A significant majority of the electronics OEM market is rapidly moving toward increasingly greater product miniaturization along with higher levels of functional integration.
The best examples are the new crops of smart phones and mobile/wireless devices. And whether the industry is ready or not, 0.3 millimeter (mm) ultra-fine pitch micro CSPs (chipscale packages), micro BGAs (ball grid arrays), and other active devices are quickly being ushered in to lead the designs of newer generations of thinner, faster, and sleeker portable, handheld electronics and communications devices.
Consequently, printed circuit board (PCB) assembly – with or without the benefit of proven 0.3 mm pitch technology know how — is at the top of this totem pole with the burden placed on contract manufacturers (CMs) and electronic manufacturing service (EMS) providers to resolve related issues.
Or, you can think of it this way. Numbers of naïve electronics sub-assembly houses and OEMs are inadvertently forging ahead using earlier generation layout rules, but remaining innocent of the consequences.
Let’s start out by saying that currently there are no formal design guidelines or layout rules specifically tailored at supporting 0.3 mm pitch CSPs. The electronics industry hasn’t come up with the highly important specifications and expertise to perform 0.3 mm pitch design and layout projects.
As a result, today, many PCB design and layout engineers largely rely on traditional 0.5 mm pitch IPC design guidelines and layout rules to develop new 0.3 mm pitch devices-based designs.
For example, the older design guidelines allow PCB design and layout engineers to design a solder-ball-joint pad with the diameter of 20 percent less than the diameter of a BGA/CSP solder-ball. Figure 1 below shows a solder-ball-joint pad with the diameter of 20 percent less than the diameter of a CSP/BGA solder-ball (A = B = 80% x ball-diameter).
Figure 1. A solder-ball-joint pad with the diameter of 20 percent less than the diameter of a CSP/BGA solder-ball (A = B = 80% x Ball-Diameter). (Source: Agilent application note)
As show in Figures 2a through 2c below , the solder-ball-joint pad with a 20 percent less diameter of a CSP/BGA solder ball allows PCB designers to design a dog-bone style layout, NSMD non-solder-mask-defined (NSMD) pads with non-solder mask covered clearance between the copper and solder mask, and to design solder-mask defined (SMD) pads with low-accuracy solder mask defined pad dimensions and low-accuracy pad center position.
Fig. 2a – A dog-bone style layout for CSP/BGA pad. (Source: Actel Corporation application note)
Fig. 2b – Side view of NSMD CSP/BGA land pads and SMD CSP/BGA land pads (Source: Altera Corporation application note).
Fig. 2c – Side view of NSMD CSP/BGA solder joints and NSMD CSP/BGA solder joints (Source: Altera Corporation application note)
Now, consider that the solder-ball dimension for 0.3 mm pitch mini-CSP components is extremely small – less than 8.2 mil. After a 20 percent reduction as used in 0.5 mm pitch design guidelines, the CSP pad dimension is considerably smaller.
It becomes obvious that the stage is set for creating innumerable solder defects, resulting in a rather dismal final yield. PCB designers use solder-mask defined pads when following the 0.5 mm pitch design guidelines.
There is a +/- 3-mil tolerance for the solder-mask layer aperture and very small pitch 11.8 mil (0.3 mm) as shown in Figure 3 below. Therefore, the very small solder-mask (less than 4 – 5 mil) cannot securely attach and exist between the two pads.
Figure 3 – Bottom view and cross-section of an example of two-layer CSP. The 0.3 mm pitch means that the distance is 11.8 mil between the center of solder-ball and the center of next solder-ball. (Source: Actel Corporation application note)
Distance is 11.8 mils between the center of a pad and the center of next pad. This is a 40 percent pitch reduction. Lost here are 7.1 mils to accommodate the mask tolerance and the gap to securely holding the mask-layer to prevent a solder bridging defect. Traditional design guidelines require at least 6-mil solder mask gaps between the two nearby solder-pads to achieve solder-joint quality and reliability.
Now, when it comes to the new 0.3 mm ultra-fine pitch technology, PCB design engineers aren’t able to find sufficient room for a 6- mil solder mask gap. There is no room for dog-bone style pad layout; and no room for non-solder mask covered clearance between the copper and solder mask.
It is too risky to allow solder-paste above the SMD mask layer for the SMD pad layout and to allow solder-paste above the non-mask areas for NSMD layout. Both SMD land pad and NSMD land pad have very high defect tendency for solder-bridging, solder-splashing, insufficient solder and open solder defects. Basically, the DFM requirement is in conflict with the layout requirement from the 0.3 mm pitch semiconductor manufacturer’s spec.
Current PCB fabrication limitations cause a lot of problems for 0.3 mm pitch technology. For example, there is +/- 3-mil mask-layer aperture tolerance and a +/- 1-mil copper-pad tolerance. Standard mask technology cannot securely apply a mask-layer within the very small areas (less than 4 – 5 mil).
What is needed is at least 6 mils for the solder mask attachment. The +/- 3-mil mask-layer aperture error will cause the pick-and-placement machine (P&P) (with even with best machine vision alignment capability) to miss the fab ball-pads. The fab ball-pad of the CSP is too small (4 – 6 mil per 20% reduction rule) for the P&P machine. The resulting solder-joints will have open solder and insufficient solder defects due to standard fabrication technology limitations.
The 0.3 mm pitch assembly process also has a numbers of issues associated with stencil limitations, which greatly reduces the final assembly yield. One is solder-paste release from the stencil is not effective due to the small stencil aperture and mask/pad tolerance.
There are some of paste on the top of mask-layer because the mask-layer placement error, because the Gerber-defined stencil aperture positions (which are very accurate) are not the mask-defined pad positions (which are usually +/- 3 mil off from the Gerber-defined X-Y positions).
As a result of the extremely tight 0.3 mm pitch, solder-paste can easily form solder-bridging defects between solder-ball joints. Distance between the two joints is reduced by 40 percent (Compared to 0.5 mm pitch technology, 0.3 mm pitch is 0.2 mm less than 0.5 mm.
That means 0.3 / 0.5 = 60%). Moreover, the missing or un-secured solder-mask between the two joints makes things worse, which is another root cause of solder bridging.
In effect, a series of issues at the PCB design layout when using 0.3 mm pitch devices trigger a domino effect at the fabrication and assembly stages. Those include highly probable pitfalls relating to stencils, solder paste, and pick and place.
Most pick and place machines today can only handle 0.4 mm pitch components and not 0.3 mm pitch micro CSPs, thus creating such reflow problems as insufficient solder at the joints and solder bridging. Today, there are no effective measures to prevent mis-placement on pads with a +/- 3-mil error.
Answering the 0.3 mm Ultra-Fine Pitch Challenge
However, not all is doom and gloom when it comes to resolving the 0.3 mm pitch challenge the industry faces. Some leading edge CMs and EMS providers are successfully creating innovative new PCB design layout rules, which meet both DFM requirements and 0.3 mm pitch CSP component requirements.
Rule one is not to follow current design guidelines – calculating CSP ball-joint pad from the 20 percent reduction formula. However, certain numerical experiments and yield improvement process developments are at the forefront to achieve the optimized pad dimension, which can meet both fabrication requirements and CSP SMT assembly requirements.
At our company, we’ve been involved in more 40 PCB projects using these proprietary 0.3 mm pitch device guides have resulted in 100 percent yield, thus proving they are successful.
A major consideration for these new developments is the board fabrication approach, itself, so that a PCB to be populated with 0.3 mm pitch micro CSPs and micro BGAs is properly fabricated with two objectives in mind. One is fabricating it within reasonable cost; two is meeting final solder-joint quality and reliability demands.
The route to get to these objectives involves a combination of methodologies to solve both small pad and mask-layer quality issues. The idea here is that the pad should be large enough and a solder-mask layer must exist between two pads.
Pad dimension is an especially crucial element for controlling the overall 0.3 mm project success and helps to ensure a high success rate for the rest of the steps. If pad dimension is less than the required critical dimension, the remaining processes won’t be successful, no matter the adjustments made. In short, a fixed-diameter pad dimension is the linchpin for both PCB design layout engineers and OEM customers to focus on, thus eliminating ambiguous pad dimension ranges.
One solution to the problem
At Nexlogic , we’ve implemented a new way to achieve +/-1 mask-aperture accuracy that we think effectively resolves 0.3 mm pitch fabrication issues. A number of factors are involved in attaining this goal.
They include reducing issues associated with fiducial mark tolerance, CSP pad position tolerance, mask-aperture accuracy, using laser direct imaging (LDI), small mask gap quality and others within a high-end fab house’s capability range.
A major part of this workable formula is a unique stencil approach for applying solder paste to 0.3 mm pitch micro CSPs. Traditionally, most assembly houses rely on a gel-flux only recipe for ultra-fine pitch CSPs. However, they fall short on acceptable yields and reliability. However, the right solution ensures there is sufficient solder paste for all CSP joints within the current area ratio (below 0.70.)
This specially developed stencil produces a highly efficient paste release. A key benefit is it prevents solder ball/solder splash normally existing around ultra-fine joints during the paste assembly process. Moreover, paste registration and volume control are also excellent, thus precluding solder bridging and insufficient solder problems.
Another part of this 0.3 mm pitch formula is adjusting the pick and place and reflow processes with proprietary processes in order to effectively handle 0.3 mm pitch devices. This normally comes as a result of working on a number of PCB projects involving 0.3 mm pitch devices.
Michael Yu is senior manufacturing engineer at NexLogic Technologies, Inc ., San Jose, CA with extensive experience in process control, SMT process and manufacturing resources management. He has been in the EMS industry for over 16 years with work experience at Bema Electronics and Pactron Electronics. He has a BS in Materials Engineering from Shanghai Jiao Tong University and an MS in Mechanical Engineering from South Dakota School of Mines & Technology.