INVESTIGATING THE EFFECTS OF ASPECT AND AREA RATIO IN THE STENCIL PRINTING OF SOLDER PASTE
The Electronics Manufacturing industry is continuously moving towards miniaturization, better reliability and better functionality which are bringing in new challenges to the industry.
1.1 Surface Mount Technology:
Surface Mount Technology is an important method used to mount electronic components on the surface of printed circuit boards or substrates. The need for smaller size, light weight and high performance for printed circuit boards has led to the trend of electronic packaging away from through-hole technology towards surface mount technology (Jianbiao Pan & Gregory L. Tonkay).
A typical Surface Mount Assembly (SMA) line consists of a stencil printer, Pick-and-Place machine and a reflow furnace for populating PCB's using reflow soldering.
* During the first step, solder paste is deposited on the pads of the printed circuit board by the stencil printer.
* Then, the surface mount components are placed on the pad by the pick and place machine.
* Finally the solder paste is reflowed and solder joints are established through the reflow furnace.
This study mainly deals with the stencil printing process of the surface mount technology. This is the first and the most important stage of SMT because if the correct amount of solder paste is not deposited on the board, this error cannot be rectified by the later stages.
1.2 Stencil Printing:
Stencil Printing is the most popular method of depositing solder paste onto the printed circuit boards in surface mount technology. In stencil printing process, the solder paste is transferred into the stencil aperture by the action of a squeegee (T A. Nguty & Ndy N. Ekere 2000). This process consists of three main stages, which are as follows:
(a) In the first stage, the squeegee forces the solder paste to roll in front of the squeegee blade and travel across the surface of the stencil.
(b) In the second stage, the paste gets deposited in the stencil apertures due to the high pressure generated by the squeegee blade.
(c) In the last stage, the paste is left on the pad of the substrate after the stencil is released (Taho Yang et al., 2004).
1.3 Challenges in Stencil Printing:
Stencil printing process is the most critical stage in Surface mount technology. If the correct amount of solder paste is not applied at this stage, the following stages cannot correct this situation. It is reported that 50-70% of solder defects in electronics assembly are attributed to stencil printing (Joe Belmonte et al., 2007; Jianbiao Pan, et al., 1999; T.A. Nguty & N.N Ekere 2000; Tsung-Nan Tsai 2007). This shows that this process is not yet completely understood and plenty of research is yet to be done. This is because stencil printing exhibits a non-linear behaviour due to the nature of the process and the unavoidable random variations.
The main parameters affecting the performance of stencil printing in solder paste deposition are identified as:
* Aperture Size and Shape
* Stencil Thickness
* Print Speed
* Squeegee Pressure
* Solder paste type and
* Board finish
1.4 Stencil Printing Defects:
The defects that may occur due to improper stencil printing may be classified as smearing, skipping, ragged edges and misalignment (R.P. Prasad 1997).
Smearing: It occurs when solder paste is deposited on unwanted areas, resulting in formation of ‘bridges' of solder balls. The incidence of bridges increases as the printing is done at finer and finer pitches.
Skipping: It is caused when insufficient amount of solder paste is deposited on the surface mount pads. Skipping causes insufficient solder joints.
Ragged edges: It is a defect which causes non-uniform solder joints.
Misalignment: Misalignment occurs when the pad pattern and the opening of the stencil are not properly aligned.
1.5 Project Aims:
The aim of this project is to achieve a better understanding of the stencil printing process in surface mount technology and investigate the effect of area and aspect ratio on its performance.
1.6 Project Objectives:
The objectives of this project are as follows:
* To study the stencil printing process used in Surface Mount Technology.
* To design experiments using the Taguchi Approach (Orthogonal Arrays).
* To find out the effect of area ratio and aspect ratio individually on the printing performance.
* To find out any interaction between the area and aspect ratio.
* To find optimum setting (process window) for both the factors.
The methodology to be used in this
2.1 Current Research:
Stencil printing is considered to be the most important stage in the surface mount assembly. Industry reports that 50-70% of solder defects occur due to improper solder paste deposition during the stencil printing process. So it is necessary to understand the factors affecting the printing performance and if there is any interaction between these factors. Plenty of research has been carried out in this area and there have been mixed results. Some of the literature which have attempted to identify the factors affecting the solder paste deposition and ways of improving the printing performance have been studied, which are:
2.2 Critical Variables of Stencil Printing:
Jianbiao Pan et al., (1999) investigated the critical factors affecting the solder paste printing to identify the most important of them. The effect of the process parameters including aperture size, aperture shape, board finish, stencil thickness, solder paste type and print speed were examined. The experiment were conducted using solder paste of type3 and type 4 on BGA (Ball Grid Array) and QFP (Quad-Flat Pack) of five different pitches ranging from 0.76 to 0.3mm and the deposited solder volume was tested by a 3D-laser triangular inspection system. A total of 16 experiments are conducted at 2 levels four factors factorial design. The data was analysed using the Analysis of Variance (ANOVA) which drew out the following important conclusions:
* The most important process parameters affecting solder paste deposition in stencil printing are aperture size and stencil thickness.
* The transfer ratio does not necessarily increase with the stencil thickness.
* An area ratio of 0.6 is recommended for the selection of proper stencil thickness.
* A strong interaction between print speed and solder paste type is shown; selection of print speed should depend on the type of solder paste.
* A perpendicular aperture prints around 8-9% more volume than a parallel aperture.
2.3 Process Development for Ball-Grid Array Assembly:
L. Gopalakrishnan & K. Srihari (1999) conducted experiments on circular apertures or ball-grid arrays of different sizes to investigate the effect of various process parameters. Experiments were conducted using the Taguchi-approach of design of experiments and process capability tests done to evaluate the capability of solder paste deposition. Considering the volume and quality of the solder paste deposited as the response variable, Response surface plots were designed to create process windows for paste deposition. The important conclusions made in this research are as follows:
* The thickness of the stencil would be influenced by the other peripheral leaded SMCs present on the board.
* A solder paste volume of 0.032-0.049mm3 was found to be optimal for BGAs.
* The area ratio must be less than 1.5 to allow adequate paste deposition.
* Modification of printing parameters such as print pressure and print speed provided better control over the volume of paste deposited.
2.4 Neural Network-Based Prediction Model:
Taho Yang, et al., (2005) attempted to solve the solder paste printing quality problem with a neural network approach in this paper. A neuro-computing approach allows multiple inputs to be used to produce multiple responses. In this paper, eight factors were used to study the non-linear behaviour of stencil printing and to predict the deposited paste volume. The 3K-P fractional factorial design is conducted to collect the structural data used for neural network training and testing. The important conclusions made in this study are as follows:
* This research proposed a neural network-based quality control system for solder-paste stencil printing.
* This system could be used to predict quality problems early in the process providing a warning and allowing early remedy before an error occurs.
* The proposed system could be used as an in-line process control system.
* The neural network could include other product types like ultra fine-pitch processes, ball-grid arrays and flip-chip packages.
2.5 Hybrid Intelligence and Response Surface Methodology:
Tsung-Nan Tsai (2007) compared two hybrid intelligence approaches with RSM as methods of solving the problem of stencil printing optimization involving many process variables. A 3K-P factorial design is used for stencil printing process that contains multiple responses and quality characteristics. This research made the following important contributions to this field:
* The empirical evaluation revealed that two hybrid solutions provide satisfactory performance for stencil printing optimization problem.
* Use of larger set of experimental data to closely examine the non-linear behaviour of stencil printing process.
* A customised program was developed to enable the connections of stencil printer with AOI/AVI system via generic equipment model to get real-time operating settings and inspection data.
2.6 Area Ratio Rule for Type 4 and Type 5 Solder Paste:
The area ratio rule identifies an area ratio of 0.66 or higher to produce a stencil that will print with good solder paste deposition. Joe Belmonte, et al., (2007) in this paper discusses the experiments conducted by Speedline Technologies and Indium Corporation to determine if an area ratio of lower than 0.66 can be used when using the type 4 and type 5 solder pastes. A two level three factorial design is experimented with the factors as solder paste type, stencil thickness and stencil technology. The important conclusions made by this research are:
* This study shows that the use of type 4 and type 5 solder paste will only marginally reduce the aperture area ratio requirement of the stencil.
* An area ratio of 0.58 would give acceptable solder paste deposition by the use of type 4 and type 5 solder paste.
* The area ratio requirement can be further reduced by slightly increasing the size of the stencil aperture or decreasing the stencil thickness.
* There is a marginal advantage in using the electroformed stencil's transfer efficiency when compared to laser cut stencil's transfer efficiency.
Enclosed Printing systems:
L.C. Zou et al., (2003) conducted experiments to evaluate some of the aspects of enclosed printing system. The Enclosed print heads have been recently developed as an improvement over the squeegee system as they provide a bigger printing process window and reduced process defects.
2.1 Solder Paste:
Solder paste is one of the most important materials in surface mount technology which connects the electronic components to the PCBs using the reflow soldering process. It is a dense suspension of solder powder particles in a flux binder. It provides electrical and thermal conductivity between the component and the substrate. The factors which influence the solder paste selection include size and shape of the solder powder, metallurgical composition and rheological property of the paste. The rheological property of the solder paste is a very important variable to predict the printing behaviour of the solder paste (Daniel J.Braunstein 1994). The factors affecting the rheology of solder paste include particle size and distribution, storage, handling and temperature.
Due to the environmental concerns, the European Commission prepared the RoHS Directive (Restriction of use of Hazardous Substances) and WEE (Waste from Electrical and Electronic Equipment) which led to the introduction of lead-free solder paste (A.E. Marks et al., 2008). The biggest challenge to lead-free solder paste is the availability of surface mount components which can withstand the high melting temperatures.
It is a very important factor affecting the solder paste deposition on the substrate. The print volume and consistency could be improved with the proper stencil selection (Joe Belmonte, et al., 2007). The following factors needs to be considered when designing the stencil for optimum solder paste deposition.
It is defined as the ratio between the area of the stencil aperture and the area of the aperture walls. It is a primary factor in determining how well any stencil can print the solder paste (Joe Belmonte et al., 2007). The transfer efficiency of the solder paste increases with an increase in the area ratio. An area ratio of 0.66 or higher is desired to give acceptable transfer rates of the solder paste as confirmed by (Jianbiao Pan et al., 1999; Joe Belmonte et al., 2007).
Area ratio for circular aperture = R/2T
Area ratio for rectangular aperture = LW/ [2(L+W) T]
Where R = Aperture Radius, L = Aperture Length, W = Aperture Width and T = Stencil Thickness.
It is defined as the ratio between the width of the aperture and the thickness of the stencil. The generally accepted aspect ratio for the desired transfer ratio of solder paste is 1.5 or higher. However, for electroformed stencils, which would be used in this study, aspect ratio of 1.1 or more is acceptable (Cookson 2007). Aspect ratio depends much on the thickness of the stencil.
Aspect Ratio = W/T
Where W = Aperture Width and T = Stencil Thickness.
Aperture Shape: The shape of the aperture also affects the amount of solder deposition. Apertures are available in various shapes like square, rectangle, circle, oblong, home plate, diamond and D-shape depending on the requirement of solder joints. It has been observed that the square apertures give more solder paste volume than any other shape of aperture (Benlih Huang & Ning-Cheng Lee; Cookson2007). The height, area and volume of the solder paste deposited should be ideally equal to the aperture shape.
Stencil thickness is an important part of the stencil design and is considered to be one of the primary factors affecting the transfer of solder paste onto the PCB. It shows a non-linear behaviour to the solder paste deposition, i.e., an increase in stencil thickness.
3.1 Experimental Equipment and Setup:
This section provides the equipment and setup to be used during this study.
1. Stencil Printer: The DEK 260 series semi-automatic screen printer will be used to print the solder paste onto the substrate. This machine has a high accuracy of +/-25 microns and repeatability of +/-10 microns. It can allot a maximum board size of 500x450mm with a printing area of 440x430mm (SMT Ltd. 2010). It consists of a squeegee,
2. Stencil: An electro-formed nickel plated stencil will be used to deposit the solder paste onto the Printed Circuit Board. This stencil consists of both rectangular and circular apertures of different sizes.
3. Solder Paste: Lead-free solder paste of type 6 will be used in this experiment.
3.2 Design of Experiments (DOE) using Taguchi Approach:
Design of Experiments is a statistical technique used to study the effects of multiple variables simultaneously. This study helps us study many factors simultaneously and economically (R.K. Roy 2001). Taguchi's approach is a new experimental strategy in which he utilized a modified and standardised form of DOE.
The first step in this task is the experiment planning. In this step, the factors (and their levels) and the responses for the experiments are identified. The Taguchi approach of DOE would be used in this project to study the effect of the four factors identified as the area ratio, aspect ratio, print speed and aperture size. The other process parameters which would be kept constant include stencil thickness, solder paste type, squeegee pressure and test board. These four factors would be experimented at two levels each giving a 2-level 4-factor full factorial design.
Orthogonal Arrays would be created to derive the full factorial design of 2-level four-factor which gives us a total of 16(24=16) experiments.
Justification of the Project:
Solder-paste stencil printing is considered to be the most important stage in Surface Mount Technology, as 50-70% of the solder defects are still attributed to stencil printing. This process shows a non-linear behaviour which makes it more difficult to understand the factors affecting the performance. The introduction of lead-free solder paste has also brought in new challenges, as there is not much research done with the lead-free solder paste in the stencil printing process. Most of the literature studied derives an area ratio of 0.6 or higher and aspect ratio of 1.5 or higher for acceptable printing. This project would look deeply into the area ratio and aspect ratio requirements for acceptable solder paste deposition, and also if there is any interaction between these two factors.
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