How to Solder, Reball and Store BGA Microcircuit

The balls under the BGA IC can be lead or lead-free alloys. In 99% of cases, microcircuits with exclusively lead-free balls are now sold; there are very few lead balls and on some limited series of microcircuits. You can find out what alloy is in the microcircuit datasheet. This is almost always indicated.

Why is it important to know what alloy soldering balls are?

First of all, the soldering profile of such a microcircuit depends on the alloy:

- For a lead-free alloy, which can also be different, the melting point starts at 230°C (446°F).

- The melting window usually starts at 195°C (383°F) for lead alloy balls.

Secondly, the type of solder paste for reballing depends on the balls alloy current microcircuit.

There are important points here:

- under the lead-free BGA microcircuit, lead solder paste can be applied through a stencil. This is a mixed technology. For the best result, you can use lead-free solder paste.

- Under a lead BGA IC, you only need to apply lead solder paste because lead chips are soldered according to a profile for lead technology, the melting window of which is 195-215°C (383-419°F). Apply lead-free solder paste under a lead BGA chip through a stencil onto the printed circuit board pads, then melt it. You will have to increase the melting temperature, but you cannot increase it since, in this case, the lead alloy balls under the BGA may boil, causing splashes and sticking. That is a refusal.

BGA microcircuits are often stored under normal conditions, not in dry storage cabinets, and often even without silica gel in bags that absorb moisture. To clarify, silica gel sachets are found in boxes with new shoes. Oxide films can form on the terminals of the microcircuits, which will have a harmful effect on soldering in the future.

To this, we must add careless handling of microcircuits when taken with bare hands, leaving finger grease on them, which will also add dirt and worsen soldering.

The cleanliness of the premises where microcircuits are stored is paramount because dust particles and hairs easily get stuck between the balls of BGA microcircuits.

It’s the same with storing printed circuit boards, but things are worse with them. The fact is that printed circuit board coatings are different, and these coatings oxidize differently, but they do oxidize. You can solder on them, but not for critical electronic devices. There will be many times more defective soldering.

Before being put into operation, microcircuits and printed circuit boards that have been stored for a long time in inappropriate conditions must be dried for at least 2 hours in ovens at 60°C (140°F). Whether or not to wash them depends on their condition. In general, if it comes to the point that microcircuits and boards need to be washed in cleaning liquids, then it may be better to buy a new batch of microcircuits and order new PCB boards.

Also, about antistatic protection, ESD is one of the invisible dangers that modern delicate microcircuits are very afraid of. Complying with the requirements for antistatic protection is necessary when storing, carrying, and working with BGA chips. This includes a room equipped with grounding to connect all equipment and work tables. Before starting work, a person must also connect himself to this circuit through a bracelet and a one-megaohm ESD socket.

To the mandatory measures, you need to add compliance with the humidity level in the room, at least 50%. Air humidity promotes the flow of charges through the air. This is why you often get electric shocks in winter: The charges cannot flow into the ground through the air and accumulate on various surfaces and in a person. A person accumulates approximately 5 to 10 kV in winter while driving.

All these measures aim to prevent sparking and a sudden flow of charges from charged bodies to grounding.

I support reballing only with solder paste. There are several reasons:

The soldering paste always evenly fills the stencil apertures, and they are the same for one microcircuit as the ready balls. Applying the paste is easier and faster than filling the stencil apertures with balls.

- if you examine the reballing balls under a microscope, you can see their unevenness in volume.

- while scattering balls over the apertures of the stencil, a person inevitably stains them and damages them with tweezers.

- the alloy is written on jars with balls. Are you sure that this alloy is mixed correctly, and how pure is it?

Before reballing a BGA IC, all residues of the former solder balls must be removed. You can use soldering flux for electronics.

Next, the microcircuit must be washed. I do this using either Vigon EFM or FluxOFF. There should be no alcohol solutions because we need to wash off chemical and organic compounds and soldering flux residues.

The next step is fixing the microcircuit stencil and applying solder paste. The Lead paste will begin to melt only when it reaches 195°C (383°F) but do not forget that the molten paste must be soldered to the pads of the microcircuit, and it can only be soldered when the microcircuit itself reaches this temperature. The reflow window lasts 15-25 seconds. You can heat it with a hot air gun or melt it on a lower heating table. I’m even afraid to describe possible options for microcircuit reballing.

If everything worked out, then wait until the microcircuit cools down, and you need to rewash it. I use an antistatic brush that is durable, soft, and has long bristles, which is just what you need.

We already have a clean PCB board and a clean new BGA microcircuit or BGA IC after reballing.

You can solder them either by manual or automatically. The only difference will be that automatic soldering is accompanied by applying solder paste through a stencil to all contact BGA pads of the printed circuit board, with a layer of the same thickness depents from steps BGA pinout. I recommend using lead solder pastes from trusted manufacturers with Sn62Pb36Ag2 alloy or profrssional Lead-Free alloys.

When manually soldering, apply a thin layer of soldering flux to the board pad for a BGA microcircuit. We have already examined the melting temperatures of different alloys. For lead technology, the melting window is 195-210-215°C (383-419°F). For lead-free technology, 230-250°C (446-482°F), depending on the alloy.

To summarize, working with BGA microcircuits is quite painstaking and requires knowledge and understanding of the nuances.

Thank for reading!