PCB Technical - Quality requirements for circuit board soldering

The two major American electronic and electrical industry associations (IPC and EIA) first jointly published various specifications related to assembly and welding in April 1992. The number No. 1 is the specification of J-STD-001.For more than ten years, it has not only become the most popular quality acceptance specification in the electronic and electrical assembly industry in the United States, but it has also become a model for compliance by the global industry.

Nowadays, facing the major changes of global lead-free,J-STD-001,which is directly related to welding,must of course be revised to meet the urgent needs of many suppliers and buyers.After long-term revisions and voting by participants,001D was finally officially listed on 2005.2.Because there are too many shortcomings of lead free soldering,the general industry is reluctant to enter mass production too early.It has to wait until 2006.7 until it is illegal to continue to have lead before adopting lead free soldering.Therefore,the accumulated amount in the new 001D version There is not enough mass production,presumably the E version will be launched as soon as possible in the near future.

1. Solder paste hole soldering and wave soldering
(1) Lead-free tin bone in-hole welding
The method of "plug-in" solder paste into the hole; is to first print the solder paste on the ring or part of the first board or the second board, and then insert the foot of the part, and use the process of SMT welding. At the same time, the pins of the jack are also welded firmly. To replace the original two-step processing method of first doing the socket wave soldering and then doing the board surface solder paste soldering. Its purpose is not to save labor. The main reason is that the solder SAC or SC used in lead-free wave soldering is too high and the heat is too high, which will cause great harm to PCBA processing plates and components, etc., and it is easy A lot of copper melted from the board surface, and the copper pollution in the tin pool continued to increase, which caused the melting point to continue to rise and the fluidity to be gradually insufficient, resulting in poor solderability and deterioration of solder joint strength.

However, this method of replacing lead-free wave soldering with solder paste in-hole fusion soldering is very new, and the experience is still in its infancy, and there is still great room for improvement on how to do the best.

1. The amount of tin attached comes from the printed solder paste.

2. The 25% empty area in the through hole that is not yet full of tin refers to both the assembly surface and the solder surface.

3. The solder paste can be printed in the ring and through holes on the assembly surface or the soldering surface.

(2) Lead-free wave soldering of through holes
J-STD-001 also regulates lead-free wave soldering in section 6.3, and in 6.3.2 it is more clearly pointed out that the tin material should be through hole 1007. The hole ring on the upper and lower sides must be covered with tin.

1. The tin dipped here can be processed by any method, including the solder paste printed into the hole.

2. The wave tin or solder paste tin applied to any board surface.

3. The 25% part of the through hole that is not yet full of tin material refers to the shortcomings of the first side or the second side.

4. The tin in the vertical hole of Class2 board can be less than 75%.

(3) Explanation of acceptance of insufficient tin filling in through holes of second-level boards
As pointed out in Table 4 above, when the through-hole lead-free wave soldering of Class2 board, the tin increase in the hole is less than 75%, the exceptions are:
When the through hole is intended to be connected to a large plane for heat dissipation, it can be accepted as long as the through hole is filled with tin to 50%. However, the proviso is that the soldering of the second side (welding surface) must complete a 360° complete interconnection (100%) for the soldering of the pin and the hole wall, and the surface of the ring should also be tinned 75%. And some conditions Still must comply with the user’s regulations

2. Curved foot wave soldering of single panel
The IPC specification rarely mentions the welding of single-sided non-plated through-holes, and the bend of the non-through-hole ring surface should be at least 45°, so that the strength of the solder joint after wave soldering will be better. And the third paragraph also has requirements for the shape of the bent-foot solder joints. It is required that the solder joints formed in the bent-foot area should still be clearly visible. This means that good soldering should not add too much tin. quantity. Otherwise, it will be sloppy water with insufficient fluidity, or even the amount of tin grabbed by the tin wave that is too fast in the cold welding.

Secondly, it talks about the bent pin specifications of single-face ring welding for SMT patch processing. The main principle is that the pin protruding outside the hole must not violate the minimum spacing of electrical insulation. Other specifications regarding the protruding length of the pins in NPTH are listed in Table 2. As for the acceptability of the amount of tin in each solder joint, it is listed in Table 4.

When there is a possibility of violating the minimum insulation distance, or the pins are deformed so that they protrude excessively after welding and may pierce the anti-static package, the protrusion of the pins should not exceed 2.5mm.

1.The amount of tin attached is from the amount of tin obtained in the soldering process.

2.It refers to any PCB board surface that the solder touches.

Core points of PCB component soldering:

Welding preparation stage:

Work area: must be kept clean, dust-free, anti-static, use anti-static tools and equipment, and wear anti-static bracelets.

Tool Preparation: Ensure that there are soldering and protection tools such as soldering wire holder, component box, soldering gun, soldering station, tweezers, scissors and so on.

Circuit Inspection: Carefully inspect the PCB board to make sure there are no short circuits, broken circuits and other problems.

Material verification: Confirm that the components are correct, pay attention to the polarity requirements of the components, check whether the pads and component feet are oxidised, if necessary, use fine sandpaper to polish and apply flux.

Soldering process:

Safe operation: use the soldering iron safely and reasonably, the soldering iron needs to be grounded to prevent leakage damage to the components. Recommend the use of temperature-adjustable white soldering iron, leaded soldering temperature of about 350 ° C, lead-free soldering temperature of about 380 ° C. If the tip of the soldering iron has an oxidised layer, you need to use a high-temperature sponge to wipe clean. The soldering iron should be tinned before use, i.e. apply flux when the iron is hot enough to melt the solder, and then apply the solder evenly on the soldering iron head. When not in use, please switch off the power supply of the soldering iron.

Soldering order: follow the principle of first small and then large, first easy and then difficult, first solder small volume of resistors, capacitors and other components, then solder large volume components, and finally soldering connectors.

Component placement: components should be neat, centred, placed close to the board, pay attention to the direction of the polarity of the components.

Operating position: keep the distance between the soldering iron and the nose between 20-30cm.

Soldering requirements: Ensure that the component position is stable and does not move.When soldering,the tip of the soldering iron should not apply pressure, let the solder touch the soldering point first, and then use the tip of the soldering iron to melt the solder at an angle of 45°.When the solder is melted and submerged component pins, gently lift the iron head, welding time is about 2 ~ 3 seconds. Avoid shaking the component before the solder is completely solidified to prevent false soldering. Appropriate use of flux to assist soldering.

Control soldering time and frequency: Avoid prolonged or repeated soldering to avoid damage to the components.

After soldering is completed:

Quality check: Check for leakage, mis-soldering (e.g. wrong polarity), short-circuiting, and false soldering.

Solder joint assessment: The solder joints should be full, smooth, even, no pinholes, shiny, and the solder should surround the pins in the right amount, not too much. If there are leads or pins, their exposed length should be between 1-2mm. The surface of the solder joint should be clean without rosin stains.

Waste disposal: Clean up the soldering waste in time and throw it into the rubbish bin.

Tool Return: Put the soldering tool back to its original position for next time use.

Cleaning the PCB board:Use the board washing water to clean the residues on the PCB board, such as tin slag, tin debris, component feet and so on. Pay attention to take protective measures, because the wash water is volatile and flammable. The leftover wash water should be properly stored to avoid waste.

Power test: first use the multimeter resistance gear to measure whether the power input is short-circuited, if short-circuited should be excluded before adding power. Then check the circuit according to the schematic diagram.

Assembly and debugging:After the power-on test is completed, assemble the IC and debug it according to the list. After completion, pack the PCB boards with electrostatic bags to avoid random placement.