Implementing a high-yield alternative assembly and reflow technology process

July 2nd, 2014, Published in Articles: EngineerIT, Uncategorised articles


In a White Paper entitled “Comprehensive guidelines for the implementation of the AART process”  the authors* examine the entire alternative assembly and reflow technology (AART) process. It is designed to provide a process engineer with some of the knowledge necessary to implement a high-yield AART process. The AART principles illustrated in this paper can be designed into a product at the product design phase, or used as a “drop-in” solution to an existing PCB assembly.

The manufacturing process steps used to assemble a printed circuit board (PCB) depend primarily on the specific components used in the assembly. As more emphasis is put on smaller size, increased functionality, and increased component density, many single and double sided boards contain primarily surface mount components (SMCs).   However, due to the inherent strength, reliability, and availability, through hole devices are still chosen over SMCs for certain applications, especially for edge connectors.  Coupled with the fact that current automatic placement equipment can place through hole/odd form devices, a strong case can be made for these components.

The drawback of choosing a through hole device on a largely surface mount board is the high cost-per-joint of additional processing steps such as wave soldering, hand soldering, or other selective soldering methods. For these assemblies, it is pivotal to provide a robust process for simultaneously reflow soldering both through hole and surface mount components in a single comprehensive process.

The AART process allows for the concurrent reflow of both SMCs and through hole devices in a single step. The cost savings involved in reducing additional process steps and material is only one benefit among many. The manufacturing process steps involved depend on the specific components used in the assembly. The key material, design, and process related factors relating to the AART process are identified and discussed in this paper. This paper is based on extensive research into the AART process conducted at the surface mount technology (SMT) laboratory at Universal Instruments Corporation. These research efforts encompassed the topics that related to the materials, design, processes, modeling, and reliability aspects of the AART process. This information on the AART process has been utilized to design and develop intelligent agents that can assist with solder volume calculations, solder paste hole fill predictions, printing parameters, and stencil aperture design. A stand alone cost model and a tool to evaluate possible failure modes have also been developed. These intelligent agents are not discussed in this paper.

Prior to the implementation of the AART process in a manufacturing environment, several factors must be studied, understood, and characterised.

Commonly accepted solder joint quality criteria

Commonly accepted solder joint quality standards include ANSI/J-STD-001B (October 1996) and IPC-A-610. Depending on the classification (class 1, 2 or 3), minimum acceptable conditions are given for visual inspection.

Calculation of the required solder paste volume

Computation of the required solder paste volume needs to begin with the ideal solid metal solder joint. The ideal solder joint is a completely filled PTH with a fillet on the top and bottom of the PCB.

Solder paste related factors that can affect the volume required – aart volume model

Solder paste, in a simple sense, is a combination of metallic spheres encased in a flux binder. Methods that are used to modify solder paste characteristics include adding tackifiers and rheology enhancers, and altering flux chemistries.

Solder deposition methods including stencil printing, automatic dispensing, and solder preforms

 Stencil printing is the preferred method to deliver solder paste to the PCB for the AART process. The thickness of the stencil is a critical parameter, since the solder deposit is a function of the aperture area and stencil thickness.

Component  design and material issues

Since the through hole and odd form components that are to be soldered onto a PCB using the AART process are going to pass through a reflow profile, they must be able to withstand the temperature excursion

Stencil considerations, solder paste holefill, and overprinting

The thickness of the stencil must be carefully selected. Typically, stencil thicknesses between of 0.005″ and 0.008″ are used since these are a ‘drop-in’ for most surface mount processes. The area of an aperture is a function of the component pitch, number of rows, and the deposit-to-deposit spacing. A two-row component with a lead pitch of 0.100″, such as a 25-pin DSUB connector, is easily processed with nearly any reasonable stencil thickness. Four row memory modules with a lead pitch of 0.080“ begin to become more of a challenge as far as the stencil thickness is concerned.

Placement – options and issues

One reason for the resurgence of interest in through hole technology is the ability of automatic placement equipment to place odd form and through hole components.  Components can be shipped in tubes, reels, trays, etc. and the feeders are placed directly on the placement machines.

 Reflow profile development and recommendations

The oven used must be able to provide adequate heat (temperature) over the entire assembly at all lead locations. Many of the odd form/through hole devices are tall and/or have a high thermal mass when compared to other SMCs populating an assembly. It is generally accepted that a forced convection system is superior to infrared for these AART applications.

 Strength of AART solder joints

The segment of this research effort that compared the AART solder joint’s strength versus those produced by wave soldering had two objectives First, it benchmarked the strength of solder joints formed via the AART process against the traditional wave soldered joint. Second, it characterized the strength of solder joints that result when a less than ‘optimal’ amount of solder was used. The failure mechanisms that were observed were cataloged for different solder volumes and solder paste types. It was found that the strength of the joint decreases when the solder paste volume is less than 80% of the “ideal” volume. A transition in the failure mechanism was also observed as the solder volume decreased.

Solder joint inspection and quality criteria

Quality decisions for AART solder joints are usually based on the individual manufacturer’s requirement. There are, however, joint industry standards for typical through-hole component processes that may be used unmodified or slightly revised.  These standards include “Requirements for Soldered Electrical and Electronic Assemblies” [ANSI/J-STD-001B, 1996] and “Acceptability of Electronic Assemblies” [IPC-A-610, 1996]. The J-STD-001 describes final product requirements for PCB assemblies depicting minimum end product acceptable characteristics, as well as test methods for evaluation.

The vast majority of current PCB designs that contain through hole components are compatible with the AART process. In many instances, the same parameters that are used to assemble the SMCs on a PCB may be used for the through hole devices as well. The goal is to provide a “drop in” solution to assemble the through hole devices in the same step as the surface mount components. The AART process can be applied to a wide array of applications that cover automotive, communication, consumer electronics, computers, etc.

The full paper is available for download at http://www.smtinline.com/html-en/uicaart.pdf

*Authors: Jay B. Hinerman, Senior Applications Engineer, DEK, 8 Bartles Corner Road, Flemington, New Jersey 08822.

K. Srihari, Ph.D,  Professor – Department of Systems Science and Industrial Engineering, State University of New York, Binghamton, New York 13902.

George R. Westby, 3Director – Surface Mount Technology Laboratory, Universal Instruments Corporation, Binghamton, New York 13902-0825.

Contact Zalman Orlianski, Zetech, 011 789-3230, zorlianski@zetech.co.za

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