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Process Control Strategies for EMS Assemblers
By Frank Silva, FocalSpot, Inc., San Diego, CA
ment process contributes to 15 to 20 percent of the most common SMT defects, such as missing parts, wrong parts, misplaced parts, and wrong polarity. In addition, 50 to 65 percent of Board Assembly defects result from uncontrolled solder paste print- ing and solder formation processes which lead to defects such as opens (co-planarity, head-in-pillow), shorts, tombstoning, and excess or insuffi- cient solder. While solder voiding is not a defect but rather a process vari- ation, voids can lead to field reliabil- ity issues. By using simple tools to auto-
B
mate the inspection process, EMS providers can achieve higher yields and less defective boards being shipped to the end user. FocalSpot offers several production assembly inspection tools to aid in improving process control to improve yields and find defects quickly and easily to ensure customer satisfaction.
First Article Inspection Let’s start with the data neces-
sary to build a PCB board. EMS providers struggle with collecting the proper data from the OEM to build a board and wind up manipulating data as they prepare the programs for the screen printer and placement machines. When the customer data first comes in, it often does not include a bill of material and may even lack a proper assembly drawing. EMS providers must cobble together the necessary data to quote the job in the first place and then prepare the equipment files for the assembly machines to build the product. Much can be and often is lost in this very
esides improper design meth- ods, it is well documented that the Assembly setup and place-
first step. While the centroid file tells the placement machine where to place each part including its rotation, it does not normally include a part description and possibly even the part number physically marked on the component. A bill of material (BOM) will always include this information.
of the components to be placed onto the board, including SMD values.
The Inspection Process Every first board built goes thru
Matrix X3 Series AXI x-ray system. Now using FocalSpot’s FA
Inspector Software a simple software tool can be used during the quotation process or assembly data setup to check the assembly data file and compare it to the BOM. If the two files do not match, the EMS provider can quickly communicate the infor- mation internally or with the OEM to resolve these issues before building the assembly programs. If a PDF or JPG of the assembly drawing exists, the FA Inspector also has a tool to visualize the data and ensure that all the proper parts data from the cen- troid file, including polarities, align onto the drawing as an effective way to check component polarity and data integrity. The result of this labor is that there is a complete description
a first article visual inspection process prior to completing the full run of boards whether this is a one board build or thousand boards. In most manufacturing environ- ments this is a manual process including using hard copy print outs of the program list of each reference designa- tor and bill of materials. Operators check every refer- ence designator part visually by eye or using a microscope to check the board for pres- ence/absence of parts, position of the parts, and polarity. Each and every reference des- ignator is marked off the list using color highlighters as well, if the drawing exists.
This is where it’s important to check the drawing, using a
printed drawing showing the refer- ence id and polarity marks. This process is time consuming and after the fact only paper documentation of the inspection exists. On the inspec- tion side what is often missed at this stage is whether the right part num- ber has been mounted. Checking part values is an
important step prior to running pro- duction boards . This can be accom- plished with taking parts out of the real and probing them to measure their values. This is not completely ideal but better than not checking them at all. You can also check part values on parts which are marked using cross reference lists of stan- dard SMD components. The FA Inspector software uses a simple and
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easy-to-use drop-down SMD cross reference list to check against the part number. But that alone does not fully ensure that the proper part val- ues have e been mounted. A good technique is using the sticky board with a combination of probing and automatic part checking. Using the FA Inspector can
automate this process step including checking parts on an unsoldered board for proper values. After loading the centroid data, BOM and available drawing, the FA Inspector can scan the first board to provide a single-pass full image of the PCB. Using the FA Software each reference designator is automatically presented to inspect either visually on the screen or by simply drawing a region of interest around the part label to create a part picture template that the machine
Controlling the SMT
assembly process involves many variables, including equipment limitations and human error.
will test. These picture templates, which are linked the part number, form the basis of the inspection library together with the package models to allow for automated first article testing or AOI inspection.
Pre-Reflow Inspection Using the In-line FA Inspector
pre-reflow can automatically identify missing parts, wrong parts, off-posi- tion parts, and improper polarity. While traditional AOI machines are usually deployed after the oven as they work hard to attempt to posi- tively find solder defects — which is no easy task for even multiple cam- era systems — a machine focused on component defects takes less time and is easier to set up than for post- reflow inspection. Have you ever seen an AOI machine located post- reflow where the solder inspection algorithms have been turned off?
Post Reflow Inspection In an effort to monitor and con-
trol the solder paste process and oven profiles, EMS providers complete a post reflow inspection step of the pop- ulated SMT parts post reflow. Many facilities today have elected to use AOI and X-Ray equipment at this stage of the inspection process. Smaller facili- ties still carry out manual Vision inspection using a microscopes. Finding print-related and sol-
der composition defects after the sol- der formation process is not an easy task for an AOI machine. AOI machines simply cannot see the entire soldered area under a lead or in hidden areas (such as Gull Wing heel regions or J-Lead toe regions) completely. X-ray Inspection is more efficiently suited for monitoring and finding these defects as the entire pad/solder connection is visible to an x-ray machine. Today’s typical boards include
Continued on page 64
March, 2012
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