PCB Design Perfection starts in the CAD Library - Part 8

Part 8 - PCB Design Perfection Starts in the CAD Library

Part 8 - BGA Components 

 

The Ball Grid Array, commonly BGA, has been around since the 1980s, but the pin pitch began with 1.50 mm and then quickly went to 1.27 mm (50 mils) for about 15 years. Then in the late 1990s, the 1.00 mm pitch BGA was introduced. Every few years since then, a smaller pin pitch was introduced. Today 0.40 mm pitch BGAs are in every cell phone and 0.30 mm pitch BGAs are the next generation. Figure 1 shows the lead type for this component family.

 

Figure 1: BGA leads - a lead ball collapsed against the PCB land

 

Ball Grid Array components can have one of two types of ball leads:

  1. Non-collapsing - this is normally employed for 0.50 mm pitch and smaller BGAs, where the land (pad) is larger than the ball to allow for via-in-pad technology and to provide an adequate annular ring. The solder mask can be the same size as the land with non-collapsing balls. In some cases, the land for fine pitch BGAs is solder-mask defined where the solder mask encroaches slightly over the land. This provides protection for any trace routing between the lands but the most significant benefit is to help secure the land to the PCB. During cell phone "drop testing", the BGA solder joint normally holds better than the land to the Prepreg. i.e.: drop tests prove that the non-solder-mask land will rip from the PCB before the solder joint breaks. So the solder-mask defined land is secured better to the PCB for drop testing.
  2. Collapsing - this is normally 0.65 mm pitch and larger, where the land (pad) is smaller than the ball size to allow the ball to collapse around the sides of the land. This requires a non-solder-mask-defined land where the solder mask must be larger than the land.

 

Figure 2 illustrates examples of non-collapsing and collapsing BGA balls.

 

Figure 2: Non-collapsing ball (left) and collapsing ball (right) BGAs, highly magnified

 

The BGA land (pad) size is determined by the ball size as seen below in Table 1, from the IPC-7351B land pattern standard. Notice the correlation between the “reduction” and the “land pattern density level.” The three density levels change the land size reduction percentage, but they also determine the placement courtyard excess (See Table 3).

 

 

Table 1: Land Approximatin (mm) for Collapsible Solder Balls

Note: The IPC-7351B LP Calculator Uses this chart for calculations

 

It is very important to note that IPC prefers the Maximum Material Condition for all BGA Land sizes; they do not use the Nominal Land Diameter, but do use the Maximum Land Variation Diameter (notice the Bold numbers in the Chart “Land Variation” column). The standard ball sizes are in 0.05 mm increments until the pin pitch hits 0.50 mm and less. However, even though the world standards try to keep BGA balls sizes in 0.05 mm increments, component manufacturers sometimes do not adhere to the standard and create BGA ball sizes in 0.01 mm increments. However, I have never seen a BGA ball size less than a 0.01 mm increment. Also, the BGA pin pitches are in 0.05 mm increments. As a result, the BGA land (pad) sizes are in 0.05 mm increments including the via fanout padstacks and hole sizes.

 

IPC-7351B has a three-tier BGA formula for placement courtyard excess that uses the BGA ball size to calculate an adequate placement courtyard for BGA rework tools. If the BGA has a large ball size, larger rework equipment is necessary to unsolder the increased solder volume.

 

With a small ball size, the placement courtyard can be smaller as less heat is then required to unsolder the BGA component for rework. However, the end user may not plan to rework the BGA if it fails. In that case, there is no need to have a robust placement courtyard, but a recommended minimum placement courtyard excess is 0.5 mm.

 

“Non-collapsing” Ball BGA Components

Table 2 shows land size calculations for non-collapsing BGA balls.

 

Table 2: Non-Collapsing BGA Ball Land Calculations

 

It is very important to note that IPC prefers the Maximum Material Condition for all BGA Land Sizes, meaning that the Maximum Land Variation Diameter is used; not the “Nominal Land Diameter”.

 

Figure 3 is a 0.50 pitch non-collapsing BGA ball. Instead of shrinking, the non-collapsing land size gets larger to handle the solder volume that creates the solder joint. This technology is new to the electronics industry and was created as a solution for lead-free BGA balls and via-in-pad technology as a routing solution for fine pitch BGA components.

 

  Figure 3: Non-Collapsing 0.5 mm pitch BGA

 

Via-in-Land Technology

 

Space & Grid Data

BGA Ball Size: 0.15

 

Trace Width: 0.075

BGA Land Dia: 0.275 Trace

 

Trace Space: 0.075

Hole Size: 0.15 Trace

 

Via Space: 0.075

Thermal Relief Required Trace

 

BGA Land: 0.075

Plane Clearance: 0.425

 

Routing Grid: 0.05

Solder Mask: 1:1 scale

 

Part Place Grid: 1

 

IPC-7351A has a three-tier BGA formula for placement courtyards that uses the BGA ball size to calculate an adequate placement courtyard for BGA rework tools.

 

If the BGA has a large ball size, larger rework equipment is necessary to unsolder the large solder volume. With a small ball size, the placement courtyard can be smaller as less heat is then required to unsolder the BGA component for rework. However, the end user may not plan to rework the BGA if it fails. In that case, there is no need to have a robust placement courtyard.

 

Table 3 below represents the 3-Tier scenario and the different placement courtyard excess size determination.

 

Table 3: BGA Density Levels for Placement Courtyard Size Determination

 

Coming Up

Additional brief topical articles will appear in future newsletters. You can also read more detail in my blog, which can be found at:http://blogs.mentor.com/tom-hausherr/

 

Written by Tom Hausherr CID+

EDA Library Product Manager 

Mentor Graphics Corporation 

Reprinted by permission from iConnect007