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VOLUME -23 NUMBER 7
Publication Date: 07/1/2008
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Special Feature: Assembly and Packaging
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July 2008 Issue
Special Feature: Assembly and Packaging
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Solder Paste Analysis: Meeting New Standards
Results of a tack test.
By Graham Naisbitt, Managing Director, Gen3 Systems
In the electronics industry, analysis of manufacturing defects shows that solder problems are largest single cause of failure. And with surface mount technology now the dominant assembly method, many of these soldering failures can be traced back to poor housekeeping of solder paste creating problems during printing, placement and reflow.
The International Electrotechnical Commission (IEC) and Association Connecting Electronics Industries (IPC) standards bodies have quantitative test standards for solder paste (although the current standards only cover tin/lead pastes). Publication of equivalent standards for lead-free alloys is planned for 2009). IEC 61189-5 and IPC-TM-650 include four and five solder paste characteristic tests respectively.
These tests provide a framework for ensuring that the solder paste is up to the job, and are carried out by the solder paste manufacturer prior to dispatch. However, until now, the tests have been complex and time-consuming, requiring a separate machine for each test. But a new generation of all-in-one machines allows a semi-skilled operator to complete the entire suite of tests in 15 minutes. Now, by checking the characteristics of the paste before using it, manufacturers can minimize defects that may otherwise manifest themselves in the field — when rectification is very expensive and damage to reputation is painful.
Today's solder pastes and stencil printing equipment have advanced dramatically to meet the demands of high volume or high mix manufacture of fine pitch, densely populated electronics assemblies. Paste manufacturers go to great lengths to ensure that their products meet the specifications laid down in the IEC and IPC standards.
However, solder paste is a complex combination of solder powder, flux vehicle and other additives that are there to enhance the paste's performance during the printing process and in the time between manufacture and use, paste properties can change. Worse still, once the paste is opened and "conditioned" (allowed to reach room temperature and stirred to ensure a homogeneous mix of the paste's constituents) it begins to deteriorate rapidly. This deterioration is due to evaporation of solvents and increased oxidation of the solder spheres due to the air that has been introduced during mixing. The period during which the paste is acceptable for production is known as the "open time".
Paste that has exceeded its open time, or is in poor condition due to inadequate storage or storage beyond its shelf life will cause faults during printing, placement and reflow. Defining open time is critical for ensuring that identical products made in separate factories enjoy the same quality levels. That's not to say that open time will be equal in every location. A specific brand and type of solder paste may have a shorter open time if, for example, it is used in a warmer or more humid environment.
Apart from the open time, the paste has other key properties that determine its performance at various stages of the manufacturing process. The key property during printing is slump. Slump describes a physical property of solder paste which determines how the cross sectional profile of paste printed on a pad "relaxes" after printing. A degree of slump is inevitable, but excessive slump can allow the paste to bridge the gap between adjacent pads, especially on fine pitch PC boards.
Tack is the property of the paste that is most relevant to placement. Tack is a measure of the stickiness of the paste and is essential to ensure that placed components stay put during the placement cycle and subsequent handling prior to reflow. If the paste has insufficient tack, components fall off the PC board or become misaligned, or can tombstone during reflow. Each of these faults results in expensive rework. Maintenance of tack after the paste is applied to the board (tack decreases as the solvents in the solder paste evaporate) is essential to allow a reasonable time between placement and printing.
Manufacturer's open time test.
Other assembly faults due to substandard paste don't manifest themselves until during or after reflow. Worst still, some of these faults may not appear until the product is in the field when the cost of rectification has increased a thousandfold compared to repair at the automated test station. These faults are excessive solder balling and spreading, and poor wetting.
Solder balling is a phenomenon caused by lack of heat and oxidation and can result from insufficient cleaning of the surface to be soldered by the flux (i.e. poor solderability) and insufficient heating of the solder paste. As the solder starts to melt, small balls form around the joint, but as the rest of the solder melts the solder balls fail to coagulate with the main mass. Later, the solder balls can break away from the joint and cause short circuits. Wetting determines the integrity of the solder joint. Molten solder introduced to a contaminated or oxidized copper pad fails to wet properly and forms a weak or "dry" joint. A weak or dry joint can fail immediately or may survive for years in the field until failing catastrophically. One of the flux's tasks is to remove contaminants and oxides to promote good wetting.
Excessive spreading occurs when the flux is too active and can cause two faults: solder reflowing over the edge of the pad, and excessive residues on the boards. The flux's activity is a balance between ensuring the solder wets properly and leaving excessive contaminants on the PC board. Insufficient activity limits cleaning and deoxidization and residues can cause problems for automatic test equipment by clogging test probes and causing phantom faults. Moreover, excessive residues can cause electrochemical reactions in the presence of moisture and a bias voltage resulting in short circuits in the field.
All-in-One Test System
Although there is not yet an IEC or IPC standard for open time, IEC 61189-5 and IPC-TM-650 do include comprehensive tests for slump, tack, solder balling and wetting (and in the case of the latter, a test for spreading). Committees made up of independent technical experts from OEMs, EMSs and equipment manufacturers have devised the tests. They represent a qualitative benchmark for determining solder paste suitability with good Gauge Repeatability and Reproducibility (Gauge R&R).
Adhering to these test regimes is the best way to test and document solder paste suitability at any time, any place and by any operator. This eliminates any contention when determining who is responsible for manufacturing faults. Indeed, if the solder paste passes the test then the manufacturing faults caused by solder paste in poor condition will be all but eliminated. However, the tests are detailed, time consuming and require skilled personnel to interpret. Fortunately, equipment manufacturers have eased the process by introducing test equipment that automates the individual tests, allowing semi-skilled operatives to perform them and saving time. Nonetheless, five pieces of test equipment represent a lot of capital equipment outlay and require plenty of expensive real estate on the shop floor.
Spread test results.
Gen3 Systems, has released a single integrated machine that performs all the tests detailed by the IEC and IPC, plus an open time test. Called the SPA1000 Solder Paste Analyzer, the machine promises to improve quality, increase process yield and reduce inspection and rework. The SPA 1000 performs slump, solder ball, tack and wetting in accordance with IEC 61189-5 and IPC-TM-650 and adds an IPC-TM-650 spread test and the manufacturer's own open time test.
The open time test is performed by printing samples using a just-opened container of solder paste at regular intervals (for example, every 30 minutes) and conducting a tack test. When the tack force drops below an acceptable limit the paste is deemed unsuitable for use. The time between the first test and the test where the paste no longer meets the tack force requirement is the open time. Once the open time is exceeded during production, the solder paste is discarded.
The SPA 1000 runs from a Windows
-based PC running either Vista
and presents results in a familiar graphical user interface (GUI) with 10 and 30x magnification and image capture. The machine can be used by a semi-skilled operator in around 15 minutes and takes up a fraction of the space of the individual test equipment ordinarily required for all six tests. It provides good Gauge R&R and is simple to calibrate.
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