FPCWAY

Application of magnetic pulse welding technique

for flexible printed circuit boards (FPCB) lap joints_1_ Principle of MPW

Modern electronic devices are required to be thin, lightweight and  functionally sophisticated. Therefore, joining thin flexible printed circuit board (FPCB) in different shapes is receiving attention. FPCB and flexible cables constructed from polyester or polyimide film and such films are lightweight, flexible and thin. Varying circuit shapes, dimensions, circuit arrangements and different length cable arrangements may be constructed using FPCB. However, a disadvantage to the use of FPCB is the bonding problems. Yoonet al.(2007) studied bonding characteristics of FPCB using solder method and reported high electrical and mechanical properties, while the excessive growth of intermetallic compounds at the joint interface significantly degrades the performance and reliability of the solder joint. Maruoet al.(2004)investigated adhesive-bonding methods using anisotropic conductive adhesive or non-conductive adhesive for FPCB bonding but experimental results show poor electrical property and low mechanical reliability in joint interfaces. Unfortunately, the conventional boding meth- ods for FPCB do not appear to provide an economical and reliable solution to interconnection and construction challenges. The mag- netic pulse welding (MPW) provides an excellent and high speed method for achieving FPCB lap-joint. MPW uses magnetic pres- sure to drive the primary metal against the target metal sweeping away surface contaminants while forcing intimate metal-to-metal contact, thereby producing a solid-state weld. Several technical

research papers has been reported about MPW for exampleTamaki andKojima (1988) and Shribman et al. (2002)used conventional MPW method with solenoidal coil for joining tubular parts and investigated its feature. MPW has been theorized and tested for several decades, but equipment limits the total energy stored and this keeps weld lengths to the order of meters or less. Recently, MPW application rapidly growing in industrial application and new development make MPW method well suited for manufacture and assembly in wide range of application. Shribman andGafri (2001) introduced  MPW technique  for tube to tube applications. They studied the fundamental equations ofMPW process and illustrated some examples of similar and dissimilar weld applications with some interface microstructures. Uhlmann et al. (2005) also studied the applicability and the potentials ofMPW for joining of aluminum and magnesium  structure which is a new solutions for modern lightweight structures applications. Daehn and Lippold (2009) also proposed a new MPW device and developed it for similar or dissimi lar thin sheet metal joints application. Recently, several works also were carried out on application of MPW technique in electronics micro-devices. For example, Kashani et al. (2008, 2009) developed a new low energy MPW system which can be used for bonding of wire to terminal plate in electronic devices or making small Copper and Manganin alloys joint as a shunt resistor for using at control circuits. The goal of this work was to introduce new low energy system with modified coil structure for using MPW technique in FPCB lap-joints application which has not been reported before. The present paper examines the detail of the welding process and welds quality characteristics for FPCB lap-joints in two cases: (1) with aluminum driver sheet and (2) without aluminum driver sheet.

2.  Principle of MPW

Magnetic pulse welding (MPW) uses electromagnetic force to accelerate one metal piece (base metal) against another stationary metal piece (target sheet). When a high magnetic field  is suddenly

generated and penetrated into metal sheets, then the eddy currents (current density ) pass through them and as a result, an electro magnetic force of  acts mainly on the base metal sheet and

it is accelerated away from the coil and collides rapidly with the target metal sheet. The eddy current  and the magnetic pressure p are given as following:

Where k, μ, τ, Bo and Bi are the electrical conductivity, magnetic permeability, thickness, the magnetic flux density at lower and upper surfaces of Al sheet, respectively. The depth of skin effect (ı) can be obtained by calculation of angular frequency (ω ) and it is governed by the complete MPW system’s inductivity L and its capacity C. The skin depth becomes important parameter specially for thin sheet metal bonding process. When the thickness of the base sheet metal is the same as the skin depth, then magnetic pres- sure equals 86% of its maximum value and it reaches 98% when the base metal thickness is twice of the skin depth. The appropriate skin depth and higher magnetic pressure can be adjusted by the frequency of the discharge current.

At the moment of collision the colliding surfaces can be cleaned by a large kinetic energy getting before the collision. The velocities attained during this process range from 200 m/s to 500 m/s and the joining  process  completed  within  microsecond.  Because  of the short impact period, the extent of heating might be minimal along the joints. Therefore,  comparing to the traditional  fusion welding process, no significantheat affected zones is produced in MPW joints and it can be noticed as a main advantage (Aizawa and Yoshizawa, 2001;Aizawa and Kashani, 2004).

Kakizaki et al. (2010) found that the surface oxide of the metal interfaces is disrupted due to the jet action and metallurgical bond- ing is achieved between clean surfaces. As shown in Fig.  1, the principle of MPW technique can be summarized into three steps: (1) producing high magnetic field, (2) acceleration of base metal, and (3) impaction and bonding.

In the present experiment which has been carried out for FPBC lap-joints application, the base and target metals are thin and light and the skin depth is comparable with thickness of the FPBC copper layer. Therefore, the impactionis weaker in comparing with thick metal welding case. In this case, using Al driver sheet can increase the electromagnetic force and magnetic pressure. The impaction of Aluminum driver sheet with FPCB layers can make a solid state bonding between FPCB sheets. Fig. 2 shows the principle of welding process with using the Al driver sheet. The result of second experiment without using Al driver sheet also is reported in this paper for better comparison.