Tampilan:1 创始人: Site Editor Publish Time: 2026-03-09 Origin: Site
In the reliability research of
plastic-encapsulated integrated circuits (ICs), the problem of ion
contamination caused by water vapor erosion has been widely concerned, while
the failure cases caused by solder contamination of printed circuit boards (PCBs)
are relatively rare. This paper delves into the influence mechanism of solder
contamination on the reliability of plastic encapsulation ICs based on the
actual failure cases of new energy vehicle charging pile control boards.
Through elemental composition analysis, failure phenomenon observation and
aging test, the process conditions and failure characteristics of solder
contamination are clarified. It is found that the wave soldering process can
easily lead to solder penetration into the IC plastic encapsulation, forming a
contamination layer with a depth of not less than 6μm, which can cause abnormal
failure of IC electrical parameters under specific conditions. High-temperature
baking can temporarily restore the failed device, but the contamination itself
is irreversible and there is a risk of failure again. Based on the research
results, targeted process recommendations are made for surface mount technology
(SMT) engineers to reduce IC reliability issues caused by solder contamination.
0 Introduction
In the field of industrial control,
especially in complex electronic systems such as new energy vehicle charging
piles, multilayer PCBs often need to be soldered on both sides and integrate
surface-mount ICs and plug-in devices to complete control logic and power drive
functions. At present, the research on IC soldering quality and solder joint
reliability has been relatively mature [1-4], but there is a lack of research
on the reliability failure caused by solder contamination of plastic envelopes
after IC soldering.
A batch of new energy vehicle charging pile
control boards showed abnormalities in reliability tests: the front IC using
reflow soldering did not fail, while the back IC using wave soldering had a
failure rate of about 5%. This significant difference indicates an
under-recognized association between soldering processes and IC reliability.
This paper focuses on the impact of PCB solder contamination on IC reliability,
analyzes the probability of solder contaminated IC under different soldering
methods, and explores the manifestations of IC electrical parameter failure and
the law of failure and recovery after contamination, so as to provide a
theoretical and practical basis for solving similar engineering problems.
1 Soldering methods and characteristics of
PCB and IC
There are two main methods of soldering PCB
surface mount ICs: reflow soldering and wave soldering, which have significant
differences in solder characteristics, process and impact on ICs.
1.1 Reflow soldering process
Reflow soldering is the mainstream process
of surface mount IC soldering, and its core process is to melt the pasta solderpreset on the PCB pad through infrared heating to realize the connection
between the IC and the PCB. The solder used in this process is lead-free
solder, the main component is tin, containing a small amount of silver, copper
and other metals, and the soldering temperature range is 217~260°C.
The notable feature of reflow soldering is the precise and controllable solder distribution: the solder paste is pre-printed on the PCB pad, and after melting, only the solder joint is formed on the bottom and sides of the IC pin, and the solder climb height is strictly limited, and it does not touch the front of the IC pin and the forming bend (as shown in Figure 1). At the same time, there is a height difference of 60~100μm between the bottom of the IC plastic body and the surface of the PCB, and there is no solder under the plastic encapsulation, which spatially blocks the possibility of contact between the solder and the plastic encapsulation, so the probability of solder contaminating the plastic encapsulation body is extremely low.
1.2 Wave soldering process
In order to improve efficiency and reduce
costs, some manufacturers use wave soldering to solder PCB surface mount ICs.
The solder used in wave soldering is also lead-free solder, with the same
composition as reflow soldering, but the soldering temperature is higher,
ranging from 255~265°C.
The wave soldering process is unique: when
the PCB passes through a solder pot containing molten solder, the pump in the
pot creates a standing wave-like solder uprush, and when the PCB comes into
contact with the wave crest, the components are soldered to the PCB. During
this process, the solder wave peak soaks the entire encapsulation of the IC
being soldered, resulting in a significantly increased probability of solder
contamination (as shown in Figure 2). As can be seen from Figure 2, the wave
soldered IC pins are completely covered by solder, and the exposed points left
by the copper frame ribs on the side of the molded body are also wrapped in
solder, and the original copper color changes to the silver color of the
solder, which fully reflects the extensive contact between the solder and the
IC plastic body in wave soldering.
1.3 Comparison of soldering schemes for
multilayer PCBs
For multilayer PCBs that require
double-sided soldering and involve surface mount and plug-in ICs, there are two
main soldering options:
Option 1: Reflow soldering completes PCB
front surface mount IC soldering, reflow again to complete PCB back surface
mount IC soldering, and finally complete PCB back plug-in IC soldering through
wave soldering. This scheme requires two reflow soldering and one wave
soldering.
Scheme 2: Reflow soldering completes the
PCB front-mount IC soldering, and wave soldering completes the PCB backside
surface-mount IC and plug-in IC soldering at the same time. This solution only
requires one reflow soldering and one wave soldering, which can reduce
operating costs and improve production efficiency [6].
The core difference between the two options
is the soldering process used in the backside surface-mount ICs, while wave
soldering has cost and efficiency advantages but a higher risk of solder
contamination.
2 Failure phenomenon and localization
analysis of solder contamination
Through comparative tests and physical and
chemical analysis, the IC failure characteristics caused by solder
contamination can be clearly identified, and the source of contamination can be
accurately located.
2.1 Observation of failure phenomenon
The same IC is selected to be soldered on
the front and back of the PCB using reflow soldering and wave soldering,
respectively, to conduct high-temperature live reliability aging tests (ambient
temperature 50°C, Vin=9V, Iout=15mA). The results showed that the front-facing
reflow IC did not fail; The failure rate of the backside wave soldering IC is
about 5%, and the failure manifestation is:
VIN to ground short;
The Vout outputs -0.5V at 10mA, well below
the normal output range (3.23-3.37V);
quiescent current (Iq) exceeded the
standard, exceeding 5μA;
The I-V curves of the failed products such
as VIN-VSS and VOUT-VSS are significantly different from those of normal
products.
Dehumidifying the failed IC (12h, 0.13Pa,
40°C vacuum box) found that the IC was still failing, indicating that the
failure was not directly caused by moisture. Further verification showed that
longitudinal dissection of the failed IC and qualified product did not show any
delamination between the chip surface and the plastic encapsulation body (as
shown in Figure 3). The humidification experiment of 85°C, relative humidity of
85%, and 12h was carried out on the IC that returned to normal baking, and the
product performed normally without failure. These results rule out the
possibility of IC aging failure due to moisture erosion, and combined with the
process background of the failed part, it is speculated that the failure is
related to solder contamination.
2.2 Elemental composition analysis
In order to clarify the cause of failure,
EDX analysis of the elemental composition of the IC plastic encapsulation body
of the wave soldering failure product, wave soldering normal product, and
reflow soldering normal product was performed (acceleration voltage 30keV,
detection depth of about 6μm). The results showed that no tin (Sn) was detected
on the surface of the plastic encapsulation body of the reflow soldering IC.
The SN mass fraction of wave soldering failures is significantly higher than
that of wave soldering normal products (as shown in Table 1).
The analysis shows that there is a strong
correlation between the reliability failure and baking recovery of ICs after
wave soldering and the high content of Sn elements in the plastic
encapsulation, indicating that the plastic encapsulation body of the failed IC
is contaminated by solder due to wave soldering, and the contamination depth is
at least 6μm. At present, there are few studies on the effect of solder metal
element contamination on IC packages on performance, and the relevant
literature mostly focuses on flux residue during reflow and wave soldering [8],
and there is no systematic study on solder metal element contamination.
3 Failure mechanism and verification test
Combined with the internal structure and
contamination characteristics of ICs, the mechanism of IC failure caused by
solder contamination can be elucidated, and the reversibility and repeatability
of failure can be verified through experiments.
3.1 Failure mechanism analysis
The internal circuit structure of the
failed IC shows that the IC chip is soldered to the Vin pin, and the chip
substrate is directly connected to the Vin pin (as shown in Figures 4 and 5).
After solder contamination, a large number of metal elements such as Sn are
infiltrated into the IC plastic encapsulation, and during the aging process of
IC live reliability, these metal elements move directionally and gradually
accumulate under the action of electric field. When the metal ion concentration
reaches a critical value, a conductive path is formed that connects the Vin pin
to the GND pin, resulting in a short circuit of the Vin pin to the ground, with
a short-circuit current of up to 4.24mA.
3.2 Failure recovery and re-failure test
The failed IC was baked at 150°C and its
electrical parameters were observed
After 12 hours of baking, the locally
gathered high-density Sn and other metal ions were redistributed evenly, the
electrical characteristics of the IC improved, and the short-circuit current
dropped to 1.18mA (Fig. 6b).
After 24 hours of continuous baking, the
IC's electrical parameters returned to the normal range, and the short-circuit
current was 0.00mA (Fig. 6c).
High-temperature baking can only
redistribute metal elements such as Sn in the molding body, but cannot remove
them. The IC failed again after 24 hours, showing that the IQ exceeded the
standard: the IQ of the three samples was 3.273, 4.269, and 3.763μA,
respectively, and rose to 15.011, 5.499, and 8.206μA after aging, all of which
exceeded the normal range of 5μA. The test confirms that the failure of IC
electrical parameters caused by solder contamination is reversible and
repeatable, and the contamination itself cannot be recovered, and there are
always potential reliability risks in ICs.
4 Conclusions and Recommendations
Studies have shown that in order to improve
production efficiency, some SMT manufacturers use wave soldering equipment to
process surface mount ICs and plug-in ICs on one side of the multilayer circuit
board at the same time, which has cost advantages, but wave soldering has a
significant risk of solder contamination for surface mount molded ICs. The
contaminated IC plastic body cannot remove contaminants, which is irreversible
and permanent pollution, and the contaminated IC will have electrical parameter
failure under certain conditions, and the failure is reversible and repeatable,
and there is a continuous reliability hazard.
Based on the above research, it is
recommended to use the standard reflow soldering process for sensitive
surface-mount plastic ICs and avoid the wave soldering process to reduce the
possibility of solder contaminating the IC molded body, thereby ensuring the
reliability of electronic devices.
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