General Purpose Lead Free (GPLF) Process Optimization Study

Much of the lead free volume printed circuit board assembly operations to date have been carried out by manufacturers producing products that fall within the scope of EU RoHS and similar national directives/legislation.  These circuit boards are typically characterized as being simpler in component parts list complexity and smaller and thinner in size. In addition, reliability expectations of these products are not as critical as many types of equipment currently exempted from lead Free solder legislation. One of the big challenges facing many of the member companies within the HDPUG consortia is that their products are larger, thicker, use a broad variety of component packaging styles and have long field life/reliability requirements. This pushes the limits of current assembly technology at a soldering material, equipment and component level; both for primary component attachment and for rework.

 

A key issue in adopting lead (Pb) soldering technology to manufacture more complex and thermally heavy products is in the control of the temperature settings during the soldering reflow process. The tin silver copper alloy adopted by most of the industry as a Pb-free substitute for tin lead solders has a much higher melting point and this drives the need for higher reflow processing temperatures. For high complexity designs with a wide range of component formats, sizes and weights, maintaining similar peak reflow temperatures across all the components is difficult. In extreme cases, ensuring enough heat to satisfactorily solder large components can lead to smaller components being excessively overheated, threatening the functional reliability of products. Pb-free soldering therefore presents significant challenges to PCB assemblers forcing them to work in a tighter process window compared to that defined for tin lead soldering operations. Understanding the extremes of this window, in terms of maximum and minimum peak temperature, as well as time above the melting point of the alloy is critical to optimize the surface mount assembly process.

The objectives of this project were therefore to:

 

·        Characterize temperature profiles across complex PCB designs.

·        Determine the minimum peak temperature needed to guarantee acceptable quality solder joints.

·        Develop optimized time/temperature reflow profiles to minimize temperature peak and variation across components on a printed circuit board.

·        Assess the reliability, using accelerated temperature cycling techniques, of products manufactured with these optimized profile process parameters and at the temperature extremes of the process window.

 

This report details the findings of an extensive collaborative research project, into the performance of Lead Free solder joints connecting a wide range of electrical components to printed circuit boards. The study includes major contributions from 10 HDPUG member companies. It describes in detail the results of reflow profile optimization studies and evaluates the solder joint performance under accelerated thermal cycling testing conditions of a wide range of component types. It is accompanied by an in depth database of analytical results with over 400 photographs detailing failure analysis and solder joint microstructures.

 

The report contains valuable data for all manufacturers and producers of all printed circuit assemblies and in particular for those in the process of converting designs to be Pb-Free compliant.

  

NOTE:  This is a very extensive document package with many pictures and tables that are located in several files.  A link system is provided within the report structure that allows the reader to drill down through the data to pictures of the specific joint in question.  Because of the number of files, the document package is presented as a 225 meg compressed (zip) file.  You will need a copy of winzip, winrar, or similar decompressing software.  Copies are available on the web.