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      Quzhou Chuante Electronic Technology Co. Ltd.

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      • Double-sided single-sided tin plate tin bad how to deal with
      • Release time: 2015-03-27  
      • In the production of a product, (it is a tin-sprayed board), when the first side is produced, the tin is good and there is no problem at all. However, when the second side is produced, the PCB board refuses to solder. According to the analysis, it should be the PCB. The question, but what is the reason for Xiliang on the first side? Answer: 1. Due to design restrictions, only the first side (TOP side) can be produced first, and the second side (BOT side) can be produced afterwards. However, some scrap PCBs are used for testing. The first side of the board is not produced, and the BOT surface tin is directly printed. After the furnace is OK, there is no solder rejection. I have also changed the solder paste of different brands and tried to adjust the furnace temperature several times. But defective products are just the difference between more and less. 2. If the furnace temperature setting and solder paste problems are eliminated, the possible reasons are PCB board material problems, mainly due to insufficient / uneven PCB solder thickness, which causes solder rejection problems. Generally, large copper surface solder thickness is required to be above 100u ''. The recommended solder thickness for QFP and pass parts pads is 200 ~ 350u '', and the recommended solder thickness for BGA pads is about 450 ~ 550u '', which can basically solve the problem of tin rejection. However, while increasing the thickness of the tin, pay attention to the problem of the short-pad being squashed. 3. The amount of solder is thin and some of the solder rejection should be the problem of pad plating. Through the analysis of the picture, the solder paste and the tin of the plating layer are fused with each other, but the fusion of tin and the pad is poor. A can first use an empty board to pass through the furnace for wave soldering to see the bonding of the pad to the tin. Pass the wave furnace on both sides. B. You can use a soldering iron to gently add tin to the good A panel. If there is an empty pad, knock out the tin. Use a microscope to see the effect of tin. The effect of the experiment cannot be viewed. C. Apply flux on the TOP side during production on the A side, and then see how it works in the furnace. 4. The thickness of pcb sprayed tin is too thin, and the storage time is too long, and the environment is not good ... and other factors have caused the IMC thickness to grow rapidly. CU6SN5 is formed on the top layer of the IMC. After reflow heating, IMC (CU6SN5) finally breaks through the surface layer. It can be seen that the pads have been discolored in the photo. Such "near" oxide layers are no longer traditional fluxes and the temperature can be improved (especially reflow soldering). Therefore, it is necessary to return to the PCB factory to use higher temperatures and stronger fluxes. , After the surface layer is removed again, the tin is sprayed again. As the thickness of the sprayed "thin and thick" has its own advantages and disadvantages, it still needs to meet the overall needs (environment, smt process equipment ... etc.) 5. Lead-free tin spray board: A: The tin spray on the PCB surface is pure tin, and its melting point is higher than the lead-free solder paste currently used, so the furnace temperature should be adjusted higher. B: Many lead-free solder pastes now contain no CL-element. 6. Weldability decreases due to deeper oxidation or surface contamination in the second furnace. IMCs in the second furnace do not agree with the viewpoint that the solderability is reduced after extending to the surface. The reasons are as follows: 1 We are often plagued by unevenness and excessive thickness of sprayed tin. Do manufacturers not want to level the dots? Why is it difficult for manufacturers to do leveling? This is the difficulty in the PCB manufacturing process. Generally speaking, the smaller the pad, the narrower the tinned the more Thick, that is, the smaller the PITCH is, the thicker the IC and 0402 pads are, the thicker the tin is (see those test points, they are almost becoming convex lenses). If the tin on the IC has become Cu6Sn5, then the other pads will not play. Welding broke out on a large scale. 2 Once IMC is generated, its thickening speed is gradually attenuated. It is well understood that there is a layer of IMC between Cu and Sn, and it is much more difficult to "combine" them. Therefore, it has been generated during the tin spraying process. With a layer of IMC, the IMC added when the SMT passes through the furnace is much thinner than that produced when spraying tin. Such a thin IMC is not enough to "consume" the sprayed tin on the board. Right or wrong, welcome to shoot bricks. 7. Yes, "theoretically," the smaller the solder pad thickness, the more difficult it is to control the thickness. This is also the "difficulty" and "technique" in soldering control. It must be "thick enough" and "flat enough". Early use of "vertical spray" Tin is more difficult to control, but experienced manufacturers still choose the "medium" in the dilemma. Most modern tin spraying has been changed to "horizontal" spraying. The thickness is easier to control, especially the "flatness". However, the disadvantage is that it is usually because The flatness is better and the tin thickness is "thin" (the principle will be discussed later). Under normal circumstances, tin and copper will produce IMC when the PCB is sprayed. Its thickness depends on several processes and environmental conditions including oxidation / oxide. , Temperature, speed .... Although a layer of solder is attached to the surface of the sprayed thick copper to isolate "oxidation" and pollution, copper ions will continue to disperse to the surface IMC layer, although the speed is very slow, but if the PCB is stored at ambient temperature, humidity Poor control (especially in high temperature and high humidity in PCB factories), and the substrate itself also easily absorbs moisture. The IMC changes become very "sensitive" and easily deteriorated under multiple influences. Another important factor is that Reflow soldering, when the spray is too thin, the reflow soldering temperature makes the original After good tin "remelting", the IMC can't be completely protected because the upper layer solder is too thin, so it can accelerate "deterioration". In severe cases, it can even accelerate the release of copper ions to form copper oxides and inferior IMC. Such substances can be used through flux. Clear, but because the reflow soldering is "static soldering", the solder has insufficient fluidity, so the oxide can only be removed by "penetration", but when the oxidation is too severe or the oxide "structure" is too strong or the inferior IMC is too thick, it depends on tin The flux contained in the paste can't play a role, especially if there is no flux. Xx can do an experiment to add soldering iron to the solderless pad and use a soldering iron tip to push the solder On the surface of the disk, the solderability should be improved. In addition, the principle of erasing the surface of the pad with an eraser is the same. When this type of PCB is returned to the PCB factory for re-spraying, a strong flux is usually used, plus a solder bath. The internal high temperature solder can "completely remelt" the solder and remove the surface oxides, and then eat the tin again, but if the IMC layer has been severely damaged, some will not be sprayed even if it is resprayed, or it will form. "False attachment", this type of PCB is only There is a way to "scrap" 9. Because when you pass REFLOW for the first time, the side without the patch is also passed through the furnace once, and it is easy to oxidize after being subjected to a high temperature. Therefore, the welding effect on the second side will be poor when passing REFLOW a second time. Change the gold plate, don't be afraid to go twice 10. The reason is that when the first side is passed, the furnace temperature is too high and the second side is oxidized and discolored. The entire surface of the second side of the PAD does not eat tin well. After the furnace temperature is lowered, production on both sides is no problem 11. PCB TOP surface and BOT surface production time is too long, resulting in moisture, this phenomenon I encountered in 2007. Recommendation: TOP surface and BOT surface production interval is not more than 4H. 12. If it is the first side of the welding rejection, the manufacturer has not dealt with it, you can return to the manufacturer and clean it again. At present, there is a little effect after cleaning with an eraser according to experience, but if you want to solve this problem, it is still a matter of PCB material. Because when the reflow passes the first side, the tin spray on the other side will produce tin crystals and dirt. Lead to solder rejection.
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