermal shock conditions, the “blistering” of copper-clad laminates is the result of severe interface damage.
Paper-based copper-clad laminates are composite materials formed by curing and molding solution or molten resin with paper fiber reinforcement materials and copper foil through gluing (including gluing of roughened copper tubes) and pressing. From the structure of the board, after the above processing, multiple components form multiple interface structures. The interface refers to the interface between the resin and the paper fiber reinforcement material, the interface between the adhesive and the copper foil, and the interface between the resin and the adhesive cross-linked by chemical reaction through hot pressing and the reinforcement material and copper anchor on each side.
When the resin and the reinforcement material are cured into one, the resin will basically shrink, and the thermal expansion coefficients of the resin, paper fiber and copper foil are also very different.
Therefore, additional stress will be generated on each interface during the curing process. On the other hand, the cured paperboard will also produce uneven stress distribution in the board under the action of external force and heat, and even concentrate higher stress in certain parts of the interface. The above two stresses will destroy the local chemical bonds of the interface and cause microcracks in the internal material of the board.
During the board making process, water molecules and some low molecular weight substances remaining in the pores of the board interface also greatly promote the destruction of the interface.
The additional stress generated by the above interface and the cracks and pores of the interface generated by the residual low molecular weight substances between the interfaces will produce greater concentrated stress under thermal shock conditions (i.e., placed in high-temperature solder), destroying the chemical bonds or mechanical interlocking between the interfaces, causing the bonding strength of the interface to drop rapidly, while the volume of the low molecular weight volatile gas remaining between the interfaces continues to increase, and the active energy is constantly replenished and strengthened. At this time, the bonding force on the damaged interface cannot resist the internal destructive force of these volatile gases radiating outward through the cracks and pores on the interface, and local delamination and blistering will occur between the copper foil and the substrate, or between the substrate layers, on the weak interface.
The “blistering” problem of paper-based copper-clad laminates under thermal shock conditions is the result of severe damage to the interface.
Depending on the type of board, resin formula, and manufacturing process conditions, the form of interface structure damage is slightly different. In the solder immersion test, the shape and distribution of the “blistering” are also different. When the molecular weight distribution or curing cross-linking of the resin used is relatively uniform, and the resin is relatively pure, the interface within the board should be uniform, and the time for the sample to maintain
no bubbling in the solder is relatively consistent. Bubbling generally occurs in individual large bubbles, which are accompanied by sounds at the moment of bubbling and have great explosive force.
When the resin contains a small amount of impurities, individual large bubbles will appear during the dip soldering test. Impurities destroy the continuous interface layer, and even a deep depression can be padded on the glue paper layer. In the gaps around it, more gas and stress are easily accumulated, resulting in large defects in the interface, and the measured data is more dispersed. This phenomenon of destroying the interface layer will also occur when the reinforcing material contains large impurities (such as non-ferrous chemical fiber blocks, larger and harder pulp blocks, etc.). If the sample copper is generally uniformly distributed at the same moment during the dip soldering test, bubbles with little difference in diameter are generally present at the interface. Too many low molecular weight (volatiles); or the resin is generally not well cured and cross-linked, and the interlayer adhesion is poor. In this way, the chemical bond has no ability to resist its destructive stress under high temperature conditions, and the interface structure is quickly severely damaged. In the dip soldering test, if a row of small bubbles appear on the edge of the sample, it may be caused by the semi-finished or finished product of the board absorbing moisture and water entering the interface pores. The water molecules that enter not only expand the space of the pores, but also produce steam under thermal shock conditions, causing the pores on the interface to tend to increase and expand. When some samples are placed in a smaller solder tank for measurement, the solder temperature suddenly drops by 2-3’C. This may be caused by the large number of cracks and pores on the interface of the board, which rapidly expand stress after heating and absorb active energy. Generally, the dip soldering resistance of this type of board is low and unstable. Studying the different characteristics of the interface of the above-mentioned board being severely damaged under thermal shock, that is, “bubbling”, is of great help and inspiration for us to analyze and find out the main factors of various destructive factors, adjust or change production process conditions, and design the resin formula of new product boards.
3.Some measures to improve and stabilize the dip soldering resistance
To improve and stabilize the dip soldering resistance of paper-based copper-clad laminates, it is necessary to try to eliminate and reduce the factors that will destroy the interface structure during the molding and high temperature of the board. In the process of manufacturing copper-clad laminates, the following points should be paid attention to.
① The copper foil should have a stable and good roughening treatment layer.
② The copper foil adhesive should have a certain heat resistance and high adhesion. Sometimes the two are contradictory in the selection and development of adhesive formulas, but both must reach a certain high degree.
③ The resin should have good uniformity during curing and cross-linking, with less condensation water and less low-molecular volatiles. And it should have a higher
crosslinking density. For this purpose, it is advisable to use two or more composite catalysts, and the resin manufacturing process should be strictly controlled. During the resin manufacturing process, especially during hot processing and dehydration, the glue liquid must not be pumped into the condenser to block the condenser. If the condenser is blocked or partially blocked, the dehydration time is very long. At this time, it is difficult to get rid of the upper water, which will cause glue-in-water and water-in-glue. If this resin is used to glue the press plate, it will not only affect the resistance to solder immersion, but also cause serious warping of the copper clad laminate. How to solve this problem? The first method is that the operator must operate the resin carefully and must not be paralyzed at all, and the glue liquid is pumped into the condenser; the second method is to install a “glue blocker” on the reactor evaporator or a buffer tank on the reflux device; the third method is to check the condenser frequently to see if there is any glue blockage. If there is any blockage, it should be cleared and cleaned in time. If a two-time gluing process is used, the volatile matter of the first gluing paper should be kept as low as possible, and the second gluing must be hydrophobic.
④ The additives in the resin (mainly flame retardants, plasticizers, curing accelerators, etc.) should be selected with low volatility and high heat resistance. The reactive type of flame retardant is better than the additive type. A suitable coupling agent can be selected and added to the resin to improve heat resistance or wet adhesion.
⑤ Consider the curing time (gel time) of the resin and the copper foil adhesive, and the matching of the reaction cross-linking. For paper-based copper-clad laminates with glue fiberglass cloth on both sides, attention should also be paid to the matching of the gel time and cross-linking structure of the two resins.
⑤ The size of the volatiles of the glue paper has a great influence on the resistance to solder immersion of the board. It is necessary to ensure that the glue paper has a low volatile index, but at the same time, it is not possible to simply reduce the volatiles and control the solubility (or fluidity) too small, which will affect the adhesion of the interface. The temperature design of the glue oven should have a temperature level that is more suitable for the characteristics of different resins. At the same time, attention should be paid to the uniformity of the soaking and drying of the glue paper, and residual debris should be prevented.
⑦ Ensure the storage conditions of the glue paper (generally the temperature should be 20-25’c, and the relative humidity should be below 35%) and reduce the storage period. The glue paper containing a certain polar group absorbs water molecules in the air during storage (moisture absorption,) which is directly manifested in the increase of the volatiles of the glue paper, which will pose a great threat to the resistance to solder immersion of the board.
⑧ The mixing of impurities should be prevented in the resin manufacturing, the production of glue paper, and the ingredients and lamination process in the pressing process. The purification and cleaning of the production site environment is also crucial to the intrinsic quality of the product.
⑨ The pressure and temperature during the pressing process should be appropriate.
If the pressing temperature is too high, the chemical bonds of the interface will be destroyed, causing thermal decomposition reactions. If the pressing temperature is low and the pressure is small, the curing and cross-linking will be poor, which will affect the resistance to dip soldering of the board. When pressing, it is also necessary to pay attention to the fact that the temperature in the middle and the periphery of the hot plate of the press should be basically consistent. In the humid climate conditions in summer, if the prepared blanks are not pressed in time, the edges of the materials will also absorb moisture, affecting the resistance to dip soldering.
⑩ The paper-based copper-clad laminate with glued glass fiber cloth on both sides has unique properties for dip soldering:
First, the resin interface between the dewaxed glass fiber cloth and the paper fiber of this type of board is mainly cross-linked by bonding in the form of “points and grids”, which is weaker than the “sheet and layer” bonding adhesion of the paperboard. Second, the dewaxed cloth is easy to absorb moisture. The permeability of the glass fiber cloth is not as good as that of the paper fiber. Third, the glued glass fiber cloth and the glued paper are generally two formula resins, and it is limited to achieve uniformity of the entire resin interface. Due to the above reasons, after such boards are made into finished products, the resistance to soldering will decrease greatly under humid conditions. Therefore, on the one hand, attention should be paid to moisture-proof packaging, and on the other hand, the measured value of the resistance to soldering of such boards should be several times higher than the index to have a “safety margin”.
2.Peel strength
- Determination of peel strength
Peel strength is an important quality indicator of copper clad laminates and a routine inspection item. It is an indicator of quality problems that often occur in the production of copper clad laminates. Low peel strength will cause the copper foil to fall off during the processing of the printed board or the installation and welding, and even affect the normal operation of the entire electrical appliance. According to national standards, the peel strength determination method should simulate the various harsh conditions that the printed circuit board may experience during the processing process, and measure the adhesion between the copper cylinder and the substrate. These simulation conditions include the following points.
① After dry heat (500 h);
② After exposure to trichloroethane vapor treatment;
③ After heat shock (5s/260 ‘C);
④ After exposure to simulated electroplating conditions;
⑤ After immersion in solvent (immersion in solvent for 10 min);
⑥ Peel strength test at high temperature.
- Understanding of improving bonding strength
The bonding strength is generally determined by two major parts: one is the adhesion, that is, the copper-based adhesive and the base resin are pressed and heated during pressing to produce the melt-crosslinking force, and the force between the above two resins and the reinforcing material. Second, the cohesive strength of the copper pin and the reinforcing material resin. Although the resin referred to here includes the main resin of the reinforcing material immersed in the gluing process, it also includes the adhesive applied by the copper base, but the key is the adhesive of the copper base. The necessary condition for strong bonding is that the copper tube adhesive has good wetting on the roughened surface of the copper foil. This wetting is not only achieved by uniform gluing in the copper foil gluing process, but also by high pressure and high temperature at the beginning of pressing. Since the adhesive can be well wetted and penetrated into the roughened surface layer of the copper foil on the side close to the copper foil, and can be well chemically cross-linked with the main resin on the side close to the substrate, a high peel strength of the copper clad laminate is guaranteed. The former connection is formed like many small hooks after curing, connecting the adhesive and the adherend (copper anchor) together. Some people attribute this formed adhesion to mechanical action, and this bonding theory is the mechanical force combination theory. According to this theory, higher surface energy and high specific surface area are beneficial to bonding strength. - Destruction of peeling resistance
From the cross-section of the copper foil bonding structure of the copper clad laminate, it is composed of a substrate layer (a composite material of resin and fiber reinforced material), an interface layer between copper foil adhesive and resin, a copper foil adhesive layer, a roughened layer treated with copper foil, and a basic layer of copper foil. Their mechanical properties are very different from each other. For example: the copper cylinder and its roughened layer are rigid elastomers, while the adhesive is an elastomer. Therefore, the stress distribution of the bonded joint is very complex when subjected to external forces. Due to the different thermal expansion coefficients and curing shrinkage rates of various materials, as well as the influence of environmental media such as water, solvents, thermal oxidation, etc., stress will be generated in and between the bonding layers, and the distribution of internal stress is uneven. In addition, there are internal defects in the bonding structure. In the peel strength test, the damage of the peeling resistance will always occur, but there are high and low peeling resistance. There are four forms of this peeling resistance damage: ① Destruction of the interface between the adhesive and the resin; ② Destruction of the cohesive force of the copper foil adhesive; ③ Destruction of the copper foil roughening treatment layer; ④ Mixed damage. 4 Main measures to ensure the stability of the peeling strength Ensuring the stability of the peeling strength includes two meanings: on the one hand, the peeling strength must reach the various indicators specified in the standard. On the other hand, the peeling strength should be uniform in all parts of the copper clad laminate. To achieve the above requirements, there are two aspects to ensure the conditions: better adhesive (strong heat resistance; high adhesive strength; and certain cohesive strength; good wetting ability; good chemical resistance; good moisture resistance; good storage stability; good matching with resin, etc.). On the other hand, the process technology is guaranteed in the processing and production of the board. The following points should be noted.
① The roughening quality of the electrolytic roughened copper foil is stable, uniform, without scratches and wear.
② The resin should have a certain cross-linking density and should match the copper foil adhesive.
③ The additives in the resin (including plasticizers, flame retardants, etc.) weaken and destroy the curing cross-linking elements between the adhesive and the resin.
④ The content of the copper foil coating and the paper coating should not be too small, the solubility and fluidity should not be too small, and the content and solubility should be uniform to ensure the storage period of the semi-finished product. The semi-finished product materials should be prevented from getting wet.
⑤ The preheating and hot pressing insulation stages of the pressing should meet the process requirements in terms of time, temperature and pressure.
⑤ The copper foil adhesive should meet the process requirements.
III. Warpage
- The importance of the warpage index and the determination method (1) The warpage of the copper clad board The warpage of the board usually refers to two deformations: bowing and twisting. The so-called bow is that the four corners of the copper clad board are in the same plane, and the two straight lines along its edge are in the same plane. The so-called twist is that the three corners of the copper clad board are located in the same plane, and the other corner is suspended and warped. When studying the warping problem of the board, it can be divided into static warping and dynamic warping. Some standards only specify the measurement method of static warping.
(2)When unloading and stacking, the steel plate is not cleaned properly, and resin residue is attached to the steel plate. Therefore, the steel plate and the blank should be cleaned before stacking. Cleaning and grinding of stainless steel plates should be carried out frequently, which is the key link to ensure the appearance quality of copper clad laminates.
In summary, the current paper-based copper clad laminate manufacturing enterprises are large in quantity and wide in scope. Joining the WTO is both an opportunity and a challenge; it is particularly important to strengthen internal management and work hard to improve internal skills. People say that the production and manufacturing of paper-based substrates is three-point technology and seven-point management, which is not without reason. Take the process as a breakthrough point to achieve two improvements (improve quality and increase output) and one reduction (reduce various consumption mainly based on raw materials) to pursue profit maximization. Make the large-scale and wide-ranging paper substrates better and bigger. At the same time, it is necessary to strengthen the service to downstream customers of PCB enterprises and complete electronic enterprises. Send those excellent personnel who have been professionally trained and understand both the copper clad laminate manufacturing process and the PCB process and the complete electronic processing process to the front line of the market to serve customers well is the obligation and responsibility of copper clad laminate manufacturing enterprises. At the same time, it is also a reliable guarantee for the copper clad laminate manufacturing industry to remain invincible.
