Modern electronics products have been developing towards fine line and ultra thinness with applied electronics components becoming increasingly smaller. Moreover, increasingly more electronics components with fine-pitch IC (integrated circuit) packaging have been assembled on PCB (printed circuit board), especially BGA (ball grid array) and CSP (chip-scale package) components. Component pitch has converted from 0.65mm and 0.5mm into 0.4mm or less; PCB thickness from 1.6mm and 1.2mm into 1.0mm, 0.8mm or 0.6mm or less; PCB layer count from double side or 8 layers into 12 layers, 18 layers or more; BGA mounting mode from single mounting into POP (package on package). All of the development mentioned above has been challenging our PCB manufacturing and PCBA capabilities among which, however, soldering quality of BGA is such a key element that BGA cracks tend to be caused as soon as one procedure receives insufficient concern or an unsuitable measure is made. The commonest position where cracks occur on BGAs is soldering connection of pad and pad bottom. Generally speaking, cracks take place most at four corners of a BGA component and then at its four sides because they suffer from the most stress.
The reasons for leading to cracks in BGA soldering will be explained in the following paragraphs.
Low Quality of PCB Leading to BGA Cracks
• Incorrectly-Picked Tg and Td of PCB Substrate Material
During the process from lead manufacturing to lead-free manufacturing, reflow soldering and wave soldering temperature has to rise up due to SMT (surface mount technology) assembly requirement. Some people simply believe it OK that substrate material with high Tg (glass transition temperature) is selected for PCB board. They just think that it's essential to manage and control Z-axis expansion. The primary aim is to stop delamination from taking place of thick circuit boards and 14-layer or more PCBs and to stop cracks from occurring to PTH (plated through hole) because a large extent of Z-axis expansion of PCB tends to lead PTH hole wall to break during reflow or wave soldering. Nevertheless, Tg fails to defeat cracks generated during lead free process unless Td (temperature of de-composition) is considered to totally solve PCB cracks issue. Three levels of Td are regulated in IPC concerning PCB substrate material: 310°C, 325°C and 340°C.
In conclusion, during the process of substrate material determination, the higher Tg and Td are, the better. But PCB fabrication cost is an essential consideration based on which substrate material with agreeable Tg and Td should be picked up.
• Insufficient Gel Content in Prepreg
Insufficient gel content in prepreg used on external layers and between internal layers tends to lead copper foil to generate bubbles under high temperature.
• Unsuitable Copper Profile Selection
Generally, ordinary profile is classified into three categories: standard profile, lower profile and very lower profile. Standard profiles contain no regulations on copper sheet because adhesiveness is high but too high profile tends to cause bad etching, which further reduces stability of line width and impedance control. Lower profile regulates that maximum Profile SPEC is 0.4mil (10.2μm). Up to now, lower profile has been leveraged by most PCB manufacturers. Very lower profile regulates that maximum Profile SPEC is 0.2mil (5.1μm), which is generally only leveraged in PCB fabrication with special fine line requirement such as 2mil trace width.
• Low-Performed PCB Lamination
Whenever low-performed PCB lamination takes place, insufficient melanism or brownization will lead to bad adhesiveness.
• Low-Performed Solder Mask Developing or Surface Finish
Low-performed solder mask developing or surface finish will cause soldering defects. For example, surface oxidation tends to be generated when OSP film is too thick or too thin, receives unsuitable pre-process or goes through too long holding time.
• Too Small BGA Pad Size
During the design phase, when BGA pad size is too small, it takes place possibly due to over etching or absence of compensation value of etching factor.
Untrusted Incoming Material and Layout of BGA
• When substrate Z-axis expansion of BGA incoming material is too large, it'll feature low peel-off strength and Td will be too low, both of which possibly lead to tin cracks.
• Vacuum packaging isn't implemented after IQC (incoming quality control) check, vacuum package suffers from breakage prior to baking or BGA adhesive to board surface for over two hours prior to soldering, all of which will lead to bad soldering.
• During BGA layout, its pad size can never be too small and pad size can never be lower than half pitch of BGA minus 2mil, except for pad used in special situations. Moreover, pad on four corners of BGA should be 1mil larger than its pads in terms of size.
• Four corners of BGA had better be designed to be SMD (solder mask define) because the enlarging of BGA base and the presence of solder mask cover around pad will dramatically improve crack resistance of pad. As solder mask define soldering is used, soldering can only cover the surface with side neglected, which leads intensity of soldering connection worse than copper define soldering.
• PCB with ENIG surface finish applied causes cracks at BGA soldering connections more easily. ENIG can never be leveraged on BGA whose pad is 11mil lower and OSP is more agreeable.
Insufficient Process Control or Low Assembly Condition
• During stencil design phase, four corners and each side of BGA component should be 1mil to 2mil larger than those of pad. Stencil opening size should be designed based on specs of BGA components including pitch, soldering balls on BGA and ingredient of soldering balls.
• During the process of printing, support pin shouldn't be against BGA to stop fake soldering and pillow effect from taking place due to contamination of BGA pad. Furthermore, especially much attention has to be paid to printing scraper pressure and printing quality control.
• Wafer positions of pickup BGA, component thickness setting and pressure amount of pickup should be emphasized during mounting stage.
• There are more opportunities for cracks during IR reflow and special attention has to be paid:
a. During the process of double-side PCB fabrication, PCB deformation degree has to be considered. Fixtures can be used during reflow soldering and substrate of fixtures has to be carefully considered for its possible shrinking due to high temperature and cooling.
b. Incoming BGA components have to be carefully inspected to see whether sinking does take place on soldering balls. Moreover, alloy ingredient of soldering balls and compatibility between Z-axis expansion of substrate material of BGA and PCB board.
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