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Core Tip: Dip resistance is a common problem in China. It is also a process technology problem 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. In the process, if the blistering of the copper foil occurs, even the pad, the copper tube line is lifted and the wire is detached, in addition to the unreasonable processing technology of the printed circuit board, it is also related to the soldering resistance of the copper clad laminate. The upper limit of the time and temperature of domestic wave soldering is 55/260"c, usually 2.5~4.5
5/230-250"c, the best condition is generally around 35/240"c. According to China's national standard, the index of dipping resistance of paper-based copper clad laminate is 105/260"c, that is, it does not stratify within 105. No bubbles.
The importance of improving and stabilizing the wettability of a copper clad laminate is as follows.
1 Most components such as TV sets and recorders are inserted on the printed circuit board. If the solder damp resistance of the board is poor or unstable, the entire component and the printed board will be damaged or scrapped and the whole assembly will be affected. The production line is working normally.
2 If the quality problem of the printed board is not checked after the automatic welding, the quality problem of the printed board will not be checked after the whole machine is assembled, even after the automatic welding, resulting in greater waste and loss.
3 Some whole machine factories have found that some printed boards have such quality problems in wave soldering, and sometimes they have to take measures such as lowering the soldering temperature and reducing the depth of the board. The solder is not sufficiently contacted with the pad, the solder fluidity is poor, and the defect of poor wetting is caused, resulting in a decrease in the quality stability of the whole machine. Each CCL production plant attaches great importance to the quality of dip soldering. The stability and flatness of this performance is almost an important item to measure the two sensitivities of the quality of similar paper-based CCL. It directly affects the product reputation of each production plant.
2. Under thermal shock conditions, the "foaming" of the copper clad laminate is the result of severe damage to the interface. The paper-based copper clad laminate is smeared by a solution or a molten resin (including the coating of the roughened copper cylinder), and pressed. A paper fiber strong material, a composite material in which copper foil is cured and molded together. From the structure of the plate, after the above processing, a plurality of components form a variety of interface structures. The interface referred to is the interface between the resin and the paper fiber reinforced material, the interface between the adhesive and the copper foil, and the interface between the resin and the adhesive which are chemically reacted by hot pressing, and the reinforcing material and the copper anchor are each side.
When the resin and the reinforcing material are solidified, the resin basically shrinks, and the thermal expansion coefficients of the resin and the paper fiber and the copper foil are also greatly different, and therefore, additional stress is generated at each interface during the curing process. On the other hand, the solidified formed paperboard will also produce uneven stress distribution in the plate under external force and heat, and even concentrate higher stress on some parts of the interface. The above two kinds of stresses generated will cause the local chemical bond of the interface to be destroyed, causing the internal material of the plate to form microcracks.
During the plate making process, water molecules and some low molecular substances remaining in the pores of the plate interface also greatly contribute to the destruction of the interface.
The additional stress generated by the above interface and the cracked pores at the interface due to residual low molecular substances between the interfaces, under thermal shock conditions (ie, placed in high temperature solder), will result in greater concentrated stress, destroying the interfacial interface. The chemical bond or mechanical fitting makes the bond strength of the interface decrease rapidly, and the volume of low molecular volatile gas remaining between the interfaces is continuously increased, and the activity energy is continuously supplemented and strengthened. At this time, the adhesive force at the interface that has been damaged cannot resist the internal breaking force of the volatilized gas passing through the crack on the interface and the pores, and the copper foil and the substrate or the substrate are generated at the weak interface. Local delamination and foaming between layers.
Under the thermal shock condition, the paper-based copper clad plate has the problem of "foaming", which is the result of severe damage to the interface. According to the type of the plate, the resin formulation, the manufacturing process conditions are different, and the damage form of the interface structure is slightly different. In the dip-resistance test, the change in the shape and distribution of the "foaming" is different. When the molecular weight distribution or curing cross-linking of the resin used is relatively uniform, when the resin is purer, the interface inside the plate should be uniform, and the time for the sample to be placed in the solder to maintain no foaming is relatively uniform, and the foaming generally appears individually. Large bubbles, accompanied by sounds at the moment of foaming, and explosive power. When the resin contains a small amount of impurities, individual large bubbles will appear during the dip soldering. The impurities destroy the continuous interfacial layer, and even the rubberized paper layer can be padded with a deep depression. It is easy to accumulate more gas and stress in its surrounding space, resulting in a large interface defect, and the measurement data is more dispersed. Big. Such damaging interfacial layer phenomena can also occur when the reinforcing material contains large impurities (such as untwisted chemical fiber blocks, larger and harder pulp blocks, etc.). If the copper matte of the sample is uniformly distributed at the same instant during the dipping process, the bubbles with little difference in diameter generally have too many low molecular weights (volatiles) at the interface; or the resin is generally cured and crosslinked is not good. Poor adhesion between layers. In this way, the chemical bond has no resistance to stress damage under high temperature conditions, and the interface structure is quickly severely damaged. In the dip soldering, if the sample has a row of vesicles at the edge, it may be caused by the moisture in the semi-finished or finished product of the plate and the moisture entering the interface pores. The incoming water molecules not only enlarge the space of the pores, but also generate steam under thermal shock conditions, and the pores at the interface tend to increase and expand. When some samples are placed in a small solder bath, the solder temperature suddenly drops by 2-3'c. It may be that the interface has more cracks and pores. After heating, it rapidly expands the stress and absorbs the active energy. of. Generally, such a plate has low solderability and is unstable. Studying the different characteristics of the above-mentioned plates under the thermal shock, the "bubble" is seriously damaged. For us to analyze and find out the main factors of the damage factors, adjust or change the production process conditions, and design the resin formula of the new product board. There is great help and inspiration.
3. Some measures to improve and stabilize the dip resistance
To improve and stabilize the wettability of the paper-based copper clad laminate, it is necessary to eliminate and reduce the factors that may damage the interface structure during the forming of the sheet and the high temperature. In the process of manufacturing a copper clad laminate, the following points should be noted.
1 Copper foil should have a stable and good roughening treatment layer.
2 Copper foil adhesives must have a certain heat resistance and high adhesion. Sometimes the two are contradictory in the selection and development of adhesive formulations, but both must reach a certain high level.
3 The resin should have good uniformity during curing and crosslinking, less condensation water and less low molecular volatiles. And should have a higher
Crosslinking density, for which two or more composite catalysts are suitable, and the resin manufacturing process should be strictly controlled. In the resin manufacturing process, especially in the hot processing and dehydration, the glue should not be pumped into the condenser. Block the condenser. If the condenser is clogged or partially blocked, the dehydration time is very long. At this time, the upper layer of water is difficult to escape, which causes glue water and water encapsulation. If the resin is used to remove the rubber platen, it not only affects the dip resistance, but also the copper clad plate. There will also be severe warpage. How to solve this problem? The first method is that the operator must operate carefully when the resin is operated, and the glue should not be pumped into the condenser; the second method is to install a "resistance breaker" on the reactor evaporator. Or install 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, clear it in time. If two times of sizing process is used, the volatiles of the sizing paper should be controlled as low as possible, and the second must be hydrophobic.
4 additives in the resin (mainly flame retardants, plasticizers, curing accelerators, etc.), to choose low volatility, high heat resistance. The reaction type in the flame retardant is superior to the additive type, and a suitable coupling agent may be added to the resin to improve heat resistance or to improve wet adhesion.
5 to consider the curing time of the resin and copper foil adhesive (gelling time), the matching of the reaction cross-linking. Paper-based copper clad laminates with plastic fiberglass cloth on both sides should also pay attention to the matching of the gelation time and cross-linking structure of the two resins.
6 The size of the volatiles of the adhesive tape has a great influence on the wettability of the board. It is necessary to ensure that the adhesive paper has a low volatile index, but at the same time, it cannot suppress the volatile matter at all, and the solubility (or fluidity) is controlled too small to affect the adhesion of the interface. The temperature of the gluing oven should be designed to have a temperature level suitable for different resin characteristics. At the same time, pay attention to the uniformity of immersion and drying of the adhesive tape and prevent residual debris.
7 to ensure the storage conditions of the adhesive paper (general temperature should be 20-25'c, relative humidity below 35% is appropriate), reducing the storage period. The sizing paper containing a certain polar group of awake resin absorbs water molecules in the air during storage (moisture absorption), which visually shows the increase of the volatile matter on the adhesive tape, and the dipping resistance of the plate is very Big threat.
8 resin production, the production of sizing paper and the mixing and lamination process in the pressing process must prevent the incorporation of impurities. The cleanliness and cleanliness of the production site environment is also critical 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 at the interface will be destroyed, resulting in a thermal decomposition reaction. Low pressing temperature and low pressure will cause poor curing cross-linking, which will affect the dipping resistance of the board. When pressing, pay attention to the temperature in the middle and the periphery of the hot plate of the press should be basically the same. In the humid summer climate, the wells are not pressed in time, which will also cause moisture absorption at the edge of the material, which will affect the wettability.
10 paper-based copper clad laminates with plastic fiberglass cloth on both sides, the dip soldering property has unique properties: First, the resin interface between the dewaxed glass fiber cloth and the paper fiber of this kind of board is mainly through "point The "form" form is cross-linked, which is weaker than the "sheet, layer"-like adhesion of cardboard. Second, the dewax cloth is easy to absorb moisture. The fiberglass cloth is less permeable than paper fibers. Third, the sizing fiberglass cloth and the sizing paper are generally two kinds of formula resins, and it is limited to achieve uniformity of the entire interface of the resin. For the above reasons, after the finished product is finished, the wettability of the plate is greatly reduced under wet conditions. Therefore, on the one hand, we must pay attention to the moisture-proof packaging, and we must pay attention to the fact that the measured value of the wettability of such plates should be several times higher than the index to have "insurance margin".
Second, the peel strength
1. Determination of peel strength
Peel strength is an important quality indicator in copper clad laminates and a routine inspection item. It is an indicator of quality problems that often occur in CCL production. The low peel strength will cause the copper foil to fall off during the processing of the printing plate or during the welding of the machine, and may even affect the normal operation of the entire electrical appliance. According to the national standard, the peel strength measurement method is to simulate various harsh conditions that the printed circuit board may be subjected to during the processing process, and determine the adhesion of the width between the copper cylinder and the substrate. These simulation conditions include the following points.
1 after dry heat (via 500h);
2 after exposure to trichloroethane vapor treatment;
3 after thermal shock (5s/260'C);
4 after the simulated plating conditions are exposed;
5 after immersion in the solvent (immersion solvent for 10 min);
6 Peel strength test at high temperature.
2. Understanding the improvement of bonding strength
The bonding strength is generally determined by two major parts: one is the adhesion, that is, the copper paste adhesive and the base resin are subjected to pressure and heat during pressing to generate a melt-crosslinking force, and the above two resins. The force between the tear compound and the reinforcing material. Second, the cohesive strength of the copper pin and the reinforcing material resin. The resin referred to herein includes the main resin of the reinforcing material immersed in the sizing process, and the adhesive coated with the copper slab, but the key is the adhesive of the matte. The necessary condition for strong bonding is the good wetting of the roughened surface of the copper foil by the copper cylinder adhesive. This wetting is not only uniform coating in the copper foil coating process, but also high pressure and high temperature in the initial stage of pressing. Since the adhesive is well wetted on the side of the copper foil, penetrates into the surface layer of the roughened surface of the copper foil, and is chemically crosslinked with the main resin on the side of the substrate, thereby ensuring high The peel strength of the CCL. The former is joined to form a state like many small hooks after curing, and the adhesive and the adherend (copper anchor) are joined together. Some people have attributed this formed adhesion to mechanical action. This bonding theory is the theory of mechanical force bonding. According to this theory, higher surface energy and high specific surface area are advantageous for bonding strength.
3. Destruction resistance
From the cross section of the copper clad bond structure of the copper clad laminate, it is composed of a substrate layer (composite of resin and fiber reinforcement), an interface layer of copper foil adhesive and resin, a copper foil adhesive layer, and a rough layer of copper foil treatment. The basic layer of copper foil is composed of five layers. 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 when subjected to an external force is very complicated. Due to the different coefficients of thermal expansion and curing shrinkage of each material, and the influence of environmental media such as water, solvent, thermal oxidation, etc., stresses in and between the layers are formed, and the distribution of internal stress is not uniform. In addition, the internal defects of the bonded structure. In the peel strength test, the damage of the peeling resistance always occurs, but there is a high and low peeling resistance. There are four forms of this damage resistance:
1 damage to the interface between the adhesive and the resin;
2 cohesive force destruction of copper foil adhesive;
3 copper foil roughening treatment layer damage;
4 mixed destruction.
4 main measures to ensure the stability of stripping strength
Guaranteed peel strength stability has two meanings: on the one hand, to achieve the peel strength in the standard specified indicators. On the other hand, the peel strength should be uniform at all parts of the copper clad laminate.
To meet the above requirements, including two aspects of guarantee: better adhesive (strong heat resistance; high adhesive strength; and a certain cohesive strength; good wetting ability; good chemical resistance; good moisture resistance; stable storage Good sex; good match with resin, etc.). On the other hand, the process technology is guaranteed in the processing of the board. The following points should be noted.
1 The roughening quality of the electrolytic roughened copper foil is stable, uniform, scratch-free and wear-resistant.
2 resin should have a certain crosslink density, and should match the copper foil adhesive.
3 Auxiliaries in the resin (including plasticizers, flame retardants, etc.), weakening and destroying the curing crosslinks of the adhesive and the resin.
4 Copper foil coating, the content of paper sizing can not be too small, the solubility, fluidity can not be too small, and the content and solubility are uniform, ensuring the storage period of the semi-finished products. The semi-finished material prevents water from entering.
5 Pressing pre-temperature, hot-pressing and heat-insulating stage should meet the process requirements in terms of time, temperature and pressure.
6 copper foil adhesive to meet the process requirements.
Third, the degree of tilt
1. Importance and measurement method of warpage index
(1) The warpage of the copper clad warpage plate generally refers to both bow and twist deformation. The so-called bow is that the four corners of the copper clad plate are all in the same plane, and the edges of the two straight sides are in the same plane. The so-called distortion is that the three corners of the copper clad plate are located in the same plane, while the other corner is suspended to warp. When studying the warpage problem of the board, it can be divided into static warping and dynamic warping. Some standards only specify the method of static warping.
(2) lEC standard recommended platform measurement method Europe, the United States and many countries in the world use this method, China's national standards also use this method. The test method is briefly described as follows: the copper clad plate is concavely facing upward and placed on a platform having a length and a width of not less than 460 mm. Gently touch the edge of the ruler at the lower edge of the specimen, read the span L (mm) from the main ruler, measure the maximum distance h between the plate and the upper surface of the main ruler, and subtract the thickness of the main ruler with h. That is, the bow value of the plate is D. Converted to the bow value d at 1000mm span as shown below,
d=Dx10002/L2
Because the warpage index d in the national standard is large, this indicator loses its practical significance. Generally, we must respect the opinions of users and achieve a smaller degree of warpage. Another measurement method is the Japanese]IS suspension measurement method.
The bow value of the single-sided copper clad plate, the copper is in the upward direction, and the plate is convex and warped. On the contrary, it is negative warpage.
The warpage of the copper clad plate has a great influence on the quality of the board. In the processing of printed boards, if the warpage is large, the accuracy of the positioning holes of the processing line will be affected, and the net will be broken due to warpage in the screen printing, and even the warpage will be too large, and it will not pass through the assembly line. . It also causes trouble in punching. In the whole assembly process, due to the large warpage of the printed circuit board, it will affect the automatic insertion of the components controlled by the computer; it will affect the quality of the automatic "beheading" when the wave soldering and the component legs are welded. Even due to the large degree of warpage, it will cause serious "bumping" when the wave soldering is over, and the solder flows into the non-copper surface to make the whole belt
The printed circuit board of the component is scrapped.
2. Causes of copper clad warping
(1) Static Warpage and Dynamic Warpage Static warpage refers to the warpage of the produced copper clad laminate itself. Dynamic warpage refers to the warpage caused by the transient thermal shock during the process of wave soldering during the process of processing the printed circuit board during the process of processing the printed circuit board. To create high-quality, high-level boards, it is necessary to achieve these warpings. From the research and the solution of the two warpings, the two warpages can be divided into the following situations, as shown in Figure 6-6-1. In the figure, "+" means generally positive warping, and "one" means negative warping. Static warpage is related to the reinforcement material, resin formulation and the condition of the board in the manufacturing process. The static warpage is as small as possible, because it has great benefits for the reduction of dynamic warpage. Therefore, the paper base copper foil Plate, it is necessary to carry out a leveling process after production.
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Label: Common quality problems and solutions for copper clad laminates
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