Retained ratio of reinforcement in SAC305 composite solder joints: Effect of
reinforcement type, processing and reflow cycle
Guang Chen
1, 2
, Li Liu
2
, Vadim V. Silberschmidt
2
, Y.C. Chan
3
, Changqing Liu
2*
,
Fengshun Wu
1*
1. — State Key Laboratory of Materials Processing and Die & Mould
Technology, Huazhong University of Science and Technology, Wuhan 430074,
China.
2. — Wolfson School of Mechanical and Manufacturing Engineering,
Loughborough University, LE11 3TU, UK.
3. — Department of Electronic Engineering, City University of Hong Kong, Tat
Chee Avenue, KowLoon Tong, Hong Kong.
Contact details
Guang Chen : G.CHEN2@lboro.ac.uk
LiLiu: L.Liu2@lboro.ac.uk
Vadim V. Silberschmidt:V.Silberschmidt@lboro.ac.uk
Y.C. Chan:EEYCCHAN@cityu.edu.hk
C.Q. Liu: C.Liu@lboro.ac.uk
Fengshun Wu: fengshunwu@hust.edu.cn
1
Abstract 1
Purpose – The effect of reinforcement type, processing methods and reflow cycle on 2
actual retained ratio of foreign reinforcement added in solder joints was 3
systematically studied. 4
Design/methodology/approach – Two kinds of composite solders based on SAC305 5
(wt.%) alloys with reinforcements of 1 wt.% Ni and 1 wt.% TiC nano-particles were 6
produced using powder metallurgy and mechanical blending method. The 7
morphology of prepared composite solder powder and solder pastes were examined; 8
retained ratios of reinforcement (RRoR) added in solder joints after different reflow 9
cycles were analysed quantitatively using an Inductively Coupled Plasma optical 10
system (ICP-OES Varian-720). The existence forms of reinforcement added in solder 11
alloys during different processing stages were studied using SEM, XRD and EDS. 12
Findings –The obtained experimental results indicated that the RROR in composite 13
solder joints decreased with the increase in the number of reflow cycles but a loss 14
ratio diminished gradually. It was also found that the RRORwhich could react with 15
the solder alloy were higher than that of the one that are unable to react with the 16
solder. In addition, compared with mechanical blending, the RRORs in the composite 17
solders prepared using power metallurgy were relatively pronounced. 18
Originality/Value –Present study offer a preliminary understanding on actual content 19
and existence form of reinforcement added in a reflowed solder joint, which would 20
also provide practical implications for choosing reinforcement and adjusting 21
processing parameters in the manufacture of composite solders. 22
2
Key words: Electronic materials; Composite materials; Solder; Retained ratio; 23
Reflow cycles 24
1. Introduction 25
Lead-containing solders have been continuously replaced in electronics packing 26
because of the environmental and health concerns; thus, lead-free solders 27
demonstrated a rapid development (Abtew and Selvaduray, 2000; Zhang et al., 2012; 28
Shen and Chan, 2009). To further enhance the performance of lead-free solder joints 29
in harsh service conditions，incorporation of reinforcements into a solder matrix is 30
widely regarded as a feasible method (Chellvarajoo, 2015; Fouda and Eid, 2015; 31
El-Daly et al., 2013; Hu et al., 2013; Bukat et al., 2013; Gao et al., 2010). 32
At present, there are two common methods to prepare composite solders with 33
added reinforcements: mechanical blending and powder metallurgy (Shen and Chan, 34
2009; Liu et al., 2013; Tsao et al., 2012). In the former, a solder paste and 35
reinforcement are directly mixed together through mechanical stirring. In the latter, a 36
solder powder and reinforcement are blended by ball milling before compacting, 37
sintering and subsequent extrusion or rolling. However, no matter what method is 38
used, most of the reinforcement added was excluded outside of solder joints in the 39
soldering process (Liu et al., 2008; Chen et al., 2015). In such a case, the amount of 40
reinforcement retained in the final state of solder joints is quite different from the 41
initial one, leading to reduction of an enhancing effect due to limited doping with 42
reinforcement. To date, although the effect of foreign reinforcement on microstructure 43
3
and performance of lead-free solders was widely studied, a retained ratio of 44
reinforcement in composite solder joints was only mentioned in few works (Chen et 45
al., 2016; Haseeb et al., 2014; Tay et al., 2013; Haseeb et al., 2011). It is expected that 46
a type of reinforcements, a number of reflow cycles and a method of processing of 47
composite solders have important impacts on RRoR in solder joints. 48
In this paper, Ni and TiC nanoparticles were chosen as reinforcements to 49
strengthen a SAC matrix since Ni is known as an active reinforcement that could react 50
with molten SAC solder, while TiC is a relatively inert reinforcement (Chellvarajoo, 51
2015; Tay et al., 2013; Kennedy et al., 2001). To understand the effect of processing 52
and reflow cycles on RRoRs in solder joints, mechanical blending and powder 53
metallurgic routes were adopted to produce composite solders while the number of 54
reflow cycles was controlled when preparing solder joints. In addition, the 55
microstructures and chemical compositions of prepared composite solders at different 56
processing stages were contrastively investigated. 57
2. Experimental procedures 58
The SAC305 (wt.%) solder paste (Beijing Compo, China) and powders (Suzhou 59
EUNOW Electronic Materials, China) were used as matrix materials, while the 60
as-purchased nano-sized Ni (with an average diameter of 20 nm, JCNANO) and TiC 61
(with an average diameter of 25 nm, JCNANO) were employed as reinforcement 62
materials. 63
The initial weight fraction of both reinforcements was chosen as 1 wt. %. In this 64
4
paper, mechanical blending method (Method A) and a powder-metallurgy method 65
(Method B) were utilised to prepare composite solders. Specifically, in Method A, the 66
pre-weighed solder paste and the reinforcements were first mechanically blended 67
prior to printing onto an aluminium oxide chip using a steel stencil and further 68
soldering into solder balls in a reflow oven (see Fig 1a). In Method B, a mixture of a 69
solder powder and reinforcements was first ball-milled for 20 hours before uniaxial 70
compacting into solder billets and sintering at 180℃ for 3 hours under vacuum 71
atmosphere. Subsequently, the sintered solder billets were rolled into solder foils (200 72
μm in thickness) and then cut into solder flakes with dimension of 1 mm×1 mm×0.2 73
mm using a rotary cutter; solder balls with an average diameter of 750 μm were 74
prepared through the reflow process (see Fig. 1a). To ensure the stability of reflow 75
process, same reflow parameters were adopted for both of method A and method B; 76
the reflow curve is shown in Fig 1b. According to the type of reinforcement added and 77
the processing method, these prepared composite solder balls are denoted as follows: 78
SAC/Ni-A, SAC/Ni-B, SAC/TiC-A and SAC/TiC-B. 79
To study the characteristics of treated composite solder (including solder paste 80
and powder) before reflow process, the morphology of and the distribution of 81
reinforcements in composite solders were observed using an environmental scanning 82
electron microscope (ESEM Quanta 200). To measure the extent of RRoRs in 83
composite solder foils and pastes before sintering and reflow, 50 mg mixture for each 84
solder were ultrasonically dissolved in aqua regia; the resultant solutions were tested 85
using an ICP-OES Varian-720 with test precision at a ppm level. The RRORsin 86
