I
was asked by many PCB designers about BGA design guidelines, say ball PAD size,
pitch size, annular ring, layer counts, solder mask opening, etc. There is no
ready answer for it, because every BGA is different from one another, yet there
are some rules and tips to follow to make better results. In this article, I am
going to introduce them step by step.
There are 4 parts in the post as
showing below:
1.
The advantages of using BGA
2.
How to avoid BGA design mistakes
3.
BGA design rules
4.
BGA PCB feature definitions
The advantages of using BGA
A
BGA package is more advantageous at design layout because it provides increased
functionality at the same package size compared to the Quad Flat pack (QFP). It
improves the ratio of pin count to board size, hence providing more connections
and increased area for routing.
A
BGA package also offers better thermal and electrical performance. It does that
by providing ground planes for a low impedance power system and controlled
impedance on traces.
BGAs
also offer reduced package thickness than those with pins. Moreover, BGA balls
are considerably stronger than QFP leads and therefore can tolerate rough
handling. This can potentially reduce the cost of ownership by virtue of their
re-workability.
How to avoid BGA design mistakes
Today’s
smaller packaging can create confusion to PCB designers. When designing with
BGA with pitch sizes of 0.5mm
and above, there are certain rules of thumb related to pad sizes and solder
mask opening. One is to maintain pad size to about 85 % of a BGA’s ball size.
Another is to use a non-solder mask defined (NSMD) pad. In this case, the
solder mask is larger than the BGA pad.
The
solder mask is usually opened to a diameter of about the ball size of the BGA.
The recessed solder mask provides stress relief to the NSMD pad during reflow.
In short, it creates a protective barrier and lets solder go around it.
Then,
there are solder mask defined (SMD) pads, which are used for 0.4mm and below pitch BGA PCB designs. NSMD and SMD pad definitions and photos are
shown in the bottom of this article. In the SMD pad case, the solder mask
opening is smaller than the pad size. Better yields and results are achieved
for these particular PCB designs.
Below
are some tips to avoid BGA design mistakes.
1.
Using NSMD pad sizes for 0.4mm
pitch BGA PCB design may provide favorable results at low volumes, but chances
are low yields result at high-volume production runs.
2.
Keep in mind that ball size of 0.4mm
pitch BGAs are smaller than those of 0.5mm
pitch BGAs. Reducing pad size further may cause an insufficient solder-able
area on the BGA pads. This can lead to PCBs being rejected at assembly
inspection or latent failures in the field.
3.
Bridging can be created between pads when using NSMD pads for 0.4- and 0.3mm pitch BGAs. Bridging is created
because there’s not enough solder mask webbing between pads.
4.
Since pad sizes are so small, and there is no solder mask webbing to provide
adhesive strength, the pad may peel off, during reflow, or in the field.
It’s
best to have no trace between two BGA pads. Having them introduces shorts at
pitches of 0.4mm
and smaller on external layers.
5.
On internal layers, there should be just one trace between pads. Also, PCB
fabrication problems surface when running multiple and thin 2 to 3 mil traces
between two BGA pads.
6.
Make all vias solder mask covering; for the vias at BGA area, they should be
via plugged. This is better your assembly purpose.
BGA design rules
|
PCB
Typical Feature Sizes For Standard BGA Arrays
|
|||||
|
Ball
Pitch
|
Via
Diameter/PAD
|
Via
Hole Size
|
Trace
Size
|
Clearance
|
Micro
Vias?
|
|
|
18
mil
|
10-8
mil
|
4-5
mil
|
4
mil
|
No
|
|
|
12
mil
|
6
mil
|
4
mil
|
4
mil
|
Yes
|
|
|
10
mil
|
5
mil
|
3
mil
|
3
mil
|
Yes
|
|
|
10-8
mil
|
5-4
mil
|
3
mil
|
3
mil
|
Yes
|
Conversion
between mm and mils
•
0.075mm = 3
mils (0.003” )
•0.1mm = 4
mils (0.004” )
•0.125mm = 5
mils (0.005” )
•0.2mm = 8
mils (0.008” )
•0.4mm = 16
mils (0.016” )
•0.45mm = 18
mils (0.018” )
•0.5mm = 20
mils (0.020” )
•
•
•
•
•
•
BGA PCB feature definitions
Pitch-distance
between the center of one ball to the center of the next one.
Via
diameter - the full width of the via annular ring, also called PAD
Ball
pads-Also called BGA "lands". This is the copper that the BGA ball
solders to.
Micro
via - a via that is laser drilled.
HDI
- High Density Interconnect. This is the type of board that uses very small
vias like micro vias. When a board uses micro vias it is called an HDI board.
NSMD
pad - Non-Solder Mask Defined ball pad. This kind of pad has an opening that is
defined only by the size of the ball pad. The solder mask is applied around the
ball pad and does not cover it. This kind of pad is more popular lately and has
the advantage of increased ball adhesion because the ball flows around the
sides of the ball pad and grips on the top and the sides. This kind of pad is
usually recommended for BGA designs 0.5mm and above.
SMD
pad - Solder Mask Defined ball pad. This kind of pad has a large copper area,
but the solder mask covers the edges, so the opening is known as solder mask
defined. This kind of pad is sometimes best for very small pitch pads where
ball shorting is an issue.
Baggio WANG FAN
SHENZHEN JAAPSON TECHNOLOGY CO LTD
skype: baggiowang0214
JAAPSON, Expert in HDI Multi-layer PCB Manufacturing
