PCB Tech - PCB problems How to locally increase the amount of solder paste or solder in the SMT process

PCB problems How to locally increase the amount of solder paste or solder in the SMT process

Today’s electronics industry is really becoming more and more developed. As electronic parts are getting smaller and smaller, products are getting thinner and thinner. Just like the big Black King Kong mobile phone at the beginning, it is now a miniature mobile phone that can be placed on the wrist. These are thanks to the miniaturization of SMD electronic parts. Some 0402, 0201, and even 01005 sizes have been tried. Even the pitch of general IC (integrated circuit) parts has been reduced to 0.5mm (fine pitch)., Micro pitch), even 0.3mm has been made, which is really 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. On the contrary, it is easier to have problems in the manufacturing process, but the same board needs to be marked with large and small parts at the same time.

How to make these tiny electronic parts welded on the circuit board without the shortcomings of empty welding and short circuit is enough to make the SMT engineer very troubled. The bigger challenge is that not only these small parts on the circuit board need to be soldered. Unfortunately, due to technical or cost constraints and considerations, some parts cannot be miniaturized until now (such as most linkers, batteries, coils, Large capacitors... etc.), then there will be the problem of large and small electronic parts being squeezed on the same circuit board.

Because large parts need more solder to be printed on the solder feet, so as to ensure the reliability of their solder; small parts need more precise and tiny amount of solder paste control, otherwise it is easy to cause solder short circuit or empty soldering problems. The control of the amount (volume) of solder paste is generally determined by the thickness and aperture of the steel plate (stencil), but the thickness of the same steel plate is basically the same, and the thickness of the steel plate suitable for small parts is not suitable for large parts. The remaining parts can only control the opening of the steel plate, but the opening cannot solve such a problem. This seems to be a problem of fish and bear's paw.

At present, the common practice in the electronics industry is to let the steel plate meet the requirements of the solder paste volume for small parts, and then use different methods to locally increase the solder paste volume, because in comparison, the small volume of tin is much more difficult to control than the large volume of solder. Here are four more common methods of partially increasing the amount of tin for your reference. In fact, most of these methods have been introduced in the previous articles, and here is just a little sorting out.

1. Manually apply solder paste

Using a semi-automatic dispenser, after printing the solder paste on the steel plate or before entering the reflow soldering, partially add the solder paste to the place where the solder paste needs to be added. The advantage of this method is high mobility.
However, there are a lot of disadvantages of manually applying solder paste:

Need to add a manpower.

It doesn't matter if this manpower can share other manpower, such as visual inspection in front of the furnace, or manual placement of parts in front of the furnace. Basically calculate the allocation of manpower.

The quality is more difficult to control.

The amount and position of solder paste can not be accurately controlled by manual soldering, and it is more suitable for parts that require a larger amount of solder paste.

Easy to work negligence.

Manually adding solder paste may touch other places where the solder paste has already been printed because of no action, causing the shape of the solder paste to be damaged, and then causing short circuits or empty soldering. It may also move to other parts that have 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 dot the red glue under the SMD parts and stick the parts on the PCB to avoid After wave soldering (Wave Soldering), the parts fell into the tin furnace. In fact, 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 applied to the place where the solder paste needs to be added locally to increase the amount of solder.

Disadvantages of automatic solder paste machine:

Because wave soldering is rarely used in the current process, most SMT production lines are no longer equipped with dispensers, so this method may require an additional machine.

3. Use step-down stencil

"Step-up steel plate" is divided into two types: [STEP-UP (partial thickening)] and [STEP-DOWN (partial thinning)]. Paste printing volume, or locally reduce the thickness of the steel plate (step-down) to reduce the amount of solder paste. This kind of STEP-UP steel plate can also overcome the problem that some parts are not flat enough (COPOLANARITY), and STEP-DOWN can effectively control the short circuit problem of FINE PICTH parts.

Disadvantages of stepped steel plate:

The price of steel plates may be 10% to 20% more expensive than normal steel plates.

Because this kind of special steel plate must use a thicker steel plate, and then use the laser method to remove the part that needs to be thinned, so STEP-DOWN should be easier to make than STEP-UP, but I have almost never seen a STEP-DOWN board. .

There is a limit to the increase in the amount of solder paste.

The thickness of this kind of steel plate cannot be locally increased too thick, usually 0.1mm steel plate 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 steel plate is thicker than the normal thickness. If the thickness is increased locally, the buffer must be lengthened, which will increase the amount of tin for nearby small parts.

4. Use Pre-forms

This kind of "solder preforms" basically turns the solder paste into a solid and pressed it into small blocks. It can be designed into a variety of shapes to meet actual needs, and it can also be used to supplement steel plates. Insufficient amount of solder paste caused by printing restrictions, and this "preformed tin sheet" is generally made into tape and reel packaging, just like small parts such as resistors and capacitors, SMT machines can be used to paste parts to save Manpower, and avoid operator errors.

Disadvantages of preforms:

This kind of "solder preforms" must be printed where the solder paste is printed.

This can prevent the PCB from moving when it vibrates, and it merges with the original solder when the solder paste melts.

increase cost.

At present, the price of such "solder preforms" is not cheap, and may be much more expensive than ordinary small resistors with no resistance. With mass production in the future, the price should get lower and lower.