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We in the SMT foundry will appear a lot of various defective phenomena, including short circuit is also one of the common defective, then the SMT foundry short circuit phenomenon is the cause? What is the cause of short circuit in SMT foundry?
SMT foundry processing short-circuit defective phenomenon occurs mostly in the fine pitch between the IC pins, so it is also called ‘bridge’. There are also short circuit phenomenon between CHIP parts, but it is very rare. Here is an introduction to the problem of bridging between the pins of fine pitch ICs and how to solve it.
SMT foundry processing common fine-pitch IC pin short circuit causes and solutions.
SMT foundry processing stencil templates are not properly designed.
SMT foundry processing short-circuit phenomenon occurs between the pins of fine-pitch ICs, mostly between the pins of ICs with a pitch of 0.5mm and below, because of its small pitch, improper template design or slight printing omission is very easy to produce.
According to IPC-7525 stencil design guidelines, in order to ensure that the solder paste can be smoothly released from the stencil aperture to the PCB pads,the aperture of the stencil is mainly dependent on three factors:
1.Area/width/thickness ratio ‘0.66.
2.Smooth mesh hole wall, steel mesh production process requires suppliers to make electric polishing treatment.
3.To the printing surface as above, the mesh hole should be wider than the upper opening 0.01mm or 0.02mm, that is, the opening into an inverted cone,so that the effective release of solder paste, and at the same time reduce the number of times the stencil cleaning.
Specifically, that is, for the pitch of 0.5mm and the following IC, due to its pitch is small, easy to produce bridging, stencil opening pipe length direction remains unchanged,the opening width of 0.5 ~ 0.75 pad width. Thickness is 0.12~0.15mm, laser cut and polished to ensure the inverted trapezoidal shape and smooth inner wall for good tinning and moulding during printing.
Improper solder paste selection for SMT OEM processing
The correct choice of solder paste is also very important in solving bridging problems. When using solder paste for ICs with 0.5mm or less spacing,choose one with a fineness of 20-45um and a viscosity of 800-1200pa.s. The activity of the solder paste can be decided according to the degree of cleanliness of the PCB surface, and RMA grade is generally used.
PCBA
SMT OEM Printing Pipes
Printing is also a very important part.
1.Squeegee type:Squeegee has plastic squeegee and steel squeegee, for pitch less than or equal to 0.5 IC printing should choose steel squeegee, in order to facilitate the printing of the solder paste after shaping.
2.Squeegee Adjustment:Squeegee running angle of 45 degrees in the direction of printing can significantly improve the paste different templates openings towards the imbalance of the phenomenon, while also reducing the fine pitch template openings of the damage.Squeegee pressure is generally 30N/mm.
3.Printing speed:solder paste will roll forward on the template under the impetus of the squeegee,fast printing speed is conducive to the rebound of the template,but at the same time will hinder the solder paste leakage printing. If the speed is too slow,the solder paste will not roll on the template, which will cause the poor defence rate of the solder paste printed on the pad,usually the printing speed range of 10-20mm/s for the fine pitch.
4.Printing pipeline:the most common printing pipeline is divided into ‘contact printing’ and ‘non-contact printing’.The gap between the template and the PCB is ‘non-contact printing’,the general gap value is 0.5-1.0mm, the advantage is that it is suitable for different viscosity solder paste.The solder paste is pushed into the template aperture by the squeegee to contact the PCB pad.After the squeegee is slowly removed,the template will be separated from the PCB automatically,which reduces the trouble of contamination of the template due to vacuum leakage.
The printing pipeline with no gap between the template and the PCB is called ‘contact printing’. It requires overall structural stability and is suitable for printing high-precision tin templates with very flat contact with the PCB, which are separated from the PCB after printing, thus achieving higher printing accuracy,especially for fine-pitch and ultra-fine-pitch solder paste printing.
SMT foundry processing improperly set mounting height
Paste height, for pitch less than or equal to 0.55mm IC in the paste should be used 0 distance or 0-0.1mm paste height, in order to avoid the paste forming collapse due to the paste height is too low, resulting in short-circuit reflow.
SMT OEM processing reflow soldering setup is not appropriate
1.Heating speed is too fast.
2.Heating temperature is too high.
3.Paste heating faster than the circuit board.
4.Flux wetting too fast.
If the SMT foundry short-circuit phenomenon, mainly due to poor processing of SMT foundry, short-circuit PCBA can not be used normally, there are many ways to view the status of the SMT short circuit.
1.In the PCBA map on the short circuit network is lit, to find out more likely to be shorted places, and pay attention to the IC short-circuit conditions.
2.If the SMT foundry processing process appears many of the same short-circuit phenomenon, you can first check the short-circuit part of the components are separated from the power supply.
3.Use the short-circuit location analyser to check.
4.Must seriously treat the SMT OEM small-scale capacitor soldering, power supply filter capacitor is too much can easily cause a short circuit between the power supply and ground.
5.If there is a BGA chip, because all the solder joints are covered by the chip and can not be seen, and is a multi-layer board, so it is best to separate the power supply in the planning of each chip, and then use a magnetic bead or 0 ohm resistor connection, when the power supply is short-circuited, the bead will be disconnected for detection, it is very simple to locate the chip.
6.Manual welding operation with a multimeter to check the circuit with or without a short-circuit condition, each time the SMT paste a IC after the multimeter to measure the power supply is short-circuited.
The above is about SMT OEM processing short circuit problem solving pipeline introduction, I hope you have some help, more PCBA information please pay attention to the content of this site updates! IPCB is a professional PCB and PCBA processing enterprises.