Publication date: 10 April 2008
Over the last couple of years engineers have started to experiment and in some cases implement stack packages, what is often called Package on Package POP.
In simple terms Package on Package POP represents the stacking of components one on top of the other either during the original component manufacture or during printed board assembly. POP packaging may include direct soldering, wire bonding or conductive adhesives for device to device interconnection.
During a SMART Group Workshop on 1st May we will illustrate stack package assembly with ball to ball assembly, area array on top of area array plus the rework process required. The details of the workshop are available at http://www.smartgroup.org/pdf/popassembly.pdf
The main industry problems associate with this technology are open joints, warping of the substrate and of course rework of the stack component or simply the top mounted part. Through modified design rules we hope to investigate warping and open joints during the assembly process with use of daisy chains circuits on the printed board and stacked packages.
With any new technology it is inevitable that engineers must consider production trials on assembly, rework and inspection. This is necessary to define process parameters, demonstrate capability to customers and allow the training of production staff with theses new devices. Often parts are necessary for equipment trials if new machines are required for product introduction. One of the major issues is the cost of fully functional or dummy BGA components for these very necessary projects.
Having faced the same financial problems in the past when setting up production lines and practical demonstrations at exhibitions a solution needed to be found. So why not just make your own devices for a few pounds using existing manufacturing facilities. It should be bore in mind that a number of BGA manufacturers effectively do this on their own products.
Demonstration BGA’s have been used successful in production for a number of years, they have also been used for the assessment of ball shear strength comparisons on different laminate types. They are an ideal solution to the problem on component availability.
Quite simply they are just 1- 1.6mm FR4 laminate circuits with 63% tin 37% lead solder terminations formed by stencil printing paste and reflow. In the last five years they have often been produced with lead-free solders like tin/silver/copper.
First a circuit panel approximately 200mm x 400mm is produced on double sided copper laminate. A multiple pattern of 361 0.025" round pads are etched on to one side of the panel with a large copper area on the opposite side to prevent warping. Obviously if different pin out devices are required they can be produced in the same way.
Remember if you are already laying out a new design you have the necessary pattern. The panels after etching are coated with solder resist and imaged to expose the round copper pads. Alternative pad sizes can be used to gain experience in processing the parts.
The final fabrication operation is "V" scoring the panel to allow each of the parts to be removed at a latter stage. The scoring operation defines the BGA size. The exposed copper pads are treated with an OSP, silver of nickel/gold coating to preserve the solderability. The OSP is often used as this is the simplest and most cost effective process.
To form the terminations panels are printed with solder paste with a stencil on a conventional printer before reflowing the panels. The solder paste apertures are 0.030" etched in a 0.010-0.012" stencil. Increasing the stencil thickness can be done but the amount of paste defined in such a small area tends to make the stencil stick to the board and lift it at the end of the printing operation.
During the reflow operation the solder paste reflows, wets the pads and due to surface tension the solder paste forms a ball like termination. Ideally this is conducted in a nitrogen environment to preserve the surface of the ball terminations. The use of nitrogen also provides a more ball like appearance.
Depending on the solder paste activity used the panels may be cleaned at this stage. A key factor of the paste is that it must not slump during reflow as it will cause multiple shorts if it does. The yield in terms of BGA devices from this manufacturing technique will be between 85-90%.
There are three common alternatives to the printing process one is simply dispensing solder paste onto the surface of the panel which may be slower but can achieve very good results and increase the volume of the balls.
A further alternative is the DEK DirEkt Ball Placement process which directly prints the balls on the surface of the board. A tack flux is used to locate and hold the balls in place during handling and reflow. Placement machines from Europlacer also have placement heads developed for ball placement.
It is difficult to achieve the same volume of solder in the balls using printing but for all practical purposes this is not so important. When the panels are broken up there are hundreds of parts available for production trials. Using this technique guarantees that plenty of the components are available for production trials and more importantly for operator training.
Often due to the cost of normal parts operators are just given one or two to play with. To gain experience before working on real products its important that repair operators get some first hand experience before moving on to the real and expensive parts.
The same technique has been used for CSP and flip chip parts. The flip chip parts were much more demanding and the yield was much less but still proved to be a cheaper option. Both tin/lead and lead-free parts have been produced using these methods. The following examples were produced for Cookson Electronics Flip Chip Hands On Workshops run in Europe.
Bob Willis is a process engineer providing engineering support in conventional and surface mount assembly processes. He also runs production lines, seminar and workshops world wide on site and at exhibitions. Bob will be running his “Process Advice and Defect Clinic” at SMT Nuremberg in June on Stand 209 Hall 9.
Bob has one of the largest collections of training videos, interactive CD-ROMs and training material in the industry. For further information on how Bob may be able to support your staff contact him via his web site http://www.ASKbobwillis.com
