Plastic Ball Grid Array

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EPE 442

ADVANCED SEMICONDUCTOR MANUFACTURING

TECHNOLOGY

Individual Assignment 2

ELECTRONIC PACKAGE : Plastic Ball Grid Array (PBGA)

PROTOTYPE SYSTEM : PC Motherboard

NAME : Chan Huan Yang

MATRIX NUMBER : 108469

COURSE : Manufacturing Engineering with ManagementLECTURER : Dr. Khairudin Mohamed

School of Mechanical Engineering

Date of submission: 7th

May 2014


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ABSTRACT

In this report, I present the overall process involved in fabricating Plastic Ball Grid Array

(PBGA) at Level-1 manufacturing and PC motherboard Assembly at Level-2 manufacturing. The

first section of the discussion is about the process of developing a PBGA. The following section of

discussion is about the process of developing a prototype system which is the PC motherboard.

1.0INTRODUCTIONA ball grid array (BGA) is a type of surface-mount packaging used for integrated circuits. BGA

packages are used to permanently mount devices such as microprocessors on a PC motherboard. The

BGA is descended from the pin grid array (PGA), which is a package with one face covered (or

partly covered) with pins in a grid pattern which, in operation, conduct electrical signals between the

integrated circuit and the printed circuit board (PCB) on which it is placed. On the other hand,

motherboards are the most complex and essential part of the modern PC. Not only do they hold the

chipsets that pass data from peripherals, drives and memory to the processor, they also provide slots

and ports for all the other system components and the circuits through which all data must pass.

2.0DISCUSSION2.1 Process of Developing a Plastic Ball Grid Array (PBGA)

Die preparation

First wafers are sorted at the assembly site and stored in a die bank. A 2 ndoptical visual inspection is

conducted to inspect for defects before the wafers are released for production. Next wafers are

mounted on a backing tape that adheres to the back of the wafer. The backing/mounting tapeprovides support for handling during wafer saw and the die attach process. The wafer saw process

cuts the individual die from the wafer leaving the die on the backing tape. The wafer saw equipment

consists of automated handling equipment, saw blade, and an image recognition system. The image

recognition system maps the wafer surface to identify the areas to be cut, known as the saw street. DI

Water is dispensed on the wafer during the saw process to wash away particles (Si Dust) and to

provide lubrication during the dicing process. Wafers are dried by spinning the wafer at a high RPM

before going to the die attach process.

Die attach

Die attach provides the mechanical support between the silicon die and the substrate, i.e. lead frame,

plastic or ceramic substrate. The die attach is also critical to the thermal and, for some applications,the electrical performance of the device. Die attach material formulations are optimized to provide

strong adhesion to the plastic substrate.

Wire bond

Bonding to a plastic substrate typically involves lower temperatures than bonding to a lead frame

alloy. Temperatures are reduced (to 160 C) to maintain sufficient strength in the substrate material

so that the ultrasonic energy is efficiently utilized.

Molding/Encapsulation

Transfer molding is used to encapsulate some PBGA packages. Emerging for other PBGA

applications is the use of liquid encapsulants. Liquid encapsulants are used where wire pitch is tight


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and for filling cavity packages. Liquid encapsulants are also formulated using epoxy resins, fused

silica filler, and other additives. Being in liquid form, these encapsulant materials have low viscosity

and can be filled with high levels of silica to impart desired mechanical properties. Liquid

encapsulants are dispensed from a syringe. Depending on the PBGA configuration, a dam resin may

be deposited as the first step. The dam resin defines the encapsulation area around the device. The

cavity or defined area is filled with encapsulant that covers the device and the wires. Finally a cureprocess is used. The lower viscosity of liquid encapsulants greatly diminishes the probability of wire

sweep.

Solder ball attach

PBGAs use solder balls as the interconnect path from the package to the printed circuit board.

Solder balls are attached to the substrate by applying a flux, placing the balls on the pads, and

reflowing the PBGA. The reflow process forms a metallurgical joint between the solder ball and the

substrate ball pad. Alignment is a key parameter during ball placement to avoid missing balls or

solder bridging.

Marking

Marking is used to place corporate and product identification on a packaged device. Marking allows

for product differentiation. Either ink or laser methods are used to mark packages. Laser marking is

preferred in many applications because of its higher throughput and better resolution.

Singulation

Individual PBGA units are cut from the substrate strip and placed in trays for subsequent handling.

Inspection

Assembled PBGAs are inspected to measure the co-planarity of the solder balls.

2.2 Process of Developing a PC Motherboard

Raw materials

Like any other electronic item, tracing the motherboard back to its roots leaves us staring at a hole in

the groundor, to be more accurate, a couple of them. The two dominant constituents of a printed

circuit board are fiber glass which provides insulation and copper, which forms the conductive

pathways, taking us back to their birthplaces in a sand quarry and open-cast copper mine respectively.

Turning sand into glass and copper ore into metal are processes that are hundreds of years old, but

what we do with the materials next is anything but ancient.

Fabricating copper-clad laminate

Molten glass is extruded to produce glass fibers that are woven to create a sheet of fiberglass fabric.

Next the sheet is impregnated with epoxy resin and heated to partially cure the resin; the resulting

sheet is called 'prepreg'. Multiple sheets of prepreg are stacked to produce a laminated sheet of the

required thickness. Sheets of copper foil are applied to both sides of the laminate and the sandwich is

placed in a heated press. This completes the curing of the resin, making the laminate rigid and

causing the layers to bond together. The result is an insulating sheet of fiberglass with copper foil on

both sides: copper-clad laminate. The overall thickness of the printed circuit board (PCB) is typically

1.6mm. This means that, for a six-layer board, the fiberglass laminates will be about 0.35mm thick

and the copper foil will be about 0.035mm thick. The fiberglass is thick enough to provide adequate


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mechanical strength and rigidity, and the copper is sufficient for good electrical and thermal

conductivity.

Etching away unwanted copper

A photosensitive material called photo-resist is applied to both sides of the copper-clad laminate,

totally covering the copper layers. This is usually a dry film process, in which thin films of solid

photo-resist are laminated onto both sides of the board using equipment that's fairly similar to an

office laminator. Now a transparent artwork showing the pattern of the PCB's pads and tracks is

placed over the photosensitive copper-clad laminate, and is then exposed to ultraviolet light.

Ultraviolet is used rather than visible light so the board can be handled safely in daylight. Where the

photo-resist is exposed to ultraviolet, the chemicals polymerize, forming a plastic. Since the board

has two copper layers, each of which has a photo-sensitive coating, this process is carried out twice

using different artworks for each side. Next, the board is immersed in a chemical solution to develop

the latent image. The developer washes away the unexposed photo-resist, leaving only material that

was polymerized and which corresponds to the pad and tracks. The areas of the copper film that

aren't protected by the remaining polymerized portions of the photoresist are etched away. In anoxidation reaction, metallic copper is transformed into a copper salt, which is water-soluble and

therefore washes off during the etching. For quick etching, the board passes through a chamber in

which the etchant is sprayed at a high pressure and at a temperature of about 50C. After etching, the

board is washed to remove surplus etchant and the remaining photo-resist is removed using an

organic solvent. The insulating fiberglass board now has a pattern of copper tracks on each side that

will form the circuit's interconnections. This assembly is called a core. However, motherboards have

a multilayer construction, which means they have more than two copper layers. This means that the

above process has to be carried out several times. In the case of a six-layer motherboard, two of these

cores will be needed to provide four of those layers. We'll see later how the other two layers are

made.

Building up a stack

Double-sided cores are now sandwiched together to start the creation of a multilayer PCB. Two cores

are used for a six-layer board (a common figure for motherboards), but they can't be stacked directly

on top of each other because this would cause the copper tracks on the top of the bottom core to short

with the tracks on the bottom of the top core. To stop this from happening, a sheet of prepreg is

placed between them. Sheets of prepreg are also applied to the top and bottom of the stack before it's

subjected to pressure and a high temperature to complete the curing of the prepreg and bond

everything together. For a six-layer board, the stack would comprise: prepreg / core / prepreg / core /

prepreg. This means that the final result will be: fiberglass / copper / fiberglass / copper / fiberglass /

copper / fiberglass / copper / fiberglass.Drilling the holes

Holes are now drilled through the board. First come the mounting holes, which will be used for

mechanical fixing (bolting the motherboard into the PC's case). Second are the holes that are used to

accommodate the leads of through-hole components when they're soldered to the board in a couple

of steps' time. Finally, there are the tiny holes that form vias (vertical interconnect access), which

make electrical connections between the various copper layers or will, when we get to routing,

testing and QA. Despite the use of a high-speed, numerically controlled drilling machine, drilling can

be a very time-consuming process, especially if lots of different hole sizes are required. For this

reason, it's common to stack boards together so that several are drilled at once, saving time and

money.


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Copper and tin plating

Electro-plating would be an obvious choice to make the vias conductive, except for one minor

problem: only already-conducting surfaces can be electro-plated. To get around this, the board is

immersed in various chemicals that coat its entire surface with a thin layer of copper. It's a slow

method and very expensive, but it provides just enough conducting metal to electro-plate over the top.Electro-plating the entire board would be wasteful because most of the copper would subsequently

be etched away to produce the pads and tracks on the outer layers of the PCB. Instead a photo-resist

is applied, exposed to UV light through an artwork and developed as when fabricating the copper

clad laminatebut with one important alteration. Here, a different type of artwork is used so that the

photo-resist remains in those areas that don't correspond to the pads and tracks of the finished board.

Now the electro-plating will only increase the thickness of the copper on the areas without the

insulating photo-resist. The board is finally electroplated with tin, which, once again, only adheres to

those areas of the board that will form the pads and tracks. The tin serves three purposes: it prevents

the copper tarnishing; it provides a surface that can be soldered to more easily than copper; and it

acts as a resist (after first removing the remaining photo-resist) in the next process etching away

the unwanted copper. We now have a PCB with copper pads and tracks on the outer two surfaces,tracks on four internal layers, and vias making the necessary connections between the various layers.

To complete the bare PCB, a solder mask and component identification are applied via silkscreen

printing. The solder mask covers all of the board where solder shouldn't adhere when the

components are fixed in place. This prevents unwanted bridges between tracks that could occur

during wave soldering in step 9. The component identification provides a visible labeling of each of

the components with their serial numbers. This is useful in manual inspection or board maintenance.

Routing, testing and QA

Steps 2 to 7 involved the processing of a panela sheet of material comprising several motherboard

PCBs. Now the individual boards are separated using a numerically controlled router, which is alsoused to create any non-plated larger holes and slots that are needed. The board is then given a going

over by a 'bed-of-nails' tester, an automated process that probes both sides of the board to ensure that

electrical pathways exist where they are supposed to and that there are no shorts. Finally, before

leaving the PCB fabrication facility, the motherboard is given a QA inspection to ensure it meets its

specification in terms of the overall board size, mounting hole tolerances and so on.

Surface mounting

The first components to be soldered onto the bare PCB are the surface mountings. Solder paste a

mixture of solder powder and flux is printed onto those pads on the top surface of the board where

the contacts of the surface-mounting components (SMCs) will be soldered. The SMCs are placed on

the board using a pick-and-place machine. The tackiness of the solder paste holds the components inplace, but they're not fixed securely and there isn't a proper electrical connection. The next stage is

reflux soldering. The PCB is placed in a reflux oven and heated to over 200C. The solder in the paste

melts and then solidifies when the board cools down again, providing good electrical connections

and fixing the components securely.

Through-hole components

Next the larger through-hole components are fitted, often on a manual production line. Included are

the processor socket, the memory and expansion card slots and the various connectors such as

keyboard, mouse, audio and video sockets. The components are fitted to the top side of the board

with their pins protruding through pads on the bottom side of the board. The board then enters awave soldering machine. This contains a tank of molten solder that's pumped across a submerged


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edge, causing a raised wave of solder. As the board progresses through this apparatus, each part of

the bottom side of the board comes into contact with the solder wave. The solder adheres to the

board wherever it's free of solder resist, thereby making mechanical and electrical connections

between the component leads and the pads.

Final testing and packaging

For final testing, processor and memory modules are plugged into their sockets. External PC

components such as a hard disk, CD/DVD drive, monitor, keyboard and so on are also plugged into

their appropriate connectors. With the motherboard now effectively built into a complete PC, a full

functional test involving every socket is carried out. This is mostly an automated process, although

humans do still have a part in the process for areas like audio circuitry. All this is followed up with a

'burn-in' test, which involves running diagnostic software on the motherboard for a protracted time

while it's subjected to high temperatures and temperature cycles. If the board passes this test, which

is designed to cause any potentially faulty components to fail, the motherboard is complete. All that

remains is for the finished board to be packaged in an antistatic bag and box, and it's ready to take

pride of place in a new PC.

3.0CONCLUSIONPBGA provides better performance in term of high density, heat conduction and low inductance

leads. However it also has some limitations such as noncompliant connections, difficulty of

inspection, difficulties with BGAs during circuit development and cost of equipment. Meanwhile,

PC motherboard It holds many of the crucial electronic components of the system, such as the central

processing unit (CPU) and memory, and provides connectors for other peripherals. Unlike a

backplane, a motherboard contains significant sub-systems such as the processor and other

components. Motherboard specifically refers to a PCB with expansion capability and as the name

suggests, this board is the "mother" of all components attached to it, which often include sound cards,

video cards, network cards, hard drives, or other forms of persistent storage; TV tuner cards, cards

providing extra USB or FireWire slots and a variety of other custom components (the term

mainboard is applied to devices with a single board and no additional expansions or capability, such

as controlling boards in televisions, washing machines and other embedded systems).

4.0 REFERENCES

[1]

http://focus.ti.com/en/download/qlty/SEMICONDUCTOR_PACKAGING_ASSEMBLY_TECHNO

LOGY-MISC.pdf

[2] http://www.techradar.com/news/computing-components/motherboards/how-motherboards-are-

made-a-miracle-of-modern-electronics-709366/2#articleContent

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