The demand of higher density PCBs is increasing day by day. The newest and latest gadgets, mobile phones, cameras and other accessories are at high pace of small and high speed multilayer PCBs. Although the multilayer PCBs have brought revolution in smaller packages and denser PCBs but this is not enough. Inventors are continuously striving to find different ways of reducing size, dimension and thickness of PCB for high density (meaning more component in lesser surface area of PCB) and high speed PCBs. Hence the technology PoP (Package on Package) is emerged.
Package on Package (PoP) means that two or more component packages are vertically mounted or stacked upon each other through a standard interface method to create electrical connection between them. This technology brings the finer pitch and small size package along with high speed processing useful for smartphones and digital cameras. Usually the logic IC is the bottom package while the memory device is at the upper package
Types of PoP Structure:
There two main types of PoP
1. Standard PoP Structure:
In this type the logic device is at the bottom package. It has the finer pitch with BGA solder ball types and having the large number of pin counts. The top package is the memory device with large pitch and small pin counts. Margin array makes the interconnection between top and bottom package.
The important technology used in this type of PoP structure is Flip Chip. The flip chip is all about a single die. In flip chip a semiconductor die is attached directly to PCB substrate by bond pad side facing downward. That is the chip is mounted upside down hence named flip chip. The solder balls are placed on circuit side. The conductive bumps present on the bond pad side are used to make electrical connections. There is a small gap that remains when the die is connected to substrate, this gap is filled by “Underfill”. This is a non-conductive adhesive to provide stress relief and robustness.
The wire bond is being rapidly replaced by flip chip technology to cope with growing demands of lesser package dimensions. As a result the solder pitch size in bottom package goes down to 0.4mm. On the other hand, top package memory device like DRAM and flash memory demands high speed and bandwidth so 0.65mm upper package device solder pitch is not sufficient and needs to be reduced to 0.4mm or less.
The main factors contributing to the PoP size are
1). Size of a logic device in bottom package
2). Number of Input / Output interface of logic devices
3). Overall bus count of power and ground;
4). Size of memory device in upper package and its I/O s and power and ground connections with mechanical ruggedness
5). High-density interconnection between logic device and memory integrated in BGA
6). The number of solder balls and pitch size in bottom package.
2. TMV PoP Structure
TMV PoP stands for Through Mold Vias (TMV) Package on Package (PoP)
This structure has the special merit of fine pitch. Due to which it is widely used in PCBs of handheld devices.
The interconnection between the top package and bottom package is achieved by Through Mold Vias (TMV). The TMV is in between the top memory device package and bottom logic device package. This via is in between the bottom package’s top solder paste and top package solder. The solders present on top and bottom package is the balls/spheres of solder that upon applying heat, melts and turns into cylindrical shape to fill the TMV.
Advantages of TMV PoP are high reliability, higher density in terms of interconnections, high speed performance due to short tracks/interconnection between top and bottom package unlike the wire bond package with turns and curls and good warpage capability
SMT Assembly Techniques of PoP
There are two types of techniques of PoP SMT Assembly.
In this, the top and bottom packages are stacked on top of other and then this joined combination works as a single component. Now this component is attached to the PCB by reflow soldering
2. On-Board Stacking:
Here, the bottom package is attached to the PCB first by means of solder paste stencil printing. After this the top package is mounted upon the bottom package by going through the dipping process of flux / solder paste. Finally reflow soldering of final stack.
We will discuss about the On-Board Stacking Procedure of PoP of Double Sided SMT PCBA. The steps involved in this procedure are
Step-1: Solder Paste Printing of Bottom Package:
The widely used 0402 package and CSP (Chip Scale Package) has the clearance range of 0.15mm and clearance for stencil printing is 0.127mm. The CSP packages are the ones in which the package area is not more than 1.2 times of die size. It is also important that it should be a single die. Another important aspect that is required to fall in the category of CSP is that the solder ball pitch should not be greater than 1mm. The two methods for solder paste printing that are commonly used are laser cut and printing stencils by electroforming. Type 3 and Type 4 solder paste are commonly used. The solder paste printing control method is the same for BGA, CSP, Bottom package of PoP and other components with fine-pitch.
Step-2: Top Package Dipping:
The top package dipping is done by means of integrated dipping system. The force caused by PoP on solder paste surface is directly proportional to the contact area. In order to efficiently pick the solder paste, the absorbing capacity can be improved or the surface area of absorbing tool can be enhanced.
Any irregular movement between the PoP and the solder paste surface can badly affect the process and resulting peripheral solder paste must be removed from soldering ends to avoid any defects in upcoming reflow soldering process.
The solder paste thickness is can be minimized by rubbing tool / blade that scrubs away unwanted solder paste on surface
The AOI or visual inspection system can be used to closely look into the unsuitability of solder paste thickness and viscosity. It can indicate the insufficiency or excessive solder paste / flux. The attributes of solder paste are also dependent on time factor that is, delayed waiting time in tray. The flux applied for visual inspection should be colored. White, black and red colors are commonly used.
The solder paste and flux dipping process has to ensure following parameters. These are
The homogeneity or uniformity is the key factor for good solder dipping. The homogeneity is measured by applying the dipping material on glass surface then inspecting by means of optical instrument to check the patterns of particles. If the grainy or wave type particles are found it is not homogenous while if the creamy / smooth texture is observed then it is homogenous.
B. Staying time on a tray
The dipping process of solder paste or flux, is done uniformly by means of doctor blade. This blade is fixed upon the rotating tray at ta particular know height that would determine the thickness of dipping material and depth of flux.
The time that is required to exposure to air and humidity is almost the 8-10 hours that is a usual working hours in an industry shifts. Also the viscosity changes are directly proportional to time spend on tray. The two important parameters that affect the consistency or amount of solder paste material transfer on bumps are dwell time and dip depth.
For example, The dwell time is about 200ms to 1000ms for dip depth of 142um to 190um for a package of 620 I/O pins with 0.4mm pitch TMV PoP 14mm x 14mm size.
C. Consistency between dipping amount and volume
It is a difficult process of measuring the consistency of dipping amount and volume of solder paste or flux. Especially for flux dipping it is very difficult. One method of checking the amount of flux is to measure the difference in weights of flux before dipping and after dipping.
The solder paste dipping consistency can measured by optical microscope.
The flux dipping consistency can be observed by following method
1) Apply flux in spinning pool and fixed rubber stand to make uniform distribution
2) Dip PoP in flux
3) Place the PoP on copper plate and then remove PoP to leave the flux on copper
4) Inspect using microscope at 45X magnification
Colored flux are much easier to determine for consistency. However coloring cannot determine the volume of dipped flux
D. Waiting time prior to reflow soldering after dipping
In case of flux, the waiting time prior to reflow soldering and after dipping will not have significant effect on soldering quality. But in case of solder paste, the waiting time has to be measured and it effect on soldering quality due to solder paste powder oxidization
An experiment can be conducted to test the flux and solder paste waiting time under extreme conditions. The aim of this experiment is to eliminate oxide prior to solder forming. The PoP dipped stack is first inverted and then exposed to experiment parameters to avoid damage of flux or solder paste. The parameters of experiment are.
1) High humidity exposure
2) Exposure to air at high temperature and 95% RH (Relative Humidity) for 2-8 hours
3) X-Ray inspection to check for bridging, holes, cracks or soaking
E. Flux dipping
Flux dipping features three leading advantages: size difference that originally occurs between solders won't be magnified; technique is controllable; material is easy to be picked.
Due to the low warpage of chip, the flux applied will not cause open soldering. The PoP has two main contact surfaces. One is in between the circuit and bottom package and other is in between the bottom package and top package. Both of them have the warpage problems. So the issue is resolved by increasing the thickness of flux that in result will not cause soldering defect but can surely create contaminant issues which can be a bigger problem in upcoming steps.
F. Solder paste dipping
Due to the warpage issues, the solder paste dipping has an upper hand advantage over flux dipping. Some of the merits of solder paste dipping as compared to flux are
1) Capability to overcome the warpage issues of substrate and chip
2) Suitable for current techniques, no extra techniques required.
3) The reliability of board increases due to larger/thicker distance between component and board
4) Easy inspection prior to surface mounting.
Step Three: PoP Component Positioning
The most critical part of PoP process is the position and control. The random vibrations in the mounting system especially in Z-Axis control can cause badly mounted component. The process of mounting SMT should be shock and vibration proof and accuracy should be maintained. Gas convection in reflow soldering machine and conveyer belt reverberations should be controlled for efficient component placement / positioning
Step Four: Reflow Soldering of PoP
The lead free soldering demands high temperature, so as a result the thin components of the order of 0.3mm can be deformed easily. Hence the requirement of delicate reflow soldering temperature. Also the lead free soldering can cause metal oxidation that tends towards bad wetting and incomplete solder. Moreover the internal temperatures of top and bottom components must not be too high. But at the same time the solder and solder paste should be melted in the bottom package effectively to form good soldering spots. It is very important that the reflow soldering temperature should be kept as low as possible to avoid thermal shocks and other thermal distortions. It is also important to mention that the humidity must be controlled to avoid popcorn because the packaging material of PoP is plastic.
Step Five: Optical and X-ray Inspection of PoP
Inspection is the step that cannot be avoided. It is not possible that a PoP assembly process is flawless. There are always some major or minor flaws / defect that needs to be identified. Some of these are excessive or insufficient solder paste, lifted pads debris, cold soldering, solder mask displacement and bridging. The inspection methods used are visual, AOI, X-Ray and endoscopy.
Endoscopic Method is less expensive than X-Ray and can detect defects that 2D X-Ray cannot. Like insufficient solder paste, open soldering and nonmetal debris. It can take snapshots and make video while soldering. However it cannot inspect BGA inside.
2D X-ray inspection is capable of inspecting defects like solder holes, bridging and tin ball. However, it is unable to find defects like cold and open soldering and inadequate solder paste under some conditions.
After the introduction of OVHM (Oblique View High Magnification) the performance become accurate and many other defects can now also be inspected. The resolutions is 8-10um and advance versions have 1um accuracy.
3D X-ray inspection combines the features of endoscopic and 2D X-ray inspection. It is capable of inspecting internal metallographic structure and solder connections. It is highly expense with slow inspection speed. It is suitable for complex technologies.
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