Ultra-fine pitch devices pose new PCB design issues
No Industry Specifications/Design Guidelines
As indicated earlier, the electronics industry hasn’t yet developed the specifications nor the expertise to effectively perform 0.3mm ultra-fine pitch design and layout. This leaves many PCB layout engineers with few options other than to base their 0.3mm ultra-fine pitch on conventional 0.5mm pitch IPC design guidelines and layout rules.
For 0.5mm pitches or greater, using non-solder-mask-defined (NSMD) pads for devices was generally preferred. This allowed for better solder-mask registration and stress relief for the BGA solder joints. Boards that followed these guidelines performed better during stress testing.
In the case of 0.3mm pitch devices, if the same guidelines are used, it can lead to potential failures during manufacturing or in the field.
Use of NSMD requires the pad size to be reduced by as much as 20% of the ball size. The solder-mask is then opened with about the same aperture as the BGA ball, size. Now consider this, ball dimensions of a typical 0.3mm package are around 8 mils.
When we talk about soldering, this is already very small. After a further reduction of the pad size, one ends up with an area that may be too small for a reliable solder joint. Also pads of such a small size don’t have sufficient adhesion to the board and may even peel off. Secondly, since the solder mask is recessed, in this case, it opens the door to the possibility of solder shorts.
A better approach is using a solder-masked-defined (SMD) pad in this case. Not only does this allow a slightly more solder area on the BGA pad, solder shorts are less likely. Moreover, since the mask now encroaches on part of the pad, it allows for better registration as well as providing adhesive protection to the pad from getting peeled. More details on NSMD and SMD are given below.
Routing Guidelines
Routing guidelines for PCBs populated with fine pitch BGAs represent another key area that demands special consideration at layout. The main concerns are BGA land size, solder mask definition, BGA fan-out, and fan-out via features.
There are two main categories of BGAs based on the pitch and solder mask definition. The first one is non-solder masked defined (NSMD) pads, discussed briefly above. This is also known as the “collapsing” category.
As shown in Figure 2 below, NSMD pads have the solder mask opening that is larger than the copper pads. Most chip manufacturers have historically insisted on implementing NSMD since it provides tighter control of copper artwork registration compared to the positional tolerance of the solder masking process.
Moreover, NSMD pad definition may introduce stress concentration points that may result in solder joint cracking under extreme fatigue conditions.
Click on image to enlarge.
However, using this method of solder mask definition is being debated in the industry for being of any benefit for BGA pitches of less than 0.5mm. In fact, many assembly manufacturers stress that NSMD causes more problems than it solves for finer pitches.
Rules followed for 0.5mm pitches may not work for 0.3mm cases, especially when working at high volumes. Typically, ball sizes of 0.4mm pitches are smaller. Reducing it even further by 15% may cause insufficient solder able area. Secondly, using NSMD in cases of 0.4mm may cause bridging between adjacent pads. Thirdly, since the pad sizes are so small, and there is no solder mask webbing to provide adhesive strength, it may peel off, during reflow, or in the field.
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