Surface insulation resistance in recent years with the atmospheric ozone layer protection, eliminate ozone depleting substance (ODS) depth and surface mount technology (SMT) is developing rapidly, more and more manufacturers choose to use or are preparing to choose to avoid cleaning welding technology, especially using a low solid content free wash flux cleaning free welding technology has been developing rapidly. The new flux caused by various advantages so as to make the user wishes, however, to ensure clean free welding quality involves many factors.
First, free cleaning welding is a comprehensive process, the printed board (PCB) and original materials of pre control, PCB to clean, less pollution, cleanliness of components and welding must ensure; secondly, the quality of Disposable flux, optimum process conditions and improve cleaning free method to detect the quality of welding must seriously, in order to obtain the best welding quality. Through nearly four years of NCF series of low solid content of Disposable flux application situation, to ensure that meet the requirements of the former, the latter is especially important. In this paper, combined with our application experience and problems encountered, is disposable flux quality, the optimal process condition and clean free welding quality detection, put forward our views.
, Disposable flux quality at present, at home and abroad have many Disposable flux has been developed and applied, the quality is uneven. As is known to all, a good Disposable flux should meet the following conditions:
1, good weldability, high welding quality, defect rate is extremely low, does not produce solder ball, solder bridge, sharp pull or tombstone phenomenon;
2, small smell, smoke less, non-toxic;
3, with the components and PCB materials used in good compatibility;
4, do not need to change the existing welding equipment, or simply add a little equipment;
5, flux itself low solid content, after welding residue minimal non sticky; PCB surface is clean, the cleanliness conforms to the American military standard MIL-P-28809A;
6, after welding without corrosion, with a high surface insulation resistance (SIR) value.
From the welding quality is taken into account, the key is good welding, after welding residue rarely and has higher SIR value. They reflect the quality of solder joints, PCB cleanliness and electrical properties, but also can reflect the product of long-term stability. In order to meet the above requirements, in the development process of the NCF series Disposable flux, we choose the low solid content, rosin, resin, halogen-free system, and add the right amount of film forming agent, corrosion inhibition agent and neutral foaming agent, flux in PCB with good infiltration, uniform coating, it can ensure the welding quality, turn the residual is reduced to a minimum. Activity of the agent in the as welded condition to sublimation and evaporation or decomposition, after welding surface clean, without leaving any loss PCB and components of the material. Therefore, the PCB after no need to wash meet the American military standard MIL-P-28809A on ionic cleanliness requirements. Through NCF-I disposable solder flux residue enrichment of infrared spectroscopy and nuclear magnetic resonance spectroscopy analysis, prove the residue does not contain acidic substances, which to avoid cleaning extremely favorable.
Second, free cleaning welding condition selection from the domestic and foreign literatures and application of low solid content of Disposable flux of experience, using spray chalk cloth way can get the best effect. However, the country has this kind of equipment manufacturers less, more manufacturers to adopt foaming coating, wave soldering or dip welding process, this process in addition to the need to ensure the PCB manufacturing quality, cleanliness, components can be welding, also must pay attention to the following aspects:
1, foaming foaming fine, foaming height is appropriate, Disposable flux in PCB to ensure uniform coating, good weldability. With fine foam tube aperture (~20 m) is the best, the height adjustment is not more than PCB 1/3. In fact, the bubble should just stick to the bottom of the PCB, do not turn on the top of the PCB, coating uniformity can minimize the flux residue, but also to obtain a better welding effect.
2, preheating can enhance the activation ability of the flux, prevent welding flux sputtering, generally using 90~110. Proper preheating temperature and time can get good welding quality, and can reduce the residue.
3, to avoid anti oxidized oil pollution alkali method can resolve the anti - oxidized oil pollution: new wave soldering machine is adopted as a, the induced electromagnetic pump can achieve the purpose to prevent oxidation of; the second is the anti oxidized solder; the third is improving wave height, the wave soldering without anti oxygen oil. This method in the existing wave soldering equipment use is most effective.
4, welding temperature to ensure that the tin pot temperature of about 250 degrees, so that it is conducive to the sublimation of the active agent, volatile or decomposition, to avoid the corrosion of the active agent residue. Tin pot temperature is too high, resulting in poor wettability, the impact of welding quality, therefore, generally not more than 262.