Lead-free selective soldering can result in extended times at high temperatures,which in turn can result in excessive dissolution of exposed copper,such as plated through holes. This phenomenon is more severe with lead-free,since the alloys have higher melting points,hence requiring longer times for the PTH to reach the higher temperatures,and the alloys typically have a greater capacity to dissolve copper.
This paper discusses a test method that characterises the dissolution rate of copper from PWBs. A PWB design was created
that allowed the time for the dissolution of a copper pad to be measured. With this quantitative method a soldering process
or an alloy can be characterised in terms of dissolution rates at specific conditions of temperature and flow rate. This methodology provided repeatable measurements that allowed the various experimental parameters to be isolated. Particular attention was paid at the flow rate of the molten solder. In fact,different alloys at the same temperature can have considerably different flow rates,due to the different viscosity at that temperature. The performances of seven lead-free alloys and a typical 60/40 Sn-Pb alloy were compared at three temperatures.
NPL worked with a number of partners using different alloys and copper types to measure the relative rate of copper dissolution. This work shows that some of the current alloy developments now offer superior performance to SnPb at the same temperature. Interestingly intermetallic formation between the alloy systems varies considerably. The copper type on the PWB is also influential,with significant differences between electroplated,electrodeposited and reverse treated.

Implementation of lead-free assembly processes results in higher copper dissolution rate than traditional SnPb alloys. The rate at which copper is dissolved could be dependent on many factors,such as solder alloy,contact time,solder temperature,flow rate,copper plating processes,copper grain structure,etc. The effect of assembly processes and process parameters on copper dissolution is well known and published. However,there is limited study on the influence of PCB (printed circuit board) fabrication processes and chemistries on copper dissolution.
This paper focuses on the effect of copper plating processes and chemistries on copper dissolution. Different Cu plating methods and chemistries were studied and compared. The paper will discuss the impact of Cu plating processes (direct current plating vs. pulse reverse plating),current density,plating chemistries and rectifiers on dissolution rate. The Cu microstructure from different plating methods is discussed.

The process of plating through holes (PTH) is inherent to modern PCB manufacturing. In an arena of increasing circuit density and layer counts,the reliability of the PTH process is under constant microscopic examination. The aim of electroless copper is to plate a conductive layer through a hole or into a blind microvia. In this context an interconnect (IC) refers to the copper to copper adhesion within the functional constraints of a circuit board. As these can include many inner layers,the inter connection quality is of prime concern. In addition the PTH process also inherits the issues from the preceding manufacture.
Some examples are highlighted below:
Base material
Material properties can result in significant Z axis mobility. Hybrid builds (different materials in one build) can be exceptionally
vulnerable and the problem is amplified in thicker panels.
Drilling
Drilling practices can affect smear residues and inner layer damage dramatically. Later processes can minimize the impact of
these but only if the manufacturer is aware of it. Unless the source of a type 1 inter-connection defect is clear,it is usually attributed to the PTH process employed.
Using electroless copper as an example,the impact of PTH on IC quality will be discussed. For this purpose it is useful to divide the PTH process into the following subsets:
- Desmear
- Cleaning/Conditioning
- Activation
- Electroless copper
This paper presents the impact of the individual PTH steps!

Fully automatic X-ray Inspection (AXI) is an established technique for inspecting component mounting and solder joint integrity on PCBA’s. It is occasionally used for the detection of faults on (or more precisely,under) press fit connectors. However,existing commercial automatic X-ray inspection systems limit the application of this technique to sample boards smaller than 610mm (24”) x 508mm (20”) and under 5mm (0.2”) thick. This is far too small an envelope to test most backplane assemblies and other large PCBA’s. These systems also use higher X-ray energies than is required to find all the types of assembly faults found on backplanes thereby unnecessarily increasing the weight and cost of the safety enclosures.
Existing manual systems can handle the largest of backplanes,but expecting a human operator to consistently recognise minor defects in the several hundred images required for a single large backplane (never mind production volumes) is unrealistic.
The authors describe a novel concept for a test system where a low energy micro focus X-ray source,and a 4 mega pixel detector,are mounted in separate robotically controlled heads. Each head is also fitted with a high resolution colour machine vision camera. The resulting RXI (Robotic X-ray Inspection) system provides both high resolution AXI for detecting faults under connectors,and full colour high resolution optical AOI for detecting faults within connectors,in the same machine,and in a single test activity. This fully automatic test system is a reliable,high speed,and highly cost effective test system for backplanes and other large PCB assemblies up to 1000mm (39.4”) x 1600mm (63”) in size and over 18mm (0.7”) thick.

Conformal coatings are used in high reliability electronics to protect the circuits from environmental contaminants. They are
applied by a variety of methods,and in varying thicknesses. Confirming that the thickness meets specifications called out by
documentation or customer can be problematic. Mechanical,ultrasonic,electrical (capacitive,eddy current),and various optical techniques are available,but all involve incurring significant limitations/penalties in capability,capacity or cost.
For optically transparent,and some translucent coatings,it is possible to accurately measure the thickness using optical (focal) techniques. This paper presents data on an innovative coating measurement process based on commercially-available low-cost optical equipment modified to make the measurements. The modified equipment is capable of making measurements on films as thin as 25µm (0.001”) and thicker than 1000µm (0.040”) with high repeatability. The method does not require a free edge and is not dependent on before/after coating differential measurement. The process has been fully developed and is used in a production environment.
The paper presents an overview of the equipment and method,Gage R&R data for the process,as well as comparative
information on other available techniques. The focal technique is applicable to measurement of all types of optically clear coatings and films,and is appropriate for moderate-volume measurement applications where direct,non-contact measurement of coated parts is desirable and where measurement in small areas is required.

During lead-free wave soldering or rework operation for through hole components,high rate copper dissolution may occur to printed wiring boards. It is widely believed that Sn-Ag-Cu (SAC) lead-free alloys,the currently most popular alloys in the electronic industry,have more than twice the rate of copper dissolution compared to the Sn-Pb eutectic (63Sn-37Pb) solder alloy. In this study,copper dissolution was evaluated with four lead-free alloys (SAC305,K100LD,SC995e,SN100C) at temperatures 245,260,280 and 300oC with time duration of 20,60,300 and 600 seconds. Results show that K100LD and SN100C have the lowest rate of copper dissolution. A unique dynamic copper dissolution testing was also performed to investigate effects of liquid solder dynamic flow speed,time and temperature on dissolution kinetics. These dynamic tests involved three different sample motion speeds of 2,5 and 15 ft/min at temperatures of 245,260 and 280oC with time duration of 20,60 and 300 seconds. Our unique test setup clearly demonstrated that alloy selection and process window definition are critical for lead-free soldering for through hole component assembly and repair operation.

Hot gas rework of BGAs with a mirrored BGA design configuration using SnAgCu based lead-free alloys is more challenging as compared to conventional SnPb techniques. Rework of BGAs using a conventional SnPb alloy system has historically required that mirrored BGA solder joints remain below the eutectic melt temperature of 183?C to avoid secondary (or partial) reflow of these mirrored solder joints. This requirement was traditionally established to maximize second level solder joint reliability performance of mirrored BGA devices. However,with the migration to SnAgCu based alloys,the approach of ensuring that mirrored BGA device solder joints also remain below the SnAgCu melting point (217?C) during hot gas rework operations presents a more difficult challenge. Increased conductive heat transfer rates through the printed circuit board (PCB) along with increased thermal exposures to adjacent surface mount components are impacts of elevated processing temperatures associated with the use of lead-free solders. As a result,secondary reflow of mirrored BGA solder joints is sometimes unavoidable – especially for thin PCB cross sections,ranging nominally from 0.050” to 0.062” (1.2 to 1.6mm).
The intent of this paper is to recommend changes in assembly materials and the process itself during hot gas rework of lead-free BGAs with a mirrored BGA configuration. The metallurgical analysis of final solder joint structures and the reliability performance of fully reflowed mirrored BGA devices will be reported. An eight month development effort indicates that mirrored SnAgCu BGA solder joints should be allowed to fully reflow when it is not possible to prevent mirrored solder joints from reaching onset melting (pasty range) temperatures. Thermo-mechanical solder joint reliability has shown improvement when these joints are processed above the alloy pasty range; when all attempts to remain below this range have been exhausted.

An experimental study was conducted to examine the impact of rework processes on quality and reliability. In this study,676 IO plastic ball grid array packages were assembled with Sn3.0Ag0.5Cu solder paste and eutectic SnPb solder paste. Selected parts on circuit boards were subjected to rework processes one time,three times and five times. X-ray inspection and environmental scanning electron microscope were used to investigate impact of part replacement on the ball grid array voids,the microstructure of intermetallic compound,and copper pads. Since the rework process includes multiple liquid solder state periods,it consumes more copper and makes the intermetallic compound growth trend an interesting topic. Copper pad dissolution was found in the samples after multiple rework processes. Lead-free assemblies consumed more copper than mixed assemblies because of higher concentration of Sn in lead-free solder. The thickness of intermetallic layer increased as the total rework time increased. Ultra thick intermetallic compound was found at the connection area between the copper pad and the copper trace after the rework processes were applied three times and five times,which may lead to reliability concerns.

The New England Lead Free Consortium,composed of many companies in the electronic supply chain in the regional area and chaired by the author; has embarked on an extensive long term reliability study of lead-free and halogen free electronic assemblies. Specialized PCB’s were built,assembled and reworked at the consortium member companies using multiple types of laminates,PCB surface finishes and various component types including through-hole and surface mount technology. The assemblies were examined for visual characteristics and subsequently tested for reliability using temperature cycling as well as vibration testing. All rework,reliability tests,and evaluations have used or will be using industry standards,methods and techniques for easy reference to other long term reliability studies. The studies will include comparison to a baseline of leaded electronic assemblies. This paper will outline results obtained so far into the long term reliability study.

The reliability of lead-free (LF) solder joints in surface-mounted device components (SMD) has been investigated after thermo-cycle testing. Kirkendall voids have been observed at the interface component/solder together with the formation of fractures. The evolution,the morphology and the elemental analysis of the intermetallic layer have been evaluated before and after the thermal treatment. Voids produced by the release of volatile species during the soldering process due to the application of flux were present. If compared with SnPb soldered systems,lead-free joints are characterized by larger and a higher amount of voids. In several electronic joints (ball grid arrays (BGA),surface-mounted device components (SMD),etc.) fractures developed after the thermal stresses generated during the accelerated thermal aging. Warpage of the PCB has also been observed. Backward and forward compatibility of SnPb and lead-free BGA connections has been performed on pads with an ENIG finish. The effect of the reflow peak temperature on the structure of the intermetallic layer has been assessed.