Download pdf - BGA Construction And Rework

8/14/2019 BGA Construction And Rework

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INTRODUCTION

Demanding space and weight requirements of personal computing and portable

electronic equipment has led to many innovations in IC packaging. Combining

the right interface and logic products with new package technology can have a

significant impact on the capabilities and form- factor of the end product.

Leaded surface mount devices are constantly pushing the manufacturing

capabilities of leading board manufacturers to finer and finer lead pitch

geometrys to increase I/O density and reduce board space.

This requirement has lead to significant interest in the JEDEC (Joint Electron

Device Engineering Council) registered Fine-Pitch Ball Grid Array (FBGA)

package.

These packages are ideally suited to low cost, high volume applications, where

package size and performance is of major

Importance.

Typical BGA applications include:

1. Notebook computers

2. Personal Digital Assistants (PDAs)

3. Mobile telephone handsets

4. High density disk drives

5. Camcorders

6. Digital cameras


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BGA comparison to Fine pitch leaded components

1. BGAs are usually smaller.

2. BGAs have no fragile leads.

3. Board assembly yields are significantly improved.

4. Board inspection can be reduced.

5. BGAs have better thermal and electrical properties.

6. In many applications, the use of BGA results in signifi-

cant system level cost savings.

BALL GRID ARRAY (BGA)

Current wire-bonding technology limits todays smaller package sizes to pin

counts of 256 or Less. The introduction of ball-grid array (BGA) package

technology allows a device to have up to 1000 interconnects on a 33 mil

package with a ball pitch of 1.5 mils. BGA package material falls

The plastic ball grid array (PBGA) has become one of the most popularpackaging alternatives for high I/O devices in the industry. Its advantages over

other high leadcount (greater than ~208 leads) packages are many. Having no

leads to bend, the PBGA has greatly reduced problems and minimized

handling issues. During reflow the solder balls are self-centering (up to 50% off

the pad), thus reducing placement problems during surface mount.

The PBGA has an improved design-to-production cycle time and can also be

used in few-chip-package (FCPs) and multi-chip modules (MCMs)

configurations. BGAs are available in a variety of types, ranging from plastic

overmolded BGAs


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called PBGAs, to flex tape BGAs (TBGAs), high thermal metal top BGAs with

low profiles (HL-PBGAs), and high thermal BGAs (H-PBGAs).

The H-PBGA family includes Intels latest packaging technology - the Flip

Chip (FC)-style, H-PB-GA. The FC-style, H-PBGA component uses a

Controlled Collapse Chip Connect die packaged in an Organic Land Grid Array

(OLGA) substrate. In addition to the typical advantages of PBGA pack-ages,

the FC-style H-PBGA provides multiple, low-inductance connections from chip

to package,

as well as, die size and cost benefits. By providing multiple, low-inductance

connections the FC-style, HPBGA offers equivalent or better performance than

an extra on-chip metal layer. The FC technology also provides die-size

benefits through the elimination of the bond pad ring and better power bussing

and metal utilization. The OLGA substrate results in a smaller package, since

there

is no cavity, and thermal management benefits since the thermal solution can

directly contact the

die.

PBGA


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BGA PACKEGING DESIGN

A BGA package consists of a wire-bonded die on a substrate. The substrate is

usually made of two metal layers of copper clad bismaleimide (BT) laminates,

but can have up to four metal layers. Four metal-layer substrate designs often

contain additional power and ground planes. The die and bonds are

encapsulated within a protective molding compound. Via holes drilled through

the board provide routing from the lead fingers to eutectic solder balls on the

underside. The array of solder balls on the underside of the BGA package

provides a high density interconnection of quality solder joints.

The H-PBGA and HL-PBGA, however, are configured differently to provide

for greater thermal and if required, electrical performance. The thermal

advantage provided by this design is based first upon attaching the die to the

bottom surface of a heatspeader or slug that also forms the topside of the

package. Secondly, because the copper heatspreader forms the top of the

package, the thermal resistance is extremely low and exposes the package

surface to available air flow. If required, this heatslug can be directly coupled

to active or passive thermal management devices such as heat sinks or heat

pipes. Improved electrical performance is achieved through additional power

and/or ground planes.


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BGA CLASSIFICATION

There are various types of BGA packages are available in the market . As the

Technological development is taking place new thermally stable materials are

produced for its manufacture .the commonly used material for BGA are plastic

Ceramic etc.

The various plastic BGAs in the market are shown below:

1) PBGA (Die up configuration )

2) H-PBGA ( Die up configuration )

3) HL-PBGA (Die down configuration )


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MANUFACTURE PROCESS OF BGA

The manufacture of BGA Ic is a large step process where after each step the

inspection is done. As the processsors ,microcontrollers or different ic of

various complex circuits are design. After wards the solder balls are placed on

these chips called ball grid array packaging.

The solder balls are previously made of uniform size and are collected in an

bowl. A vacuum ball picker instrument is used which work on the phenomena

of picking the solder balls from the container with the vaccum created in the

pipe which picks the solder balls with its nozzles. A CNC machine is used to

Handle this instrument.which has been loaded with the CAD data of that Ic So

that proper alignment of solder balls can take place with the IC.The solde balls

are dipped in the flux before they are mount on the ICs The nozzles dip a small

surface of solder balls in the flux tank.

Now these balls are taken to the ICs workbench where they are mounted on

them. After mounting these ICs are passed throw a oven for reflow process

where the solder balls having flux get attached to the ICs.

Now these ICs are send to the inspection Workbench , where the inspection by

naked eyes using high power lenses is done manually.where they check

problems wrong alignment,crack in solder balls if they pass it these BGA

packages are ready for sale or to be placed on PCB.

After this

it is send

to oven


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BGA BOARD DESIGN

To achieve maximum reliability, the design of the PCB on which the BGA is

mounted should be considered. In particular, the diameter of package lands and

board lands are very important. The actual sizes of these dimensions are

key factors, but their ratio is also of critical importance. Fig shows a BGA

Board Layout with the optimum 1 to 1 ratio for package land to PCB land. This

optimized ratio equalizes stresses, reducing the chances of a stress cracked

solder ball, which will lead to premature system failure. Ratios other than 1 to 1

will lead to unequal distribution of stress loads. For example, solder lands that

are larger than the package lands will place a greater amount of stress on the

ball at the package land to ball interface. This can cause cracking and premature

failure at the package land to ball interface.


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BGA MOUNTING PROCESS

Replacing leaded packages with BGAs offers several board assembly

advantages:

1. Improved device planarity

2. No chance to bend leads

3. Greater pad to pad spacing

With no chance to bend or deform leads, BGA products offer PCB

manufacturers a significant yield improvement over similar lead count fine-

pitch surface mount devices. Another important feature of BGA products is

their ability to

self-align over the PCB solder lands. This feature is caused by the surface

tension of the solder balls pulling the BGA over the pads. The use of solder

paste is recommended for mounting BGA devices, although it is possible to

omit the paste, and only use a flux. The advantages of using paste are:

1. Paste acts as a flux, and aids wetting of the solder ball to the PCB land.

2. Paste, being sticky, helps hold the component in place during reflow.3. Paste helps to overcome any minor variations in planarity of the solder balls.

4. Paste contributes to the final volume of solder in the joint, and thus allows

this volume to be varied to give an optimum joint.


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FLUXING

Most BGA assembly is done with a solder paste that contains flux, however,

there are some companies reporting adequate results from mounting BGAs

when using low residue, no clean or an aqueous clean flux. To obtain high

yields and reliable joints this may require monitoring the surface insulation

resistance of the board to insure there are no foreign contaminates on the board

or slder ball surface that will show up as reliability problems later on.

Fluxing without paste does come with disadvantages; the ball reflowed with

only flux will have a smaller solder volume than the initially attached ball.The

self centering ability decreases because of the smaller solder volume.

SOLDER PASTE

Assembly with solder paste has advantages over just fluxing the pads. The paste

is the vehicle to provide the flux necessary to both the PCB and solder ball

surfaces to enable proper soldering of the component to the board. A no-clean

or aqueous clean solder paste with 63Pb/37Sn is commonly used in mounting

the PBGA. Typically the choice of solder paste determines the profile and

reflow

parameters. Most paste manufacturers provide a suggested thermal profile for

their products which should be referenced prior to developing a reflow process.

Since the BGA balls consist of solder, the flux activity on the ball surface is

assured, so long as the paste reaches the ball surface. The selection of paste is

generally made to fit the entire component mix being assembled, not driven by

the use of BGA packages. It is necessary, however, to ensure that all the

thermal and environmental requirements of the paste can be met. On the

average, the BGA packages do not need any specialized solder paste. However,


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most solder suppliers have developed a BGA paste that has minimal voiding

during reflow. This paste may also be used for non BGA components as well.

PASTE DEPOSITION INSPECTION

The quality of the paste print is the single most important factor in producing

high yield BGA assemblies. Defects detected after paste print require a strip and

rescreen of the PCB. Any deviation can turn into a defect downstream requiring

rework and repair. Thus the most economic area to intensify process controls

when beginning BGA assembly is in the paste screening step.

SOLDER STENCILS

The stencil thickness, as well as the etched pattern geometry, determines the

precise volume of solder alloy deposited onto the device land pattern. Stencil

alignment accuracy and consistent solder volume transfer is critical for uniform

reflow-solder processing. Stencils are usually made of brass or stainless steel,

with stainless steel being more durable. Hole designs are dependant on the

solder ball size, squeegee type, board layout, and the paste used. There appears

to be no single hole style that is used by everyone. There are companies that

are using square, diamond, round and oval shapped holes. Round holes are

definitely the dominate design.

Ensuring proper stencil application is the most important factor with regards to

reflow yields further on in the process.


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BGA PASTE DEPOSITION


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PLACEMENT & ALIGNMENT OF BGA ON PCB

PLACEMENTBGA packages have shown excellent self-centering properties. Because of this,

wide variation in placement location is accommodated during reflow of the

solder joints. The general rule for BGA packages is that the placement be at

least 50% on pad.The self centering characteristics of the BGA are attributed to

surface tension which will pull the component onto the pad during peak reflow

temperatures.

BALL TO PAD PLACEMENT MACHINE VIEW


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ALIGNMENT

The pick and place accuracy governs the package placement and rotational

(theta) alignment. This is equipment/process dependent. Slightly misaligned

parts (less than 50% off the pad) typically automatically self-align during

reflow. Self centering on the pads is greatly reduced for grossly misaligned

packages (greater than 50% off the pad) and may develop electrical shorts, as a

result of solder bridges, if they are subjected to reflow.

PICK AND PLACE MACHINE

The main areas of concern for Pick-n-Place machines are:

Component body alignment

Component ball alignment.

Inspection of component balls before placement.

Most alignment inspections can be done from either the top or the bottom. As

there is no defining mark on the bottom of the BGA package, the only way to

ensure Pin 1 is located in proper position is from the top by using the Pin one

mark as the orientation indicator. Proper placement is then done:

From the top by alignment marks on the board (which must be done on the

stencil,

By aligning off the bottom of the component by using an up-down vision

system to accurately place the balls on the solder paste. Because the top surface

of some of the BGA packages (like the HL-PBGA types) are highly reflective,

some top side vision systems do better with their inspection process if they use

a diffuse light-ing source instead of polarized source. A round fluorescent tube

type light near the package or a light filter over the polarized fixture seems to

enhance the operation of some systems.


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SOLDER REFLOW

Except for semi-accurate placement of packages, there are no special

requirements necessary when reflowing BGA components. As with all SMT

components, it is important that profiles be checked on all new board designs.

In addition, if there are multiple PBGAs on the board, the profile should be

checked at the different PBGA locations on the board. Component temperatures

may vary because of nearby surrounding components, location of the part on

the board, and areas of package densities. Temperatures may also be different at

the edge of the package than in the center.

PCB CLEANING

Cleaning can be done with aqueous, semi-aqueous, or solvent based systems, or

not done at all. With the need to eliminate Chloroflourocarbon (CFC)

containing materials, many companies have moved to using a no-clean or

aqueous based system. The proper cleaning of solder flux residues and other

ionics left on the boards from the assembly process is necessary for long term

reliability of the finished product. NO clean fluxes simply mean that there is

no harmful residues left on the board that will cause any corrosion or damage to

the components if left on the board. This residue has sometimes shown to be a

collection point for outside contamination on the board surface. For any

application, an evaluation needs to be done to see if the remaining residue still

needs to be removed from the boards in final application.


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PACKAGE DIMENSIONS

The various PBGA packages available in the market are shown in table below :


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PACKAGE BALL ARRAY CONFIGURATION


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SYMBOL LIST FOR PBGA FAMILY


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PBGA OUTLINE DIAGRAM


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MOISTURE SESTIVITY IN BGA

Most PBGA components are highly sensitive to moisture exposure before the

reflow temperature exposure. Maintaining proper control of moisture uptake in

components is critical to the prevention of "popcorning" of the package body


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or encapsulation material. BGA components, before shipping, are baked dry

and enclosed in a sealed desiccant bag with a desiccant pouch and a humidity

indicator card.

With most surface mount components, if the units are allowed to absorb

moisture beyond their out of bag times for their moisture rating, damage may

occur during the reflow process.

REWORK OF BGA PACKAGES

SMT yields of BGA packages are very high, but there may still be a possible

need for rework of components. Component defects, SMT defects, or other

functional problems require rework of the package from the PCB

Rework is the process of removing a component from a PCB, and replacing it

with a new component. The removed component is not immediately reusable.

The shape and volume of the solder balls will not be the same as a new

package. If component reuse is desired, a separate process for replacement ofsolder balls should be used.

Rework Tooling

There are several systems currently on the market for reworking BGA

components. Some systems direct the heat under the package while other


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systems direct hot gas on the top of the package. Backside heating is a common

feature of all BGA rework tools. While some systems attempt to backside heat

with local hot gas behind the package to be reworked, this often results in large-

scale PCB warpage during processing, especially if the PCB is large. The

suggested method is to use a global heat that brings the entire PCB up to a

specified temperature (100-125 C)

\

Component Removal

The BGA package is removed with a hot gas tool, usually fitted with a custom

head that is sized to the BGA package. Proper sizing of the gas head reduces the

thermal impact on adjacent packages. Correct tool settings depend on the tool

being used, the package being removed, and the PCB. Determining the settings

is an exercise in profiling, similar to initial reflow. By using a profile assembly

with thermocouples mounted in solder joints, tool settings can be determined

which assure that all solder joints reflow properly. Monitor both the top and

bottom of the PCB.

The component is removed from the PCB by a vacuum nozzle within or integral

to the hot gas head. When profiling, shut the vacuum off so the component is


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not removed to avoid damage to the profile card. Control the pressure of the

head down onto the component during removal. If pressure is applied after the

solder balls are melted, the solder is pressed between the plates of substrate

and PCB,

ADVANTAGES OF USING BGA

BGA packages are smaller

No fragile leads

Board assembly improved

Board inspection reduce

Better thermal,mechanical & electrical properties

CONCLUSION

BGA packages provide a reliable solution for high-pin-count designs as long as

thermal,

electrical and mechanical issues are considered. BGA devices can have a

positive impact on theoverall PCB technology. Using BGA packaging

technology can result in a more robust and cost effective assembly process


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SELFCENTERING OF BGA

Self centering is one of the mot important property of solde present in BGA

Which help in proper alingment


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