Electroless Ni/Immersion Au (ENIG) is a well accepted surface finish for soldering and Al-wire bonding application in the assembly industry. The number of ENIG boards has increased over the last few years with the complexity of the PCB design and due to the beneficial features of this final finish. These include co-planarity, excellent solderability/Al-wire bondability, the ability to survive multiple soldering cycles and additionally the allowance of multiple hand reworks. Nevertheless especially with the assembly of CSP´s it was reported that some solderjoint reliability problems occurred. These problems arose due to the different expansions (elongation) between the Chip Scale Packages and the PCB material which creates a force on the solderjoint which leads in the worst case to a brittle fracture between the Intermetallic and the Ni surface. Due to the fact that the NiSn intermetallic is more brittle then the CuSn Intermetallic a solution for this problem will be to selectivly plate NiAu on all areas except the CSP areas and to protect these areas with organic solder Preservation (OSP). This technology will still have the benefits of NiAu but will not show the brittle fracture behaviour on CSP components. However, different process steps have to be added to the manufacturing of the PCB´s which influence the conditions of the NiAu plating/performance. This paper should describe the SIT process in general, with the explanation of the benefits by using a specially developed electrolyte which can cope with the challenges created by this technology such as the need for low bath temperatures and low loading factors, especially in the Ni-bath. Information is also provided for typical process interactions, occurring due to the necessary additional process steps. By understanding this, the interactions between the NiAu-process and the OSP process as well as the masking process of the CSP-areas the overall reliability for the PCB producer can be improved to the expected yields.
(1) Introduction
Second Image Technology combines the application of two final finishes, NiAu and OSP, on the same PCB.
• NiAu = key pads, contact switching and soldering
• OSP = soldering of μ-BGA´s / CSP's
With this technology it is possible to achieve a higher reliability of μ-BGA solderjoints due to higher solder joint integrity of the CuSn- intermetallic as compared to a NiSn- intermetallic. Assemblers of CSP´s and μ-BGA´s have reported solderjoint failures at the Nickel/Solder alloy interface for conventional NiAu finishes. The problem is commonly referred to "Brittle fracture". This interfacial fracture is a clean cleavage between the solder and the Nickel layer. The source for this error is the force created by, the coefficient expansion of different materials during the assembly of the PCB. This force creates internal stress in the solder joint, which in turn will lead to a brittle fracture if exposed to reliability testing (e.g. drop testing). If this force exceeds a certain level, the brittle fracture will occur at the weakest point of the solder connection which is the interface between Ni and Solder due to the brittleness of the NiSn intermetallic which is formed during the soldering process. It was also observed that a solder joint created by using OSP, thus forming a Copper/Solder alloy , is less sensitive to show this problem due to a stronger solder joint formed by the CuSn-intermetallic. So one possible way to minimize the problem would be, to go for OSP protected copper on the sensitive areas. Other factors that can impact the Brittle fracture behaviour are as follows : Quality of the NiAu-layer, PCB-Board thickness/-material, Type of CSP-component, Usage of Underfill, BGA pad size, Cu-thickness, Soldermask application (soldermask defined/-undefined).
On mobile phone boards it is essential to apply NiAu on some areas were contact switching or electrical testing is carried out (eg. Key pads, display, vibrator). On this boards it is necessary to have two final finishes combined on one board . Additional the multisoldering behaviour from OSP is less then perfect. This leads to the Requirement that only the BGA areas should be covered in OSP and the rest should be plated in electroless NiAu.
The production flow of SIT boards is different to the conventional production flow. Additional process steps are necessary such as Imaging before NiAu , stripping of the resist and OSP
coating after NiAu.
(2) Results and Discussion
Due to the need for coverage of certain board area, by using an Imaging mask and exposing this mask to the hot electroless Ni and Au electrolytes, influences on the quality of the SIT Board and/or the performance of the NiAu process have been observed. The currently used imaging masks like dry film and screen printing mask are developed for different applications (Etching or Plating resist). Even in modified versions it could be detected that the use of these Imaging masks could result in serious blistering of the mask after plating ENIG. In the worst case this blisters break through and the electrolyte could contaminate the covered pads. Additional it was observed that the leach out products from the Imaging masks could effect the performance and lifetime of the process steps in the NiAu-process. Also the exposed Cu-area and therefore the loading factor in the Ni-bath is seriously influenced. Low loading factors leads to inactivity in the Ni plating. Without adjustment in terms of Electrolyte composition, plating problems like Skip plating and Ni-thickness distribution problems are observed. Due to the above mentioned process problems a new development for a Ni-electrolyte which can cope with the need for low plating temperatures and low loading factors was necessary.
(3) Conclusion
NiAu Process performance
To run the Ni/Au process for SIT Technology with the expected performance, it is necessary to operate the Ni-bath at low temperatures to minimize the dry film leach out and at low loading factors. This which will affect the quality of the Ni-layer as well as the lifetime of the baths. Additional a stronger activation compared to current systems is necessary to avoid an uneven thickness distribution, especially on small pads. Therefor a modification of current existing bathes is necessary.
Imaging mask compatibility
All lab tests and production experiences showed that it is necessary to run the Ni-bath at lower bath temperatures with the current status of Imaging Technology. This improves the yield of the PCB´s due to less blistering of the Imaging mask. These blisters can break and Electrolyte can contaminate the pads underneath which is a possible source of critical defects.
Solderability of SIT NiAu-layers
In general a solderable NiAu –layer deposited in SIT Technology is requested by Endusers. NiAu-layers deposited with the Aurotech SIT-process exhibit good solderability, but due to the
usage of Imaging mask on the PCB-boards in the NiAu-process and OSP layers on top of the NiAu process, the solderability can only be ensured after solderability testing over the life time of each of the baths have been verified. The Imaging mask is not designed for the use in NiAu processes. During the process steps, especially in the hot Ni and Au-baths this mask leaches out and contaminates the baths. This contamination might influence the solderability performance of the NiAu-layer.
The main problem for a general assurance of the solderability of the NiAu-layers is, that different customers are using different kinds of Imaging mask or different conditions in application of these masks. Additional the OSP process can influence the solderability, due to the fact that a cleaner which will contain Cu and a microetch containing Cu and Oxidising agents contact with NiAu-layer.
If the amount of Cu in this solution is too high or the Exposure time is too long, Cu can deposit on the Ni (due to porous Au) and can also influence the solderability. According to Atotech´s recommendation it is necessary to carry out the following test procedure to ensure the solderability of NiAu-layers in the SIT-Technologie:
• During ramp up of production frequent solderability testing has to be carried out ( Must II, Metronelec,...) over the life time of the Ni and Au-bathes.
• This solderability test has to be carried out directly after NiAu and also after the OSP-Process on NiAu areas using special test coupons.
If no influence on the wettability is detected, the solderability tests will give constant results. In this case the wettability is ensured with the Imaging mask used, for the recommended life time of the Baths in the NiAu-process. If a decrease of wettability is detected it is necessary to limit the life of the baths or to adjust the parameters of the dry film process. Additional it has to be mentioned that wettabillity doesn´t give results regarding the adhesion of assembled components. To ensure a good adhesion of the components, it is also necessary to plate test coupons for shear testing together with the coupons for wettability control. This testing has to be done when all parameters for applying the Imaging mask are set and has to be carry out again if there is a change of parameters or of the Imaging mask.
