SMD electronic parts have been minimized,The sizes of 0402, 0201, and even 01005 are already available,The pitch of general IC (integrated circuit) parts has also been reduced to 0.5mm (fine pitch), even 0.3mm. This is a big challenge for the SMT process,But the bigger challenge is that there are not only these small parts and small solder joints on the circuit board,It is more prone to problems in the manufacturing process. Instead, large and small parts must be printed on the same board.

How to make these small electronic parts intactly soldered to the circuit board, and the shortcomings of empty soldering and short-circuiting cannot occur. The greater challenge is that not only these small parts on the circuit board need to be soldered, so large and small electronics will appear the problem of parts being squeezed on a circuit board.

Because large parts require more solder to be printed on the solder feet, this can ensure the reliability of their soldering; small parts need more accurate and tiny solder paste volume control, otherwise it is easy to cause solder short circuit or empty soldering problems. The control of the amount of solder paste volume is generally determined by thickness and aperture of the stencil. However, the thickness of the same steel plate is basically the same. The bottom can only control the opening of the steel plate, but the opening can not solve this problem.

At present, the common practice is to let the stencil bend the solder paste requirements of small parts, and then use different methods to locally increase the solder paste amount, because in comparison, the amount of small tin is more difficult to control than the amount of tin. Here are four common ways to increase the amount of tin locally.

1. Manually solder paste

Use a semi-automatic dispenser to add the solder paste locally after the solder paste is printed on the stencil or before reflow. The advantage of this method is high mobility.However, the amount and position of manual solder paste cannot be accurately controlled, which is more suitable for those parts that require a larger amount of solder paste. Adding solder paste manually may contact other places where the solder paste has been printed because of no action, causing the shape of the solder paste to be damaged, which may cause short circuit or empty soldering. It may also move to other parts that have already been placed, causing the parts to shift.

2. Import automatic solder paste machine

In the SMT processing plant, the early SMT production line configuration was equipped with an automatic dispenser. The purpose of this dispenser is to place the red glue under the SMD part and stick the part on the PCB to avoid the back Parts fall into the tin furnace during Wave Soldering. This dispenser can also be used to dispense solder paste. As long as the solder paste is added to the syringe, the solder paste can be placed where the solder paste needs to be locally added to increase the amount of solder.Because wave soldering is rarely used in the current process, most SMT production lines are no longer equipped with a dispenser, so this method may require the addition of a new machine.

3. Use step-down stencil

step-down stencil is divided into STEP-UP and STEP-DOWN,This special stencil is increased by locally increasing the stencil thickness (step-up),The amount of solder paste printed or the step-down of the stencil is locally reduced to reduce the amount of solder paste. This kind of STEP-UP stencil can also overcome the problem that some parts are not level enough (COPOLANARITY), and STEP-DOWN can effectively control the short circuit of FINE PICTH parts.

The price of special stencil may be about 10% to 20% more expensive than normal stencil. Because it is necessary to use a thicker stencil, and then use laser to remove the parts that need to be thinned, STEP-DOWN should be easier to make than STEP-UP, but few STEP-DOWN plates have been seen.

There is a limit to the increase in solder paste volume. The thickness of this type of stencil cannot be locally increased too much, usually 0.1mm stencil can only be increased to 0.15mm at most, and most of them can only be increased to about 0.12mm. This is because there must be a slope buffer where the thickness of the stencil is relatively thick and normal. If the thickness is locally too thick, the buffer zone must be lengthened, and the amount of tin in the vicinity will increase.

4. Use Pre-forms

This kind of "solder preforms" is basically to turn the solder paste into a solid and press it into small pieces. It can be designed into various shapes to meet the actual needs, and it can also be used to make up the stencil. Insufficient amount of solder paste caused by printing restrictions, and this "pre-formed solder sheet" is generally made into tape and reel, just like small parts such as resistors and capacitors. SMT machines can be used to place parts to save Manpower, and avoid mistakes in personnel operations.

These "solder preforms" must be printed where solder paste is printed. This will prevent the PCB from moving when it vibrates, and will fuse with the original solder when the solder paste melts.At present, such "solder preforms" are not cheap, and may be more expensive than small resistors that generally have no resistance. With mass production in the future, the price should be lower and lower.

5. Can increasing the amount of solder paste improve BGA soldering defects?

Especially for BGA parts with fine pitch, we generally call BGA a fine pitch when the pitch is below 0.8mm (some definitions may be slightly different). Increasing the amount of solder paste locally rather than all of the solder ball pads increases the amount of solder.

To change the amount of solder paste of the BGA, you can start from Stencil. The basic principle is to make the amount of solder ball printed in the outermost row or four corners of the BGA more than that of the remaining solder balls. Not too much, so that when the PCB and the BGA body are deformed, there is still enough solder paste to contact the solder balls under the BGA.When the BGA solder ball is larger than 0.4mm: Increase the printing volume of the solder ball solder on the four sides of the BGA, and open the stencil into a square to form an inscribed circle with the original solder ball pad. The other solder ball pads maintain the original the amount.When the BGA solder ball is less than 0.4mm: the solder ball solder paste printing on the four sides of the BGA remains unchanged, but the other inner solder balls reduce the amount of solder paste and become a circumscribed circle.In this way, no matter what size the BGA is, the amount of solder paste on the outermost circle of solder balls will be 16.7% more than the amount of solder paste on the inner circle. Sufficient solder paste can keep in contact with the solder balls under the BGA.