Page 262 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

• Solder Paste Printing:

Solder paste quality plays a key role in affecting soldering quality. The following aspects should be considered
when selecting solder paste: excellent printability, excellent solderability and less contaminant.
Particle diameter of solder paste should be compatible with lead pitch of components. Generally, the smaller lead
pitch is, the smaller solder paste particle diameter, the better printing quality will be. But it’s never that simple
as solder paste with larger particle diameter leads to higher soldering quality than that with smaller particle
diameter. Therefore, comprehensive considerations should be taken when determining solder paste. Because
BGA components feature fine pitch, it’s suitable to select solder paste with particle diameter below 45μm to
guarantee excellent printing effect and soldering effect.
• Component Mounting:

The essential aim of mounting is to get each solder balls on BGA components aligned with each pad on PCB
board. Because BGA component pins are too short to be easily seen by naked eyes, specialized equipment
should be used for accurate alignment. Up to now, primary equipment for accurate alignment includes BGA/CSP
rework station and chip mounter among which precision of chip mounter reaches approximately 0.001mm. With
mirror recognition leveraged, BGA components can be accurately mounted on pad array on circuit board.
• Reflow Soldering:
Reflow soldering is the most difficult-controlled phase in BGA assembly process, so optimal reflow soldering
curve achievement is a key element contributing to excellent BGA soldering. Reflow soldering curve contains four
phases: preheating, soaking, reflow and cooling. Temperature and time of the four phases can be respectively set
and modified so that optimal soldering result can be acquired.
• BGA Rework:
BGA rework after soldering is carried out on a BGA rework station that can solder and rework independently on
a BGA chip without affecting adjacent components. Therefore, a hot air reflow nozzle with suitable size can be
selected to cover BGA chip for the convenience of soldering.
BGA Component Soldering Quality Inspection:

BGA as a short form of ball grid array packaging contain solder balls under components and quality of solder balls
can hardly be known without specific inspection devices. Visual inspection alone fails to acquire soldering quality
of solder joints. Up to now, inspection devices for BGA soldering quality inspection are X-ray inspection devices
that are classified into two categories: 2D and 5D.
IC Reballing and installation:

IC Reballing is a process that uses solder paste and a dedicated stencil printer to place solder paste onto all of
the pads on the circuit board. The ICs, resistors, capacitors or diodes are then removed from the old PCB and
placed on top of the new solder paste using tweezers or forceps. The new component is placed on the solder
paste covered pad and pressed down firmly onto it. This is repeated until all of the pads have been replaced with
new components. Power is applied to the circuit board and then the resistors, capacitors and diodes are tested.
If everything checks out then the board is returned to the user for use.

Rebalancing is a process that is used to repair BGA balls that have cracked or been missing for some time. BGA
balls are typically used on motherboards and in place of solder pads on IC sockets. They are designed to be
hot-soldered into place but over time they may become loosened or damaged. This can result in instability and
intermittent operation of the socket so it needs to be repaired as soon as possible.
Reball a circuit board:
The first step in reballing is to remove the defective component from the circuit board. This can be done using a
soldering iron, tweezers or forceps. The old solder is then removed using a desoldering braid or wick. A new BGA
reballing station is used to place the new BGA component on the circuit board using a solder paste stencil and
laser head. The new BGA component is then soldered onto the circuit board. The new component is tested and
then the circuit board is returned to the customer.
The process of reballing BGA chip is as follows:
1 First remove all of the old ICs, resistors, capacitors and diodes from the PCB.
2 Clean the PCB with Acetone or Isopropyl Alcohol and allow it to dry.

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CITS : E & H - Electronics Mechanic - Lesson 130 - 145