PCB Design Perfection Starts in the CAD Library - Part 9: BGA Components

PCB Design Perfection Starts in the CAD Library - Part 9: BGA Components

The BGA or Ball Grid Array has been around since the 1980’s but the pin pitch started out with 1.5 mm and then quickly went to 1.27 mm (50 mils) for about 15 years. Then in the late 1990’s, the 1 mm pitch BGA was introduced and every couple years a smaller pin pitch was introduced. Today 0.4 mm pitch BGA’s are in every cell phone and 0.3 mm pitch BGA’s are the next generation. Figure 1 displays the lead type for this component family.

Figure 1

There are 2 types of BGA Ball Leads –

1. Non-collapsing - this is normally 0.5 mm pitch and smaller, where the Land (pad) is larger than the ball to allow for via-in-pad technology and provide an adequate annular ring. The solder mask can be the same size as the Land. In some cases the Land for fine pitch BGA’s 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 higher, 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.

 See Figure 2 for examples of non-collapsing and collapsing BGA balls.

Figure 2: Non-collapsing and Collapsing BGA Balls

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

Table 1: Land Approximation 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). Notice that the standard ball sizes are in 0.05 mm increments until the pin pitch hits 0.5 mm and less. However, even though the world standards try to keep BGA balls sizes in 0.05 mm increments, component manufacturer’s sometimes do not adhere to the standard and create BGA ball sizes in 0.01 mm increments, but 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 3-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 below is used for 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 0.5 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          Trace/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 3-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

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