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All About BGA’s

The Ins and Outs of Designing, Placing and Testing

BGAs (Ball Grid Array) on Printed Circuit Boards

Compiled by

Advanced Assembly, LLC.

June 2013

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Advanced Assembly 1-800-838-5650 www.aapcb.com 2

Table of Contents

Section 1: What is a Ball Grid Array (BGA)? .............................................................................. 4

Section 2: Designing with BGA’s ................................................................................................ 4

Section 3: Designing with uBGA’s .............................................................................................. 5

Section 4: Reflow for BGA’s ....................................................................................................... 5

Appendix 1: Examples of 3D X-Rays for BGA’s ......................................................................... 6

Magnification .......................................................................................................................... 6

Power ..................................................................................................................................... 6

QFN/Thermal Pads ................................................................................................................. 7

Defects ................................................................................................................................... 7

3D ........................................................................................................................................... 7

Limitations - Interference ........................................................................................................ 7

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About This White Paper

This white paper is the first in a series of papers written by Advanced Assembly to help design

engineers create and submit printed circuit board (pcb) designs that can be assembled quickly

and error-free. By sharing our industry knowledge, gained from nearly 10 years of quickly

assembling pcb orders of 1 to 35 boards, our goal is to help engineers avoid common design-

for-assembly mistakes, thereby ensuring all pcb projects are delivered on-time and on-budget.

The All About BGA’s whitepaper is being provided free of charge to all interested parties. This unique benefit from Advanced Assembly is designed to help engineers further understand factors at play in the market as well as provide information to assist in quick-turn product innovation, creation and delivery.

Advanced Assembly is the only printed circuit board assembly company that provides services

specifically for engineers needing fast prototype PCB assembly. Until now, engineers had a

difficult time finding a reliable partner to assemble small quantities of circuit boards. In 2004,

Advanced Assembly developed a proprietary technology to assemble a few boards at a time

and deliver them over 80% faster than the typical industry turn time. The company’s financial

success proves that it is fulfilling a previously unmet need as evidenced by continual year-over-

year, double-digit revenue growth.

For more information about the company or to quote a project online, contact Advanced

Assembly at 1-800-838-5650 or www.aapcb.com.

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Entire ball flowed into via

Section 1: What is a Ball Grid Array (BGA)?

A ball grid array (bga) is a surface mount chip package that

connects through a grid of solder balls. During reflow the

solder balls fuse with the pads on the printed circuit board and

form the joints.

BGA’s are becoming more popular because they can be easier

to align and require less space, an important consideration

when designing for electronic products that continue to shrink in

size. This is accomplished by putting all the terminations

underneath the package instead of around the edges, like is

done on QFPs.

Key advantages of a BGA package is include:

• A smaller footprint

• A larger pitch

• No fragile leads

• Better thermal and electronic properties

• Reduced lead inductance (faster speed)

Section 2: Designing with BGA’s

When using BGA’s in a pcb design, it is important

to not have via in pad. BGA’s usually fail when via

and traces leading from the BGA are not solder

masked (and in particular are not solder masked

over bare copper). This is because

during reflow, the BGA ball will become molten,

and will take any path that it can, often pulling

away from its destination. For this reason, via in

pad (unless plugged) is not recommended for any array device.

Other design considerations include:

• A HASL finish on a board is typically not flat enough for proper BGA placement.

• All traces and vias connected to a pad need solder mask (Solder Mask Over Bare

Copper – SMOBC). Otherwise, thieving or poor solder joints may result.

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• It is important to specify if you are using mixed lead/lead free components or boards, as

this can affect the reflow procedure (see reflow section below).

• Alignment indicators are extremely important for BGA’s because the package makes it

impossible to see if the grid of connections is in the right place.

Section 3: Designing with uBGA’s

Paste flux (instead of solder paste) is recommended for uBGA type packages (pitch less than .5

mm). Masking off all areas on the stencil is recommended.

The inclusion of uBGA’s normally does not preclude the board from the advantages of machine-

placed assembly. The board gets assembled using the automated Pick-and-Place machine just

like all SMT components.

However, if the uBGA has a mirrored surface, it might be hand-placed as opposed to pick-and-

place to ensure good placement. (Some pick-and-place machines can have vision issues with

mirrored surfaces).

uBGA’s are very susceptible to solder thieving if traces or vias going to these pads are not

masked. Below is a diagram of a recently-placed, small uBGA (flip chip).

Section 4: Reflow for BGA’s

The reflow profile is evaluated at the time of reflow. The thickness of the board, component mix,

and Lead/Lead free BGA’s are all important factors in determining the profile used. Typically

the first board processed will be X-rayed before reflow to ensure correct ball placement.

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Section 5: X-Ray for Quality BGA Placement

An X-ray process is recommended for BGA’s to ensure quality because the parts cannot

accurately be visually inspected. X-rays can see bridging, solder balls, excess solder, thieving,

component misalignment, parts under BGA’s and missing balls. For these reasons, Advanced

Assembly X-ray’s all BGA’s (including uBGA), LGA, and bottom terminated components (QFN

w/ thermal pad). Please note, however, that an x-ray cannot detect opens, like head in pillow

defects.

Appendix 1: Examples of 3D X-Rays for BGA’s

Magnification

This is an X-ray of a uBGA – the overall dimensions of the

part are 1.5 x 3 mm.

Note: This shows the thieving effect of placing vias in pad.

The solder ball has almost completely dissolved into the via

(creating an open).

In this 3D X-ray of a BGA, you can see that at the top of

each ball the pad is visible due to the lower density of the

pad vs. the ball.

Power

This is an X-ray of a BGA with a thick metal can. The power

(up to 160 keV) and current of the X-ray can be adjusted.

Also, the brightness and contrast on the captured image can

be adjusted to ensure that the device meets IPC-A-610

(Acceptability of Electronic Assemblies), and IPC-7095

(Design and Assembly Process Implementation for BGA’s)

Note: This board has many blocking caps on the other side of the board – not shorts.

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QFN/Thermal Pads

Thermal pads can be inspected for coverage, voids, and shorting.

Defects

Shorting under a plastic

package can easily be

determined in 3D or 2D X-rays. This picture also shows

vias under the thermal pad and fill of the vias.

3D

3D views can be taken to ensure that components

(mounted on the other side of the board) are truly

components (and not shorts).

In many cases 3D images cannot be taken. Background

metal (like POP parts) interferes with taking images. The

part can be rotated in any direction to view and analyze,

but can be difficult to image.

Note: Normal day to day production X-ray of units is typically done in 2D.

Limitations - Interference

Unfortunately when parts are placed in the same XY

location but on different sides of the board, along with

numerous metal layers, it can be difficult to image. This

is very board design dependent – images can be taken

on most boards.

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This is what a BGA looks like when it is not centered

on the pads.

If there is any shorting (ball to ball) thieving (if solder mask

is not placed on the trace to a via), missing balls, etc., then

the pad looks like an oval at the top of the ball.

X-Rays also cannot detect open connections (even with 3D

inspection and maximum magnification it can be difficult to

see the pad to ball connection).

For more information about assembling BGA’s, please contact us at 1-800-838-5650 or

email [email protected]. We will be happy to connect you with Advanced Assembly’s

engineering team.

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