Quantification of Phosphorus Content in Electroless Nickel Immersion Gold Deposits

Transcription

1 Quantification of Phosphorus Content in Electroless Nickel Immersion Gold Deposits Joe CHONG*, Geoffrey TONG, Kenneth CHAN, Dennis CHAN & Dennis YEE Dow Electronic Materials Copper is well known as the most important metal in the printed circuit board (PCB) industry owing to its ecellent electrical conductivity and cost competitiveness. However, its vulnerability towards oidation led to development of different surface finishes for protecting the copper from oidation, and providing solderable surfaces for attaching electronic components onto the PCB. Electroless nickel immersion gold (ENIG) is undoubtedly one of the most indispensable surface finishes nowadays. It consists of a two-layer coating system with a thick EN underlayer and a thin IG top layer. EN can act as a diffusion and oidation barrier for underneath copper layer. EN process is simple with advantage of low equipment cost. The obtained EN deposits ehibit long shelf life and ecellent properties in wear, soldering etc. The EN plating is commonly incorporated with phosphorus as hypophosphite based compounds are commonly employed as reducing agents. The phosphorus content (P content) can be varied between and 13 wt% by altering the chemical makeup and ph of the plating bath. The P content plays a 1 critical role in both physical and chemical properties of the deposits including hardness, ductility, corrosion resistance etc. EN layer with 6-10 wt% P is classified as medium P nickel; it has moderate corrosion resistance and therefore is one of the most widely used final finishes in PCB. High P nickel (>10 wt%) possesses superior corrosion resistance that increases its applicability in the market. However, eceptional high P content in EN may increase the difficulty of IG process, and may impact the strength of solder joint in the assembly processes. Hence, a good quantitative method of P content in electroless nickel immersion gold (ENIG) deposit is necessary. methods for determination of P content in the nickel deposits. ICP-AES is a wet-chemical method which provides absolute weight percentage of P content. However, the nickel layer requires acid digestion by concentrated nitric acid; and thus the plating substrate is limited to stainless steel panels owing to their inertness towards nitric acid. XPS is another option for quantifying P content. By conducting depth profiling analysis, information like elemental composition as well as chemical oidation states could be revealed. XPS has advantages in giving P content distribution along the coating depth. However, the instrument and operation cost of XPS are high. In addition, it is time consuming in sample preparation and measurement, so it is less commonly adopted among these techniques. EDS is a fast, convenient and non-destructive method to determine P content. However, compositional quantification using EDS is highly dependent on 7 instrumental conditions (e.g electron beam voltage ); therefore a reliable EDS quantification requires careful consideration of instrumental conditions and other processing parameters. Table 1 tabulates a comparison of these key characterization techniques. Table 1. Comparison of EDS, XPS and ICP-AES techniques In the field, inductively coupled plasma atomic emission 3 spectroscopy (ICP-AES), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray 1,4,5,6 spectroscopy (EDS) are commonly employed In this report, quantification of P content using the standardless EDS technique was systematically studied. Instrumental parameters including signal amplification time (amp. time), total number counts of PK, sample working distance (WD) as well as electron beam 4 Journal of the HKPCA / Issue No. 54 / 014/ Q4

2 acceleration voltage on the accuracy of quantification were investigated. A Ni P compound standard was used for calibration as well as error estimation. The P content obtained by the calibrated EDS was also compared with ICP-AES and XPS. Sample preparation ENIG coating with different P contents were obtained from different ENIG processes. Prior to EDS measurement IG layer has to be removed as gold contributes to strong X-ray background signal, and it has an X-ray Au M line at.16 kev which is very close to neighbourhood of phosphorus K (.013 kev). Removal of gold can be achieved via chemical etching or physical sputtering. Chemical gold stripping process is fast and simple but it has to be precisely control; otherwise it may cause over attack of the underneath EN layer. In this work, the IG layer was removed by argon ion beam sputtering (Precision etching coating system, Gatan, Model 68). The IG layer typically can be removed in 3 to 5 minutes at 6 kev using beam current of 00 ìa at an incident angle of degrees. XRF was used to monitor the gold thickness and ensure a completely removal of gold top layer. A thin layer carbon (< nm) was then deposited on Ni surface to increase surface electrical conductivity and ensure better EDS data acquisition without charging effect. EDS measurement was conducted in a Zeiss EVO-50XVP scanning electron microscope (SEM) with an energy dispersive X-ray spectrometer (Si-Li detector, Ametek EDAX). A Ni P compound standard (Geller MicroAnalytical Laboratory) was used for calibration and verification. ICP-AES (Perkin Elmer Optima 4300DV ICP- OES) as well as XPS (ThermoFisher Scientific K-alpha) were compared. EDS quantification can be determined either by standard or standardless methods. Using a compound standard is definitely the best practice to minimize the measurement errors. However, in reality, it is impossible to obtain standards for all samples such as electroless nickel deposited with different amount of P content. In addition, typical electroless NiP Quantitative measurement by EDS is not perfectly crystalline but ehibits a high degree of amorphous characteristic with increasing P content. Therefore, an ideal standard truly reflect the composition of NiP is not available. Industrially, quantification of P content by EDS was determined by standardless method with ZAF correction where Z, A and F represent atomic number, absorbance and fluorescence, respectively. Table summarized the default acquisition parameters recommended by an instrument supplier. Table. Default acquisition parameters of a Zeiss EVO-50XVP SEM Effect of Signal Amp. Time Amp. time setting refers to the time interval that the detector circuitry processes the X-ray absorption. During the processing period, X-ray signal is unable to be measured and this time period is regarded as dead time. The recommended dead time is set between 0-40%. Normally, higher spectral resolution with lower background noise could be obtained with longer Amp. time. The effect of amp. time on determination of P content (in wt%) of NiP standard (theoretical P content: 0.87 wt%) is shown in Table 3. Table 3. Determination of P content in weight percentage using different amp. time in EDAX It is found that the P content was overestimated by 6-10% if amp. time was varied from 1.8 ìs to 10.4 ìs. There are no observable systematic changes. However, the background intensity was obviously reduced if amp. time was increased to 10.4 ìs. Low background intensity can provide a better signal-tonoise ratio (see Figure 1). Therefore, amp. time of 5

3 10.4 ìs is preferable for achieving accurate measurement. working distance at 10 mm to maintain a take-off O angle of 35. Increasing working distance would indirectly increase take off angle that affects the measurement. Eperimentally, spurious peaks of iron was recorded in the EDS spectrum when increase the workingdistancefrom10to5mm(seeinfigure). Figure 1. Comparison of P K acquired 10.4 (solid red) and 1.8 (Black line) ìs, respectively Effect of total counts of P K peak Total number of counts of P K was important in P content determination. Table 4 indicates the relationship between total accumulated P count and wt% of P. Similar to amp. time, the P contents were overestimated. When the measurements were kept at total counts of 300 and 600, the P content was overestimated by 7 to 1%. When the total accumulated P count was 100, the variation of overestimation reduced. The high variation at low total count was attributed to relatively high background signal (low signal- to-noise ratio). Thus, to have a stable and reliable measurement, the total counts greater than 000 was recommended. Table 4. Relationship between different P total counts and their corresponding P content. Effect of working distance (indirect variation to take off angle) Working distance refers to the distance between the final lens and the sample. Its importance in EDS measurement is primarily related to take off angle, and the optimum working distance is instrument specific. The instrument supplier recommends to set Figure. A magnified view of EDS spectrum collected from working distances of 5 (black line) and 10 (solid red) mm. Effect of etra high tension Etra high tension (EHT) refers to the acceleration voltage of electron beam. Eperimental results revealed that increasing acceleration voltage from 15 to 0 kv resulted in an overestimation of P content from 6 to 10%. Further increasing the voltage to 5 kvoraboveyieldedanoverestimationof11%. Normally, the best-preset acceleration voltage is -10 times of the analyzing X-ray lines. For Ni and P, Ni K and P K are at and.013 kev respectively. Acceleration voltage of 15 and 0 kv can fulfill the analysis requirements of both Ni and P X-ray lines. However, the computer simulated analyzing depth of 15 kv electron beam was ~0.7 ìm (in Ni with 8.1 wt% phosphorous); which may be insufficient to give average P content over the EN thickness. Therefore, it is suggested to use acceleration voltage of 0 kv (analyzing depth: ìm) to give a stable value of P content from a larger analyzing depth. Thickness requirement simulation by electron flight simulator Electron flight simulator is able to simulate the interaction of electron beam in the sample and provides 6 Journal of the HKPCA / Issue No. 54 / 014/ Q4

4 information of analysis width and depth. The simulation was fied at 0 kv, and Ni with P from 4.6 to 0.9 wt% were simulated. A typical simulation profiles for Ni P is shown in Figure 3. decrease). The interaction radius of Ni L increases from 1.0 to 1.4 ìm, which account for a change of 40%. For plan-view analysis, the thickness of coating is more important. Typical penetration depths of for NiP with wt% P are about ìm. If it has P content up to 0.9%, the analyzing depth is about ìm. Therefore, the minimum coating thickness of NiP accurate measurement should be > ìm. Likewise, for cross-sectional analysis, the radius of interaction volume is more important. The minimum coating thickness of NiP for cross-sectional measurement should be >3 ìm. It should be done using point analysis at the midthickness of the EN layer. for Table 5. Comparison between wt% P, material density & interaction volume of Ni K, Ni L & P K by an Electron Flight Simulation Programme (SEM: 0 kv) Data verification with NiP standard Based on the investigation above, a setting for standardless EDS analysis using Ametek EDAX with Si-Li detector has been defined (see Table 6). Figure 3. Simulation of 0 kv electron beam interacted with NiP. The trajectory of X-ray generated and emitted was highlighted as green and red areas. Table 6. Recommended EDS setting in Ametek EDAX The interacting radius and depth are summarized in Table 5. It is noted that the interacting radius and depth increase when the P content increases (or density Standardless EDS measurements were conducted in samples designated as "Low", "Med" and "Hi" as shown in Table 7. The data were also reference to a Ni P 7

5 standard for error estimation. A 13% overestimation of P content in all analyzed samples using standardless EDS analysis. The percentage of overestimation was consistent. Table 7. Comparison of P content of different EN deposits using standardless and standard- methods Comparison between EDS and ICP-AES and XPS analysis A high P content EN plating was prepared on PCB and stainless steel panels. Follow the recommended instrument setting, the P content of EN on the PCB panel was 1.4 wt% by standardless EDS analysis where it was 10.8 wt% when it was reference to the NiP standard. The overestimation of P content was 13% that agreed well with the data above. For ICP-AES measurement, The EN layer was tore off from the stainless steel panel, weighed and digested by concentrated nitric acid. The P content determined by ICP-AES was 11.1 wt% which was closely matched to the EDS result using NiP as the compound standard for quantification. XPS analysis showed that the P content in the coating was only 7.9 wt%; which was low compared with EDS and ICP-AES. Calibration or counter-check with NiP standard is not feasible because it is very challenging to find a large NiP grain with diameter substantially larger than 30 ìm to meet the smallest spot size of X-ray beam. Other setting like X-ray intensity, analyzer pass energy, dwell time etc. had yet not optimized. It is believed that the underestimation of P content could be improved by restricting the instrumental setting as done in this EDS analysis. of EDS instrument parameters, the P content can also be determined with standardless method. The P content determined by standardless EDS was consistent and very close to that determined by ICP-AES. XPS, in this work, showed relatively large uncertainty in P content quantification. However, it is epected the accuracy could be improved by strict control of measurement parameters. References: J.W. Yoon et al., "Characteristic evaluation of electroless nickelphosphorus deposits with different phosphorus contents" Microelectronic Engineering 84, 55 (007). R. Parkinson, "Properties and applications of electroless nickel" Nickel Institute. G.O Mallory et al., "Quality control of electroless nickel deposits" Electroless plating: fundamentals and applications, William Andrew Publishing, p183 (1990). S. Lamprecht et al., "Impact of bulk phosphorous content of electroless nickel layers to solder joint integrity and their use as fold- and aluminium bond surfaces" Atotech Deutschland GmbH, Hugh Roberts, Kuldip Johal, Atotech USA, Inc.(004). K.H. Lee, "Identification and preventionof black pad in Sn/Pb soldering", Circuit World 37, 10 (011). C.F. Oduoza et al., "Nickel phosphorus deposition on pretreated aluminium alloys during immersion in electroless nickel bath" Journal of Materials Science and Engineering A 1, 457 (011). A. Ballantyne et al., "Advance surface protection for improved reliability PCB systems", Circuit World 38, 1 (01). Summary Quantification of P content can be achieved by either EDS or ICP-AES or XPS. From the accuracy point of view, ICP-AES provides highest accuracy, as it is a direct measurement of material concentration. By strict control 8 Journal of the HKPCA / Issue No. 54 / 014/ Q4